CA1320600C - Vertical filtering apparatus for raster scanned display - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus and method is disclosed for reducing flicker in an interlace computer generated raster-scanned display. Several circuits and methods are described for implementing vertical filtering. In general the data for a given pixel on the nth, nth+1,nth+2...nth+N line is read from memory and convolved to provide data for a given pixel on a display line before pixels data for other pixels along these lines is read from memory.

Description

BACKGROllPII) OF THE INVENTION:
1. Fiald ~t the Invantion.
The invsntion relates to the ~ield of filtering of video signals ~or a raster scanned display, particularly one employing computer ~enerated pix81 data.

2. PriorArt.
Most caihode ray tube (CRT) compu~er video displays are formed with a rast~r scan. Many of the standards used with these displays can bc traced to 1 Q television standards. For example, two interlaced fields are frequcntly used to form a frame. Many early personal computers provided compatible NTSC: si~nals to p~rmit a user to use low oost television receivers. In other instances, computers generate signals such as overlays which are used in conjunction with NTSC
signals. Thus, personal computers often generate pixel data for use on interlaced, raster-scanned displays.
Computer generated data has some oharacteristics which maks it less desirabls for an interlaced, raster-scanned display than video signals originating in a video camora. For axample, pixel data can ~xhibit chan~es (e.g., amplitude) over its entira range from pixel-to-pixel. That is, virtually any change in pixel data can occur from one pixel to the next. In contrast, video data from a traditional vidao camera uses a beam spot which encompasses more than a single pixel aroa. The data interpreted for a single pixel in this case takes into account to some ext~nt the intensity and color of the surrounding area. Therefore, there is a softening, even a blurring, that occurs as the beam scans the image in a camera.

1 3~0600 Th0 human visual system is an ~dge-det~ction system. Th~ 4~y~s are very ~ood at finding contours that delin~ate shapes. To ~ive an ~xampl~, when tlisplaying a sequence of adjacel~t ~ray bars of increasing density on a comput0r display, thQ ~dgss batween th0 bars se~m ~mphasized. Perceptually the ~ray bars 5 do not look liks solid colors, bu~ rath~r they look lik~ they have been shadedbetween thcir edges. In sther words, the border bstween th0 gray bars appear cnhanced by the edge-detection mechanisms of tha 8y~.
When a typical real world scena is displayed on an interlaced display, there are no abrupt transitions from one scan line to the nsxt. Objccts ~enerally do not 10 have very hard edges, and those that do usually do not hav~ edges lined up with a scan line. The resuit is ths eye cannot find an ed~e from ona scan lins to th~ next.
If ths eye cannot find an edge between on0 scan lina and the next, it cannot distinguish betwe~n lines. In an interlaced display a cornplete frame is drawn each 1/30th of a second, however, because of the in~erlacing 0ach 1/60th of a second,15 Qither a given scan lin~ or the next scan line is flashed. The eye perceives these multiple scan lines as thick single lines flashing at a 60 frame/s~cond rate even though thsy are in fact flashing at 30 frames/second. By this modell close viewin~
of an interlaced display should resuit in perception of flicker at 30 frames/second.
This is in fact what happens; if one is clos~ enough to view individual scan line on 20 a NTSC tel~vision, interlace flicker (i.e., 30 frame/second flashing3 is seen, even ` with a r~al world image.
In the case of a computer generatsd image such as a MACINTOSH
comput~r imag~ on a interiace display, virtually every piac~ whcre th~re is oth0r than solid whit~ or solid black th~re ar~ abrupt transitions in the vertical dim~nsion.
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(Macintosh is a registered trademark of Apple Computer, Inc.) In the cas~ of tha"racing s~ripes" (alternately black and white horizontal lines) on ~he top of a typical Macintosh window, ther~ is the most abrupt transition possible, black to white, stretchad across ~he leng~h of th~ window and repeated for several lines~ Here, i~ is 5 easy for the human oy~ to detect th~ ~dge from one sGan line to the next, so it considers the scan lines as individuals, flashing at 30 frames/second. The visual perception of the human observ~r is that where th~re are abrupt transitions on ~he display, tha NTSC im34ge flickers noticeably snough to be distracting~
One additional subtlety is worih mentioning~ Tho human eya will ~ee flick~r 10 display wherever there are transitions (i.e., edgas) in the vertical dimension. But, the degree of flicker is not uniform for each typ~ of graphic pattern. Th~ worstpattem is ~he racing stripes across the ~op of a window, mentione~ above. Tex~ and other random pattarns flicker as well, but not nearly as severely. This is account~d for by the fact that it is easier to discern vertical edges where there is a high 5 horizontal correlation ~o the patt~rn (as in the case of the racing stripes), but harder to find the ~dges when there is a low horizontal correlation (as in the case of tes(t).
(As will be sean, since the present invention provides adaptive filtering for ~he subtlety.) Numsrous prior art techniques are known including those 6mploying anti-20 aliasing filters for removing this flicker. In som9 cases, fiHers duplicate the soflening effects of tha camera bsam, that is, pixal data for a cluster or spot of pixels is ~a~Yerage~ or Pconvolved~ to producs filtered pixel data. In gensral, thsse - t~chniques require considerable computational overhead.

~, 1 32~6~0 As will be seen, the present invention provides filtered pixel data, however, only in tha vertical direc2ion. The convolving performed by the present invention to provide ~hs ~iltered pixel data is done "on the fly" since the computational demands are substantially less 2han tha~ requirad by prior art systems.

SUMMARY OIF T3 IE INVEN~ION
An improved method for generating vertically filtered pixel d~ta in a ras~er-scanned video display is dascribed. The vicleo data is stored in a frame buffer which is organized by scan lines with data being stored for each pix01 along ~ach 5 sc~n line. To ganarate th~ filtered pixel data ~or a first pixel of a first lins, pixel data is read from the buffar ~or tha first pixel of the nth, n+1, n+2. . . n ~ N scan lines. This is done b~fore pix~l data for other pixels along these scan lines is read ~rom the ~rame buffen This avoids the noed ~or a plurality uf line buffers or evsn additional ~rame buffers if all the data was read for each of the scan lines. The pixel data for 10 the flrst pixel of the nth, n+1, n+2 . . . n+N scan linss is convolved to provide ~he pixel data for the ~irst pixel of the first scan line.
In one currently preferred "hardware~ embodiment, where N=2, the pixel data is stored in two banks of video RAMs (VRAMs) with the video data for scan lines 0, 1, 4, 5, 8, 9. . . stored in ona bank and the video data for scan lines 2, 3, 6, 15 7, 10, 11. . . stored in the other bank. ll~en th~ data ~or pixel 0 of scan lines 0, 1, 2, ~`and 3 is read from the VRAMs. The data for pixel 0 of lines 0, 1, and 2 is convolved.
The data ~or pixel 0 of linc 3 is ignored. Ths procass is repeated for pixel 1, and so on to complets tho line.
In this embodiment, the convolving step consists of a r~latively simple 20 weighting with half the weight being given to the pixel data o7 the n and n+2 scan lin~s when compared to the wei~ht given to She pixel data of the n~1 scan line.
Other details of the presen~ invention will be apparent frorn ~h~ following detailed description, including "software~ embodiments.

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E~RIEF DESC:RIPTION OF TtlE DRAWINGS

Figure 1 is a general block diagram illustrating the general placement of the present invan~ion in a video system.

Fi~ur~ 2 is a diagram used to illustrata a method used by th~ present invention to r~ad data from a frame buffer.

Figure 3 is a diagram used to illustrat~ an alternate method used by tha 10 present invention to read data from a frame buffer.

Figure 4 is a block diagram illustrating an embodiment of a convolvsr used in the prasent invention.

Figure ~ is a block diagrarn illustrating another ernbodiment of a convoiver used in the present invantion.

Figure 6 is a block diagram illustrating another method for obtaining convoived data particularly useful where not many bits ar0 stored for ~ach pixel.
Figure 7A is a block diagram of a general prescaler which can be used with tha convolver of the present invention.

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Figuro 7B is a block diagram of another prescaler which can be used wi~h th~ convolver of the present inv~ntion.

Figure 8 is a block diagram of a circuit illustra~ing one pres3ntly preferred 5 hardwars ~mbodiment of the invention employing two-way interleavin~.

Figure 9 is a diagram illustratin~ pixel data flow used to describe the operation of the circuit of Figure 8.

Figure 10 is a diagram illustrating the relationship between CPU and VRAM
addrass~s for the two-way interleaved frame buffer of Figure ~.

Figure 11 is a block diagram of a circuit iilustrating another hardware embodiment of the present invention, this one employing four-way interleaving.
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Figur~ 12 is a diagram illustrating pixel data flow used to describe the operation of the eircuit of Figure 11.

Figure 13 is a diagram illustrating the relationship between CPU and VRAM
20 addresses for the four-way interleaved tram~ buffer of Figure 10.

Fi~ure 14 is a flow diagram illustratin~ an embodiment of the present invention implemented in a software program; this embodiment is for a planar . ~rame buffer.
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Figure 15 lilustratss in more graphic form a planar frame buffer.

Figur~ 16 illustrates a first step in irnplementing the present invention in 5 soRware ~r a "chunky" frame buffsr.

Figur~ 17 illustrates a second step in the implementation described in conjunction with Fi~ure 16.

Figure 18 illustrates athird step in the implemen~ation described in conjunction with Figures 15 and 17.

Figure 19 illustrates a ~ourth step in the impl0mentation described in conjunction with Figures 16~18.
~ 15 `~ Figure 20 illustrates a fi~h step in the implemen~ation described in `. conjunction with Figures 16-19.

Figuro 21 illustrates gray values load~d into thc color lookup ~aùle.

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DETAILIED DESCRIPTI :3N C3F TilE PRESENT INVENTION
A method and apparatus for providing filt0rin~ data, in a rast~r-scanned video apparatus is described. The invention provides filtering in the vertical direction (perpendicular lo the direetion o~ the scan lines). In the followin~
5 d~scription numerQus ~pecific d~tails are set forth in order to provide a b~tter und~rstandin~ of the present invention. It will be obvious, howev~r, to one skilled in the art that the present invention may be practiced without th~se de~ails. In ot~er instances, well-known circuits and computer operations have been shown in block diagram form, in order not ~o obscure the present invention in unnecessary detail.
OVERVIEW OF THE PRESENT INVENTlt: N
Referring first to Figure 1 a ~rame buffer 10 is illustrated which in th~
presently preferred embodiment may be an ordinary frame buffer, for example, onefabricated from dynamic random-access memories (DRAMs) or video random-access memories ~VRAMs). Most often, data is or~anized in tha frame buffer by 15 scan lines with data bein~ stored for each pixel along each scan line. In some casas, the pixel data is organized in planes such that the pixel data ~or a ~iven pix41 iS stored with aach bit on a differ~nt one of the planes (i.a., planar frame buffer of Figurs 15). When data is organized in this arran~ernent, a scanning address causes a bit from each plan3 to be read from the mernory, the bits ara assembled to 20 form a pix~l, and hence, the data for a giv~n pix~l is road from ths mamory for a vidso display. (Often when writing data to a memory organized by planes, an 8 or16 bit word is written to each plane; this is particularly useful fsr a black and white, or two-color display, where only a single bit is stored per pixel and henc~ data is written into only a single plane.) For some ~mbodiments of the presant invention, 1 32~600 the data is stored in an ordinary manner as described above. In other embodiments, the data is two-way interleaved between two banks o{ mamory as will be d~scribed in oonjunction with Fi~ure 8. Even with this arrangement, however, th~ data is still stored by scan lines.
An address gen~rator 11 is us~d ~o address the data in thB buffer to provide an output signal for a video display. As will be seen with the present invention the order in which th~ data is scanned from th~ buffer is differ0nt than th~ used in ~he prior art and hence, the address gen~rator 11 provides this unique addressing order. (This is referred to as "kernel-scanned~ in Fi~ur~ 1.) The speciflc ord0remployed will become apparent, particularly from th~ discussion balow for Figures 2 and 3. Ordinary circuits may be used ~o implement the ~enerator 11 and providethe order described in Figures 2 and 3. As in the cass with prior ar~ generators, the address generator 11 is g~nerally synchronized with a dot clock.
The output from the buffer 10 is convolved by oonvolver 12. The output of tho convolver 1~ is pixel data which can be us~d in an ordinary mannsr for a video display. Ths convolv~r 12 is described in conjunction with Figures 4 and 5 and portions of it are described in conjunction with Fi~ur~s 7A and 7B.
In the currentiy preferred embodiment th~ output of the convolver 12 is gamma corrected. Such gamma corrsction is well-known in th~ art and used to compensate for the non-linear light intensity curvc of CRT displays. Th~ digitalinformation on line 14 is converted to an analog ~orm ~r coùplin~ to a display.
In thc following descripticn it is assumed that the buffer 10 stores pixel data.It will be appreciated that th0 buffsr may stor~ pointers to another memory such as ~`

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a color lookup table. In this event, the output of the buffer 10 is coupled to a color lookup table and ~he output of the co70r lookup table is coupled to tha cenvolver 12.
In Figur~ 2 it is assumed that each of ths blocks in the illustrated grid rspres~nts a pixel in a bit mapped buffer. In tha horizontal direction the pixels are numbered from 0 through 9; it will ba apprsciated tha~ in a typical memory, manymors pix~ls arc us~d in th~ display. In th~ v~rtical dirsction the rows of pixal data ane numb~red by scan lin~ beginnin~ with lin~ 0, and ~nding at line 5. Again, it will be appreciated that in a typical display rnany more scan lines ara used. Figure 2 thus represents the da~a organization that is ~ound in a typical fram~ buff~r.
For tha present invention the data for a given pixel (e.g., pixel 0) is read from the memo~ for seYsral lines (~.9., lines 1, 2 and 3) befors the pixel data for pixel 1 is read from the memory. The pixel data for the several lines of a given pixel is convolved to provid~ the pixel data used by ths display.
More specifically, in Figure 2 the data at loca~ions 16, 17 and 18 is read from 1~ ths memory before tha data for pixel 19 is read from th~ memory. The pixel data from looations 16, 17 and 18 is then convolved to provide pixel data for pixel 0 of a display line. N~xt, the pix~l data at iocations 19, 20 and 21 is read from tha m~mory and convolved ~o provid~ pixel data for pixel 1 of th~ display line. Thisprocess continues 70r each of the pixels 0 through 9 for soan lines 1-3 to provide 20 pixel data for a given display lins.
For the illustrated embodiment thr~ lines of data are used in the convolution process. Any number of linus may in fact be used, for ~xample, the data from lines n, n+1 e n+2 . . . N~n may be first used to provid~ pixel data for a first .~ display line. Following this, the data from lines n+1, n+2, n+3 . . . n+N+1 is used to J
provide ~ha pixel data for a second display line. However, the data is used from th~
buffer such ~hat all the pixel data for, for instance, pixel M is read for all ~he scan lines bsing used in the convolution before the pixel da~a for pixel M+1 is read from the buffer.
In some cases, the addressing and mapping scheme used for a fram0 buffer provides more than the data for a ~ingle pixel for each address. As illustrated in Figure 3 assume that a singls ~ddress provides the pixcl data for pixel O and pixel 1 ot line 1, this is shown by the enclosing linc 23. With the present invention, the data assoeiated with line 23 is first read from the msmory, followad by tha convolution of data associated with lines 24 and 25. Then convolution is performed on tha data for pixel O of lines 1, 2 and 3 for pixel O, followed by tha data for pixel 1, for lines 1, 2, 3. Now the data associated with lines 26, 27 and 28 is read from the memory, and so on.
In a specially organized fram0 buffer such as that described in a pres0ntly preferred embodiment, a singla address provides thc data for saveral lines in the buffer~ For example, a singla address may provide ths data associated with lines~3, 24 and 2~. In this avent, the first data for pixel O is convolved, then that of pixel 1 is convolvsd. Af~er that the data associateà with lines 26, 27 and 28 is read frsm ths memo~ and tha data for pixel 2, and then 3 is conYolved. This process ~û continues for all the data along the iine for scan lines 1, 2 and 3.
Thus, in general, the data for a flrst pixel for scan lines n, n+1, n+2 . . . n+N is read from tha buffer before the pixel data for subse~uent pixels on these scan lines is read from the buffer. This data is then convolved to provide pixel data for a single pixal. This process is repaated for each of tha pixels along the scan lines n, n~1, n+2 . . . n+N. Following ~his the data for a first pixel along lines n~1, n~2. . . n~N~1 is r0ad from the buffer, again before the pixel data for subsequerlt pixels alon0 these lines. This data is then convolved to provide the fittered pixel data for the first pix91 of the next display line. This procsss is rcpeated until vertical filt~r~d data is 6 provid2d for ~he entira display.
EMBODIMENTS OF TIIF C:ONYOLVER
As mentioned above, the pixel data from "N~1" lines of data may be convolved. In the currently prefarred smbodiment N=2. (Th~r~ is a disoussion of con~olution for other kemels later in this application.) in thi~ case, N implsrnents 10 the equation aPl+bP2~aP3 where P1 is the pixel data for the firs~ pixel 2a + b of the nth scan line, P2 th0 pixel data for tha first pixel of the nth+1 line, and pixei 3 ths pixel data for the n+2 scan line. ~a" and Ub" are constallts with Yb" usualiy bein~
greater than "a~. In a typical application a=1 and b=2.
In Figure 4, the convolver (corresponding to kern~l convolver 12 in Fi~ure 1) 15 includes a prescaler 32 which receives the input pixel data from the buffer. The amount of prescaling performed by prescaler 32 is controlled by the output of the coefficient table 33. The output of table 33 is controlled by the current cycl~
number which will be discussed. Tha output of the prescaler 32 provides ons input to an adder 34. The other input to th0 adder 34, in effect, is the output of the adder 20 34 aRer bsing coupled through the latch 3~ and ths multiplexer 31. The multiplexer 31 eithcr provides as an input to the adder 34 the output of the latch 35 or ths value O. As will b~ seen at "cycle 0", the 0 input is provided to the addar 34, otherwise the ~` cont~nts of the latch 35 is the input to the adder 34. The contents of the latch 35 is normalized by a normalizer 36, the amount of normalization, typically a constant, is ~`

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shown as normali~ation ~alu0 37. Th0 ou~put o~ the normalizer 36 is la~ched by lateh 38, and the contents of this latsh provide the pixel data for a pixel along a display line.
In practic~, the prescaler is simply a digital shifter that provides digital multiplicati~n by a ~actor of 1 or 2 and ~ho normalizcr 36 is another di~ital shifter which performs division by shifling the digital data by, for example, 2 placss for division by ~our.
Assume first that a-1 and b=2 in the equation discussed abov~. Furlher assums th~t data is being scanned from a buffer in ~he manner illustrat~d and 10 described in conjunction wi~h Figure 2. The convolver can be seen to operate in a 3 clock cycle sequence. During a cycle O the data associated with cir~le 16 is coupled to the prescaler 32. Cycle number O when applied to the coeHicient table33 causes the prescaler 32 to multiply this data by one, hence, the data is dir0ctly coupled to tha adder 34. Th0 cycls 0 coupled to the multiplexer 31 selects thc zero 15 input to the adder; tharefora, O is added to the data associated with circle 16. This data is simply latched within latch 35 under control of the pixel clock. Next the data associated with circle 17 is coupled to the prescaler 32 on cycle 1. The cycle 1input to the table 33 causes the prescaler to multiply this data by 2 (a l~-shift of one) before couplingitto the adder 34. At the same time the output of the latch 35 ~0 is couplad throu~h the multiplexer 31 and is added to ths output of tha prescaler 32. Hence, the sum P1+2P2 is formed and eoupled to the latch 3~. Following thison cycle 2 the data associated vdith circle 18 is coupled ~o tha prescaler 32. Tha cycle num~r"~" coupled to table 33 causes this data to be directly ooupled to the adder 34. Tha adder 34 adds this data to the data contained within latch 35 1~

~orming the sum P1 +2P2+P3. This sum is latched within latch 35 and then normalized by norrnalizer 34. For ths described embodiment, normalizer 36 divid~s ths data by a factor of 4 (a nght-~hif~ by 2) forming the final equation2 3 The resultant pixei data is latched in latch 38. On cycl~ O this data 5 may ba read ~rom the latch 38 while new data for the nex~ piXBI iS bein~ coupled to tha prescaler 32.
A fourth cycl~ may bs usad (i.a., cycle 3), in which event cycle 3 can oontrol latch 38 with no data being shifted into the prescaler 32 during cycle 3. This can ba usad if 3 cycle timing is inconvenien~.
An altamate convolver is illustrated in Figura 5. In this ambodiment, arl addar 40 receives as a first input, tha output of the pr~scaler 43. Onca again, the prescaler 43 receives the pixel data from the buffer. Tha amount of prescaling of prescalsr 43 is oontrolled by the coefficient table 44. The output of tablc 44 is controlled by the cycle number coupled to the table. Th~ othar inpul terminal of the 15 adder 40 recaives the output of the latch 42. Th0 input to the latch is ~h0 output of tha multiplexer 41. Multiplexer 41 selects either tha output of tha prescaler 43 or ths output of the adder 40. The multiplexer 41 is controlled by the cycle O signal;
for cycle O multiplexer 41 selects the output of th0 prescaler 43, othsrwisa it selects the output o~ ths addar. The output of the latch 42 is coupled to a normalizer 46.
20 Tha amount of normalization is controlled by th~ values shown as ~normalization ~ralue 45''. Tha output of the normalizer 45 is coupled to a latch 47. Tha output of tha latch 47 provides the ~lltered pixel data.
The circuit of Figure 5 performs the sama convolution as the circuit of Figure 4.

Assum0 tha~ the data ~or line n ~or pixel 0 is coupled to th~ pr~scal~r 43.
During the cycle 0 the muitiplexer 41 selects the output of the prescaier 43 andcouples th~ data into th~ la~ch 42. The prescaler 43 does not scale the data, b~causc a=1 in the equation discussed above. Th~ data for pixel 0 of line n~1 is5 prsscaled by 2 and this da~a is then added to th~ contents of the latch wilh the sum bein~ coupled to the multiplexer 41 and latched in latch 42. Th~ process continues until the sum aP1 ~2P2~aP3 is formad. computed and stored in latch 42. The normalizer 46 divides this surn by a factor of 4 and ~he resuKant norrnalized valus is coupl~cl into the latch 47. Again, on cycie 0 (the start of new data into the prescaler 10 43 for the next pixei) the data is clocked from th~ latch thereby providing the filtered pixal data for ths display. Once again, a four cycle ~cheme may be used with the~ourth cycls (cycle 3) controlling latch 47.
In Figure 7A a general prescaler is shown comprising a multiplier 50. The input pixel data is coupled to the multiplier, the output of the multiplier provides th~
15 scalsd pixal data. Th~ amount of multiplication is controlled by tho output of the coefficient lookup tabl~ ~1. This output is determined by the cycle numb~r. The cycl~ number (e.g., 1,2, 3 . . . ) sslects the amount of mul~iplication r~quir~d for the convolution being us~d and thereby controls the amount of multiplication per~ormsd by the mul~iplier ~0.
~0 Figure 7B illustrates a prescaler which may be used when ths multiplication ussd by ths convolution step involves multiplication by one or by two. In this case a multiplexer 53 rscciYes the input pixel data at ons terminal and the input pixel data multiplisd by two (i.~., Ieft-shifted by 1 with a zero filling in on the right) at its other , . ~

,, t~rminal. Th0 cycl~ number r2quiring the "x2" pixal da~a is used to selac~ ths "O"
input to the multiplex~r ~3 and thus provides the n~ded scaled input pixel da~a.The convolvers discussed above are par~icularly good ~or a serial kernel data stream. Figure 6 illustrates a convolver implement0d in a table 71 which can 5 ba used for a parallel data stream. It is particularly useful when a limited number of bits are used; ~or example, in a 1 biVpixel dispiay where the 1-2-1 kernqi is used.
The rasults of the convolution arithmetic are precomputed and placsd in the table.
This Is used as will be s0en for software embodimen~s of the inv~ntion whcr~ thecolor lookup table is preloaded for use as a convolution lookup table.
A C:URRENTLY PREFERRED EMBODIMENT
OF THE INVENTION
In ons currently preferred embodiment of the invention, the pixel data is stored in two banks of YRAM memory which together eomprise a frarne buffer. Thisis illustrat~d in Figure 8 by the bank O and bank 1 of memory 68. The addresses for 15 the banks of memory are provided either ~rom the CPU or from a pixel address counter 55. One o~ these addresses is sel0cted by the multiplexer 56. Data to and from ths CPU is coupled to the RAM por~s of th~ memory through the bus 59. The shiR register outpùts o~ either bank O or 1 is seleoted by the multiplexer 60. (This multiplax~r can ba eliminated if the 3-stat0 f~ature of the VRAM shift register ~0 switches quickly enough.~
Th~ parallel lines from ths multipl0xer 60 are coupled to thr0e l~tches such as latch 60, and th~n in parallel ~o three shift registars such as shift reglst~r 63. The multiplexer 64 selects data from ona of th0 thre~ shift regist~rs and couples it to the color lookup table (CLUT) 65. Th~ oulput of th~ CLUT is coupl~d to a convolver such as convolver 68 which may be constructed a~ described above. The digital olutputs from the convolver ar~ converted by cligital-to-analo~ converters such as conv~rtor 69 to provide P~GB signals for a color moni~or. Normally, a gamma correction device is plac~d aft~r th~ convolver 68 in digital or aft~r the conv~rters 5 69 in ana1O9.
As indicated in Figure 8, bank 0 is used ~o store even line pairs of pixel data and bank 1 is used to stor~ odd line pairs of pixel da~a. Noto this r~ers to "lin~
pairs", not lines th~ms~lvas. More specifically, th~ Weven lin0 pairs~ refers to lines O
and 1, 4 and ~, 8 and 9, etc. The odd line pairs refers to the lines 2 and 3, 6 and 7, 10 etc. The output streams from the two banks of memory (with two-way interleaving~
are rnultiplexed together to form a single word stream as shown in Figur~ 9. This provides groups of 4 words, one word for each group which is not used. The otherthree words ar~ convolved as describQd above.
An output from bank 0 specifically word 0/lins 0, word 0/line 1, word 1/line 0, 15 and word 1/line 1 is shown in Figure 9. A correspondin~ output frorn bank 1 is also shown, specifically word 0/line 2, word 0/line 3, word 1/line 2; and word 1/line 3.
After muitipiexing th~y provide a single word stream, speoifically word 0/line 0, word 0/line 2, word 0/line 1, word 0/line 3, word 1/line 0, word 1/line 2, word 1/line 1 and word 1/line 3. Then, as indicated through convolving, word 0/line 0, word 20 0/line 2 and word 0/line 1 are used to provids data for ~he pixel or pixels (dep~ndin~ on how many pixels are stored for each word) for line 1 in interlacedfield 1. Similarlyl word 1/line Q, word 1/line 2 and word 1/lin~ 1 provide data for th~
subs~quent pixel or pixels on that lin~. At this time, word 0/line 3 and word 2/line 3 are ignor~d. On another pass throu~h ihe memory data ~or pixels 7Or line 2 in int~rlac~d fieJd ? is used; this ~ime word 0/lino 0, and word 1/line 0 are T~nored.
From the viswpoint of the C:PIJ, the VP~AM is mad~ to appear as a contiguous block. This causes th~ interleavin~ to be ~ransparent to the CPU. Theinterconnections whieh provide this ~ranspar~nce are discussed below.
In the lef~hand column of Figure 10, conv~ntional coupling of the addresses between 1he CPU and a VRAM is shown. In ~ffect, there is a 1-to-1 connection b~twccn the CPU address lincs and th~ VRAM address lines. Th~ exampl~ of Figure 10 assumes a 32-bit word size. Th~ CPU address bits A0 and A1 are thus 10 used to enable VRAM bytes and words, rather than specific VRAM address bits.
For that reasonl the CPU address bit A2 is coupled to tha VRAM address bit A0, CPU address bit A3 to VRAM address bit A1, etc. Also, since thera are 2 banks ofVRAM, one address lins must ba designated as a bank select signal. With a conventionai coupling any address bit may b~ used for the convenience of the 15 design. CPU address A19 is shown coupled to BANKSEL in the example.
The connections between the CPU address bits and the VRAM address bits for the two-way interleaving is shown in the second column of Fi~ure 10. The exampl~ assumes that there are 1,024 bytes per scan line. If this is the oase, the address bit A10 from the CPU is the bit which selects between odd and even scan 20 lines. This bit is coupled to A0 of the VRAM. The effect of this bit manipu7ation is that instead of s~lecting between odd and aven scan lines, the addrGss bit selects between odd and even words, 1hereby interleaving two lines together by word.
Also, A11 is coupled to BANKSEL to select between VRAM banks 0 and 1 ~very cther scan line. It is this bit manipulation which makes the two-way int~rleaving 1 3~0600 appsar transparent ~o th~ GPU. (It is assumad that the scan line length is a power of 2, oth~rwisc th~ addr~ssin3 is mor~ complex.) Th~ shift registers, such as shift register 63 of Figure 8 are used to enabie a single pixel to be clocked out frorn a word. Typically, more than one pixal is 5 contained in a word therefors a parallel loaded shift register is needed to perrr it the individual pixels to b0 removed from ~he word. Moreover, if muiti-bit pixels areused, then multi-bit shiR registers are needed for each word.
ALTERNATE EMBODIMENT EMPLOYING FG UR-WAY
INTERLEAVED YRAM
The results described above for a two-way interleavad systern can also be obtained from a single bank of memory where four-way interlaaving is used. This embodiment has the disaclvantage that twice the memory capacity is required since ~wo copies of ~he data are needad for the four-way interleaving, as will be described.
Figure 11 iilustrates a block diagram for the four-way interleave embodiment which again employs a dual port VRAM memory. The RAM port of the m~mory is connected through a latch 73 to the CPU to permit data to be r~ad into and r~ad from the memory. The shift register port of the mamory is coupled ~o thrse latches such as latch 74 with each latch being coupled to a shift register, such as shift 20 register 75. The multiplexar 76 selects one of the shift regist~rs and oouples its output to a CLUT 77. A digital color output frorn the CLUT, after convolving by convolver 78, provides RGB outputs. The latches, shift re~isters, multiplexer, CLUT
and convolver for this smbodiment op~rate in th~ same manner as the embodiment of Figurc 8 except as discussad below.

~ 320600 Th6 addresses to tha VRAM ar~ selected by the muHiplexer 79 either from pixel addrQss counter 80 or from the CPU after the C~PU addresses haYe been coupled to the parallel load counter and latch 81 (as will bs described). Ths rnost si~niflcant bits of ~he address ars coupled to th~ counter 817 the least significant 5 bits aro simply passed throu~h a latch on~o th~ multiplexsf 79~
As mentioned, a single bank of VRAM is used with four-way int~rleavin~ and by storing two complete copies of the pixel data in the VRAM 7~. This is necessary to ob~ain pixel data from three lines (sequentially) and to provide differ~nt ~roupings of lines as will become apparent~ The four-way interleaving causes the10 CPU to write words into VRAM in a 4-line "zig-zag~ order, however, from the CPU's standpoint this is transparent~ In VRAM each of the two copies of the pixel data is stored with its interleaving offset by two lines from th~ other copy~ That is, the word order of the first line of the sven copy is: line 0/word 0, line 1/word 0, lin~ 2/word 0, line 3/word 0, line 0/word 1, line 11word 1, line 2/word 1, line 3/word 1, etc. And, th~
15 first line of ths odd copy of the frams buffer is line-1/word 0, line-2/wo~ 0, line 01word 0, line 11word 0, line-11word 1, line-2/wor~ 1, line 0/word 1, line 1/word 1, etc~ (Llnes -1 and lines -2 denote black lines above th~ first line on the display~
Thsy are used to avoid flicker at the perimeter of the display~ They ar~ initially loadsd with data representing tha color black and ne~d not be accessed by the 20 CPU, once loaded.) Even though two completo copies of the pixel data are stored in th~ memory, the CPU only has access to the even copy. When ths CPU writes data into the msmory, the addresses and data are latched and the write operation is repeated to write th~ data into the odd copy. Moreover, only the ~ven copy is r3ad by the CPU.

This additional writing ~or the odd copy is done by ordinary cirouitry in the frarne buffsr. Th~ even and odd copies are distinguished by the rnost si~nificant bit in the VRAM, thereby placing the even frarne copy in the lower half of the memory and the odd copy in the upper half.
The four-way interieaving ~r the even copy is obtained by taking the address bits from the CPlJ which select between odd and even scan lines and conneoting them to bit 0 of the VRAM. Ths addrsss bit from the CPU which selectsbstwean cven and odd soan line pairs is connected to bit 1 of ths VRAM and thsn all other bits from ths CPU are shifted "upward as shown in Figure 13 in ~he righthand column. Again, in Figure 13 the conventional interconnecl is shown in the leflhand column for purposes of comparison as it was in Figur~ -0. The eff0ct of this bit remapping for the 1024 bytes per soan line display is that the odd and even scan line ~bit" (e.~., bit 10) selects between ocid and even words, and the odd and even scan line pair bit (e.~., bit 11 ) selects between odd and even wQrd pairs.
1~ From the CPU's viewpoin~, onca again, the memory appears as a contiguous block.
The four-way interlsaving for tha odd ~rame buffer copy is obtained in ths same mann~r as the ~ven copy, axcept that before the bit remapping an add0r adds the length of two scan lines to the address generated by the CPU ~for example, for 1024 bytes per scan line, th~ number 2048 is added to th0 CPU
address prior to the bit rsmapping). The effect of this addition is to oHset thecontents of the odd copy in the buffer by two scan iines (i.e., after remapping by 2 words). (It is assumed the scan tin~ 10ngth is a power of 2, otherwise addr~ssing is more complex.) The two scan line addsr is easily impl0manted since the only time th~ odd bank copy is addrassed is a~ter the even copy has been addressed. This addition is obtained from thB coun~er 81 of Figur~ 11 by incrementing the counter. Assum~that th~re ar~ 1024 bytes per scan line, than the CPU address bits A0-A10 are 5 latched in ths latch of counter and latch 81, and ths address bits 11 and higher ar~
coupled to the counter. To wnte the even eopy tha address bits ar~ simply latched by the countsr and la~ch 81. To write tha odd copy, the counter is incremented and the data which has been latched is written into the memory with a two scan line offset.
Another effect of ~he address manipulation is to cause ths output of the memory (shiR register port) to be in scan lines of ~line quads". The sven line quads ars 0/1/2/3, 415~17, 819/10~11, etc. The odd line quads are -2/-1/0/1,2131415, 6171819, etc. Each convolution triplet needed by the convolver is then available in on0 of the~e line quads. The line quads are sequenced in the sams ordar for both ~ields:
15 cdd lina quad -~-1/0/1, even line quad 0/1/213, odd line quad 2131415, even line quad 4/51617, etc. The particular field displayed is determined by which three lines are convolved from each line quad. For example, in field 1 for lines -1, 1, 3, and 5 th~ tripies -2/-1/0, 0/112, 21314, and 41516 are needed. Then in field 2 for lines 0, 2, 4, and 6 the tnples -1/0/1, 1/213, 31415, and 516n are needed resulting in the same 20 line quad sequence.
The output of the memory is latched and then the shift registers ars used in the manner described in conjunction with the previous embodiment, except that the line order and the line quads is different, this difference being shown in Fi~ure 12.

~ 320600 E;OFTWARE EM130DIMENTS OF THE PRESENT INVENTIC)N
The method of ~h~ present inv~ntion can be raadily impl~menl~d in so~war~
to provid0 real tim~ csnvolution. Two ~rnbodiments of the inv~ntion ar~ described bQlow, one, ~or a planar frame buffer and ~he other for a "chunkya frame buffsr. The 5 mathod for the planar frame buffer is par~icularly useful where a single bit per pixel (e.g., indicating black or whit~) is stored in the buffar.
A planar fram~ buff~r for purposes of this applica~ion (as well as being the g~nsrally accepted d~finition), is one made up o~ bit planes, where aach plan~
providas one bit for each pixel of the display. This arrangemcnt is used in 10 numarous commercially available computers such as those manufacturad by Sun Microsystems, Inc., Apollo, SGI, etc. To specify a color for a given pixel, each plane contributes one bit. Thus, if ther~ are eight planes, the maximum color depth is 8 bits pQr pixel; if there ars 24 planes, the maximum color depth is 24 bits per pixel, QtC. The CPU accesses words in a pianar frame buffer only one plane at a tim~.
15 For Qxample, a 3~-bit read or write from th~ GPU would allow access to one bit of 32 horizontally adjaccnt pixels at once. For the CPU to acc~ss all bits in a single pixQI, it must perform as many access cycles as there are planes.
In contrast, with a chunky ~rame buffer, all the bits for a given pixel are stored as adjacsnt bits o~ a m~mory word. For ~xample, if color depth is 4 bits p~r pixal, ~0 and lhc CPU word size is 32 bits, then 8 pixels ar~ stored in each CPU wnrd.
UnlikQ thQ planar frame buffer, a given CPU access will always access all the bits in a givsn pixel, and in some cases, ~he bits in adjac~nt pixels. Chunky frame buffers are also used in commercially availabl~ computers such as Appl~
Computer, Inc.'s Macintosh 11 cornput~r.

~ 320600 A. Software Embodlmen~ of the Present InVQntlOn for a Plan~r Fr~3ms BulFf~r Assume a on~ bi~ p~r pixel display is stored in the frame buffer, for cxample, in plane 0. Referring to Figure 1 q, and i~noring for a mom~nt, s~ep 83, srdinary 5 block moves are empioyed to mov~ tho pixel data from plans 0 to pl~n~ 1, howsver, when this data is moved into plan~ 1, it is plaoed on0 scan lin~ down as shown by block 84. Th~ data is again block moved from plane 0 to plane 2 of the buffsr, but this time when writing into plane 2, it is written one scan line up when compared to plane 0 as indicated by step 85.
As indicated by step 86, the data from planes 0, 1 and 2 is scanned out of the memory. The block moves assure that the proper bits are available in bits 0, 1, and 2 of each pix81 to immediately do a convolution as desoribed above (typically a "1-2-1~ convolution). For any ~iven pixel, plane 0 contains the line n bit, plane 1 contains the line n-1 bit and plane 2 con~ains the line n+1 bit.
In the currently preferred embodiment of this method, the convolution is, in affect, precomputed for each possible 3 bit combination output from ths m~mory and is storad in a color lookup table. This is shown by the first step 83 where the CLUT is preloaded with a gray scale data. Tha output of the memory during scanning is coupl~d to the CLUT and tha output of the CLUT providcs tha 20 convolved pixel data as shown at step 87.
This ombodimant is again illustrated in FiQure 15 where it is assumed that thc planar frame buff~r has 4 planes, plan~s 88, 89, 90 and 91. In typical opsration, the four bits defined in each pixel are provided one from ~ach plane, as shown within the dotted line 92. Also, during typical color op~ration, th~ 4 bit outpu~ ~rom th~ buffer is coupled to th~ color loo~up table 93 to, ~or exampl~, ;
provid~ a 12 bit RGB output.
When the method of tho present invention is implemented, a orle bit per pix61 display is stored in plane 0. At system startup th~ ~ight ontries of the CLUT
5 (since thsr~ ar~ thr~ bits per pixel, thsr~ ar~ ~ ~ntries) load~d ~o th~ ~abl~ of gray scales (see Figure 21). Then, the CPU rep2atedly does block moves of th~
entir~ty of plane 0 to planc 1, however, one scan line down. The data from block 0 is also moved to block 2, ~his tim0, on~ scan lin~ up. Once this is done, th~ data ~rom the planes 88, 89 and 90 is coupled to the color lookup table to provide the 10 gray scal~ output. (The data, if any. from plane 3 is ignored for this mode.)Steps ar~ taken to prevent flickering at ~he edge o~ the screen or display, thatis, to prevent abnJpt transitions at the top and bottom. It is generally best to have the border of the display smoothly convolve into black at th~ top and bottom. This is achieved by allocatin~ one line initialized to black at the top and bottom of plane 15 0, two iines at the top of plane 1 and two lines at the bottom of plane 2. These ~guardbands~ assure that the top and bottom borders are smoothly convolved to black.
In the currently preferred embodiment, gamma correction is used as shown by block 13 of Figure 1. The gamma correction can be built into the gray scal0 data 20 and hence, loaded into the CLUT on inilialization.
In a typical application, ths CPU may not be fast ~nough to complete the block movcs to planes 1 and 2 in a single frame Ume. It has been found that it is visually acceptable to complete the block moves in several frame times since plane 1 32()600 O is always up-to-dat~ and it contributes on~-half of th~ int~nsity for a ~iv~n pix~l vaiue.

13. Sottwsra Embod~ment for the Chunky Frame Bu~fer In this embodiment, a one bit per pixel real-time convolution with a chunky fram~ buffar is r~alizad. Unlike tha method described abov~ ~or the planar ~ramebuffer, the ~xact numbsr of bits per pixsl cannot be ob~ained whsn rearranging the 5 data, hence, ~he next power of 2 ~reater than ~he number of bits needed is used.
For the described embodiment, thr~e-lines are used for the convolutisn and hence, four bits of pix~l are s~ored in a buffer for each pixel. Th~ mathod described b~low places th~ bits in their proper position.
First, H should be noted that a one bit per pixel frame buffer "off scre~n" in O RAM is used by the CPU for drawing. This frame buffsr is separate from the four bi~
p~r pixel frama buffer that is ac;ually scanned to provide ~h~ display. Tha method described below reads data from the one bit per pixel frame buffer, expands the data to th6 four bits per plxel, ~hen writes ~he data into the four bit per pixel frame buffer. The method merges the pixel information from tha two previous lines before 15 it writes tha results into th0 four bit per pixel frame buffer. When the four bit pixel is presanted to th~ color lookup tabla, the three bits for lines n-1, n and n~1 areavailable to lookup th~ proper gray scale for the 1-2-1 convolution. Again, as with the pravious ambodiment, the color lookup table is loaded with ~ray scale information to provide the convolution. (Thr~e of the four bits read from the four bit ~O par pixel fram~ buffer are used by the CLUT to provide the output convolvad signal for tha display.) Step O
Four 32-bit words (A, B, C, and D) ars initialized to zero. (A, B, C, and D
oaoh refer to 32-bit registers within the CPlJ.) A 32-bit word R is raad starting from ths laRmost pixel position of the top scan line of ths one bit par pixel ~rame buffer.
A B, C and D ara ali stored a~ adjacent laf~ to right locations starting from tha ~op scan lins of the four bit psr pixel frame buffer.
Step 1 R is read from th~ next 32 bits in ths ons bit per pixel fram~ buffer immediately below the last 32-bi~ word read from the one bit per pixel frame buffer.
This is shown in Figure 16 where two words, words 93 and 94, are shown ~or linesn and n~1 in the ons bit per pixel frame buffer.
~;t~p ~
As shown in Figure 17, one byte of R is expanded into a second 32-bit word M such that sach of the 8 bits is placed at 4 bit intervals in the 32-bit word starting at bit 1 (i~e., bit 0 to bit 1, bit 1 to bit 5, bit 2 ~o bit 9, etc.~ and a 1 is placed in every 4th bit startin~ at bit 0. All o~her bits are set ~o zero. For example, the byte 0111 0101 is converted to (shown as groups of 4): 0001 0011 0011 0011 0001 0011 0001 0011. This is dona by using a 256x32-big pre-loaded lookup taùle in RAM.
Step 3 A is leR-shifted by 1. In some microprocessors such as the Motorola Part No.
68020 this can bà accomplished mors quiokly by adding A ~o itself. In the upper part of Figure 18, A is shown before the shift and in the lower part of Figure 18 after tha shift.
Step 4 M is bit-wise ORed into A as shown in Figure 19. First, this serves to merge th~ byte from R into A since it is known that the bits in A corresponding to the bits from the byte from R are all zero (anything-ORed with zero retains its value).

.

'' :` ' :
.

Second, this serves to for~e every 4th bit starting with bit 0 in A to one (this sets up for tho merg~ operation in s~ep 10, below).
S~ep ~
A is stored in th~ ~our bit per pixel fram~ buff~r immediately below ~hs last 5 plac~ A was stor~d as shown in Figur~ 20.
~;t~p 0 Steps 2 through 4 are repeated for the thre~ other bytes from R. This time, however, B, C, and D ars usad instead of A.
Step 7 R is read for the next 32-bit word in the one bit per pixel franne buffer immcdiately below the last 32-bit word as in Step 1 above.
Step 8 As shown in Figure 17, one byte of R is ~xpanded into M with each of th~e eight bits placed at 4 bit intsrvals startin~ at bit 1. Also, a 0 is placed in CVQry 4th bit 15 starting at bit 0 and all other bits ara set to 1. For exampl~, the byte 0111 0101 would be converted to 1100 1110 1110 1110 1100 11101100 1110. This is accomplished by means of a second 256x32-bit pre-loaded lookup tabl~ in RAM.
Step ~
As shown In Figurs 18, A is left-shifted by 1. Once again, as mentioned for 20 step 3, addition of A to itself may be used.
S~ep 1 0 As shown in Figure 19, M is bit-wise ANDed into A. First, this serves to merge tha byt~ from R into A since it is known that the bits in A corresponding to the bits from the byte from R ara all on~s (anything ANDQd with on~ retains its vaiue).

Second, this serves to force ~very 4th bit starting with bit 0 in A to zero (this will set up the merge operation in step 4, above).
Step 1 ~
A is stored in th~ 4-bit frame buffer immediately below the last place A was 5 stored. See word 95 of Figure 20.
Step 1 2 Steps 8 through 10 are repeated for the 3 other bytes from R. They are me~ged in B, C, and D instead of A. See words 96, 97 and 98 of Figure 20.
Step ~3 Steps 1 through 12 are repeated until the bottom of th~ frame buffer is reached, then R is read for the pixels on the top scan line of the 1 biVpixel frame buffer just to the right of where it was loaded at the start of the last pass. A, B, C, and D are all stored at adjacent left-to-right locations on the top scan line of the 4 biVpixel frame buffer just to the right of wher~ they were loaded at the start of thc 15 last pass.
In summary, th0 pixels in the 4-bit per pixel frame buffer 100 of Fi~ure 20 are coded with line n+1 in bit 1, n in bit 2, and n-1 in bit 3 (this resulting bit configuration is shown in Figure 19). ~it 0 is i~nored by th~ CLUT 101 of Fi~ure 20. The on~ bit per piXQI frame buffer of Fi~ur0 16 is scanned vertically with a new bit added into 20 oach four bit pixQI for each scan line by left shifting the existing bit for th~ pix~l by one and merging the new bit into bit 1 of the 4-bit per pixel word. The shift op~ration serves to adjust th~ pixel from its previous centering on lin6 n-1 (the line above) to its current centaring on line n. In other words, when th~ operation begins the four bit pixei data contains bits ~rom lines n-2, n-1 and n since the data was .

used for the line abova. The left shif~ operation changes the configuration of the four bits to n-1, n, and a ona or a zero in bit 1 (bit 0 is ignored). Than, the new bit from the one bit per pixel frame buffer is merged into bit on0 for line n~1. The new assembled four bit word is stored in the four bit per pixel frame buffer and as mentioned, the CLUT is used ~o provide th3 convolution.
In detail, the method star~s in the upper-left of the fram~ buff0r and works down a 32-pixel column. The read into R loads the 32 1 bit pixels then each 8 pixels of the 32 are operated upon ~eparately. Th~ tirst 8 pixels (a byte) ar~ used as a lookup table index to fetch a 3~-bit word, M. M holcls the 8 pixels, spread out 10 at 4-bit intervals so that they are ready to merge for ~ha 4 biVpixel frams buffer.
M also is set up with the rest of its bits prepared for eithsr a bit-wise AND
merge or an OR mer~e. The reason it alternates between AND and OP~ is that i~
saves the step of clearing (or setting) th~ bits in A which are th~ destination for the 8 pixels from R. Since A will be left-shifted just prior to the AND or OR mer~, th~
15 bit immediateiy to th~ right of the destination of th~ R bits is ~orGed so that at the next step they are already prepared for merging. AND prepares for th~ OR by forcing zeroes, and OR prepares for ths AND by forcing onas.
A is lefl-shiRed by one to update th~ pixei from being centered for the pr0vious line to being centered for the current line. Th~ leR-shift moves the 20 pr0vious line n~1 to the current line n and the pr~vious line n to ~he curr~nt lin~ n-1.
Previous line n-1 (current line n-2) is shifted out. Notice that this shift applies to all aight pixols contained in the 32 bits of A so it is an 8-way parallel op~ration. Notice also the bits from previous line n-1 shifts into the unused bit of th~ next 4-bit pixel to the left (or off tha left edg~ of the 32-bit word).

Than, M is merged with A by ~ither an AND or an OR. Bits from n and n-1 ara left alone, new n~1 bits are merged in, and the unused bits are set to knownstate (0 if an AND, 1 if an OR). A is finally stored in tha 4 biVpix~l frame buffer.
The other 24 pixels in R are handled the same way, with 8 pixels each for B, 5 C,andD.
The same steps ar~ performed for ~ach SUCCeSSiVQ scan line below until ths bottom of the frams buffsr is reached. Then, the next column of 32 pixols immediately to the right is scanned-down, and so on until the entire frame is scanned.
10The CLUT 101 of Figure 10 is loaded in a similar manner to that of the planar frame buffer implementation above shown in Figure 21. The diffsrflnces are that the bit ordering is different and that since bit 0 in the 4-bit pixels is indeterminate (it alternates depending on whether tha last mer~e was with an ANDor an OR), the same gray value for every two CLUT ~ntries is stored.
15OTIIER CONVOLUTION KERNE~ S
In the previous section, most of the emphasis has bsen on thc 1-2-1 kernel.
Experiments have shown that neith~r a 3-line convolution nor on-off-on-off reduction of 50% gray is assential in all situations for effe~ive interlace flicker r~duction. If the constraint that on-off-on-off horizontal line patterns are reduced to 20 a 50% gray is maintained and other kernel sizes are tried other than 1 x3, for each k~rnel size there is one set of coefficients to meet the on-off-on-off constraint.
ThssQ coefficients match Pasqual's triangle (i.e., 1; 1, 1; 1, 2, 1; 1, 3, 3, 1; 1, 4, 6, 4, ~; QtC.)~

ADAPTIVE CONVC)LLITION
As mentioned above, the worst flickQr patterns are the ones which have hi~h hori~ontal coherence ~i.e., repeat horizontally). Horizontal solid lin~s, hsrizontal dashed lines, and gray dither patterns ar~ examples of patl erns with high 5 horizontal coherence. Taxt is an exampl~ of patterng without such cohsr4nce. The ~nvolution discussed above may be adaptive, that is, it may be varied depending on the type of patterns bein~ displayed. First, it is determined whether a repeating pattern is occurring in a local horizontal group of kernels, for ~xample, 8 pix~ls across. If there is a panern in the kernels, th~n all of th~ kernels are convolved, for 10 example, with the 1-2-1 coefficients. If ther~ is no such pattern, then the 8 pixels are convoived with coefficients making a sharper filter ~e.3., 1-3-1 or 1-~4-1 )o The test to determine whether a pattern is repeating must be applied continuously in a moving horizontal window, kemel by kern~l. Sino~ the test windows ov~rlap, some kernels may be part of a pattern in one test window but not in ano~her. For ~hese 15 kern~ls, the 1-2-1 oonvolution is us~d, since they are at the edga of the pattern.
Different lests may be used for determining whelher a pattern is b~ing repeat~d, for example, 1h~ left four ksrn01s may be oompared with the right four kem~ls withintha window.

Claims (18)

1. In a raster-scanned video display for a computer where data for the display is stored in a frame buffer, and where the data is organized in the frame buffer by scan lines with data being stored for each pixel along each scan line, an improved method for generating filtered pixel data for a first display line, comprising:
(a) storing the pixel data in a frame buffer having a first and a second bank of video RAMs with pixel data for scan lines 2, 3, 6, 7, 10, 11... being stored in said first bank and pixel data for scan lines 0, 1, 4, 5, 8, 9... being stored in said second bank;
(b) reading from said buffer the pixel data for a first pixel of the n, n+1, n+2... n+N scan lines before reading from said buffer the pixel data for subsequent pixels of said n, n+1 n+2... n+N scan lines;
(c) convolving the pixel data for said first pixel of said n, n+1, n+2... n+N scan lines to provide said filtered pixel data for said first pixel of said first display line;
(d) repeating steps (b) and (c) for each subsequent pixel on said first display line.

2. The method defined by claim 1 wherein N=2.

3. The method defined by claim 2 wherein said reading step comprises the reading from said first bank the pixel data for said first pixel for said n and n+1 scan lines, and the reading from said second bank the pixel data for said n+2 and n+3 scan lines, before reading from said frame buffer the pixel data for subsequent pixels of said n, n+1, n+2...n+N scan lines.

4. The method defined by claims 1 or 3 wherein said convolving step comprises performing the following computation:

where P1 is the pixel data for said first pixel of said nth scan line, P2 the pixel data for said first pixel of said n+1 line, and P3 the pixel data for said pixel of said n+2 scan line; a and b are constants.

5. The method defined by claim 4 wherein b is equal to 2a.

6. The method defined by claim 1 wherein steps (b) through (d) are repeated for each display line in said display.

7. The method defined by claim 1 wherein pixel data is stored for 2 scan lines prior to the first displayed scan line in the frame buffer.

8. The method defined by claim 7 wherein said pixel data stored for said 2 scan lines prior to first displayed scan line represent all black, thereby providing a smoother transition at the top of said display.

9. In a raster-scanned video display for a computer where data for the display is stored in a frame buffer, and where the data is organized in the frame buffer by scan lines with data being stored for each pixel along each scan line, and additionally where each of the scan lines comprises adjacent groups of pixel data, an improved method for generating filtered pixel data for a display line, comprising:
(a) storing the pixel data in a frame buffer having a first and a second bank of video RAMs with pixel data for scan lines 2, 3, 6, 7, 10, 11... being stored in said first bank and pixel data for scan lines 0, 1, 4, 5, 8, 90.. being stored in said second bank;
(b) reading from said buffer the pixel data for a first group for the n, n+1, n+2... n+N scan lines before reading from said buffer the pixel data in the next group of pixels along said n, n+1 n+2... n+N scan lines;
(c) convolving the pixel data for the pixel in said first group to provide said filtered pixel data;
(d) repeating steps (b) and (c) for the remaining groups of pixel data along said display line.

10. The method defined by claim 9 wherein said convolving step comprises performing the following computation:

aP1 + bP2 + aP3 2a + b where P1 is the pixel data for a first pixel of said first group for the nth scan lines,P2 the pixel data for said first pixel of said first group for said n+1 scan line, and P3 the pixel data for the first pixel of said first group for the n+2 scan line; a and b are constants.

11. The method defined by claim 10 wherein b is equal to 2a.

12. In an apparatus for generating a raster scanned display from pixel data stored in a buffer and organized by scan lines with pixel data being stored for pixels along each scan line, an improvement for providing vertically filtered pixel data comprising:
said buffer having a first and a second bank of video RAMs with pixel data for scan lines 2, 3, 6, 7, 10, 11... being stored in said first bank and pixel data for scan lines 0, 1, 4, 5, 8, 9... being stored in said second bank;
address, yeneration means coupled to said buffer for generating address for said buffer such that the pixel data for a first pixel on the n, n+1, n+2... n+N scan lines is addressed and read from said buffer, before all the pixel data for the pixels along the n, n+l, n+2... ntN scan lines is read from said buffer;
convolving means for convolving the pixel data for said first pixel of said n, n+1, n+2... n+N scan lines to provide vertically filtered data for said first pixel said convolving means coupled to said buffer.

13. The improvement dedefined 12 wherein said convolving means comprises an adder; a multiplexer coupled to a first input terminal of said adder; a prescaler for prescaling an input to said adder, said prescaler coupled to a second input terminal of said adder; the output of said adder being coupled to a latch the output of said latch being coupled to said multiplexer and to a normalizer for normalizing the output of said latch.

14. In a raster-scanned video graphics apparatus having a buffer where pixel data is organized in planes such that the pixel data for a given pixel is stored with each of its bits in a different one of the planes, method for providing filtered pixel data for a display stored in a first one of said planes, comprising the steps of:
reproducing the pixel data for the n, n+1, n+2...
n-l, n+N lines of said first plane, as the n+1, n+2, n+3...
n+N+1 lines, respectively of a second plane;
reproducing the pixel data for the n, n+1, n+2...
reproducing the pixel data for the n, n+1 n+2...
n+N lines of said first plane, as the n-1, n+2, n+1... n+N-l lines, respectively of a third plane;
scanning the pixel data from said first, second and third planes;
convolving the pixel data as it is read from said buffer.

15. In a raster-scanned video graphics apparatus having a buffer where pixel data is organized in planes such that the pixel data for a given pixel is stored with each bit in a different one of said planes, a method for providing filtered pixel data for a display line stored in a first one of said planes, comprising the steps of:
reproducing the pixel data in said first plane and storing it in a second plane with each scan line of pixel data being shifted in a first vertical direction;

reproducing the pixel data in said first plane and storing it in a third plan~ with each scan line of pixel data b~ing shifted in a second vertical direction, opposite to said first direction;
scanning the pixel data from said first, second and third planes such that the pixel data for lines n-1, n and n~l are read at the same time;
convolving said pixel data as it is read from said buffer.

16. The method defined by claim 15 wherein said convolving step comprises the step of preloading a table with gray scale data.

17. In a raster-scanned video apparatus having a memory where pixel data for each scan line is stored in adjacent locations in said memory, such that each word accessed from said memory includes the data for at least one pixel, a method for providing filtered pixel data for a display, co~prising the steps of:
(a) reading said pixel data from said memory and writing it into a second memory such that the pixel data for a first pixel of n, n+1, n+2... n+N scan lines are located in adjacent locations in said second memory;
(b) repeating step (a) for each pixel along the n, n+1, n+2... n+N scan lines such that pixel data for the n, n+1 n+2... n+N lines are interleaved in said adjacent memory locations;
(c) reading from said second memory said pixel data for said first pixel of said n, n+1, n+2... n+N scan lines;
(d) convolving said pixel data to provide said filtered pixel data;
(e) repeating steps (c) and (d) for each pixel along the scan lines.

18. In a raster-scanned video display for a computer where data for the display is stored in a frame buffer, and where the data is organized in the frame buffer by scan lines with data being stored for each pixel along each scan line, an improved method for generating filtered pixel data for a first display line, comprising:
(a) storing first a first copy and a second copy of the pixel data in the frame buffer, said buffer comprising a plurality of video RAMs;
(b) reading from said first copy the video data for a first pixel of each scan line in the following scan line order so as to provide four-way interleaved groups; 0, 1, 2, 3; 4, 5, 6, 7; 8, 9, 10, 11..., and reading from said second copy the video data for a first pixel of each scan line in the following scan line order so as to provide four-way interleaved groups: 2, 3, 4, 5; 6, 7, 8, 9;
10, 11, 12, 13;...
(c) convolving the pixel data for said first pixels to provide the filtered pixel data for said first pixel of said first display line where either said first or fourth scan line of pixel data in each of said four-way interleaved groups is not used for said convolving;
(d) repeating steps (b) and (c) for each subsequent pixel on said first display line.