US RE37755 E1 Abstract An interpolation method in which a threshold value used for determining a pixel value of a pixel generated by interpolation according to a context (state value of adjacent pixels). In the interpolation method, the ambiguity between the interpolation value and the threshold value is removed by using the context (state value of the reference pixels around the interpolated pixel), thereby reducing the blocking and smoothing phenomena in the restored binary image.
Claims(10) 1. A computer usable medium having embodied thereon a computer program for restoring a reduced binary image, reduced from an original binary image by down sampling, into the original binary image, comprising the steps of:
(a) calculating a predicted value of an interpolated pixel based on values of object pixels A, B, C and D around the interpolated pixel using the following equation:
where h and v of each term represent horizontal and vertical distances between the object pixel and the interpolated pixel included in the corresponding term;
(b) calculating a context C
_{P}, representing a state value of reference pixels around the object pixels, by concatenation of the reference pixel values using the following equation: wherein P represents the position of the interpolated pixel, C
_{k }represents the reference pixel and k is an index of reference pixels;(c) obtaining a threshold value, THRc(t), corresponding to the context C
_{P }which is changed in each interpolated pixel based on its context; and (d) determining the interpolated pixel as “1” if the predicted value in step (a) is greater than the threshold value in step (c), and setting the pixel value of the interpolated pixel as “0” otherwise.
2. The computer usable medium of
3. The computer readable medium of
4. The computer readable medium of
5. The computer usable medium of
(c1) defining arrays h(c)(t) and THRc(t), where c is an index of the predicted values in step (a), and t is an index of proposed threshold values;
(c2) initializing the array h(c)(t) to zero, and receiving the original binary image and the reduced image;
(c3) determining the location of the interpolated pixel to be interpolated, according to a raster scanning, and calculating a context of the reference pixels;
(c4) initializing the threshold index t of proposed threshold values;
(c5) calculating interpolation value INP(P) of the interpolated pixel as in step (a);
(c6) comparing the current proposed threshold value THRc(t) with the calculated predicted value INP(P), and setting the pixel value of the interpolated pixel to “1” if the interpolation value INP(P) is greater than the proposed threshold value THRc(t), and setting the pixel value of the interpolated pixel to “0” if the interpolation value INP(P) is less than or equal to the proposed threshold value THRc(t);
(c7) comparing the pixel value of the interpolated pixel with the original pixel value, and increasing the value of the array h(c)(t) and the index of proposed threshold values t by “1” if there is a hit, where a hit occurs when the pixel value of the interpolated pixel is equal to the original pixel value;
(c8) checking whether the comparison of the step (c6) has been performed on all proposed threshold values, and returning to the step (c6) if the comparison has not been performed on all candidate values;
(c9) checking whether the interpolation has been performed on all pixels of the binary image, and returning to the step (c3) if the interpolation has not been performed on all pixels of the binary image; and
(c10) setting the proposed threshold value having the highest frequency of hits as the threshold value of the corresponding interpolation value, if the interpolation is performed on all pixels of the binary image.
6. A computer usable medium having embodied thereon a computer program for restoring a reduced binary image, reduced from an original binary image by down sampling, into the original binary image, comprising the steps of:
(
a) calculating a predicted value of an interpolated pixel based on values of object pixels A, B, C and D around the interpolated pixel; (
b) calculating a context C _{p1} , representing a state value of reference pixels around the object pixels, by concatenation of the reference pixel values using the following equation: wherein p represents the position of the interpolated pixel Ck represents the reference pixel and k is an index of reference pixels;
(
c) obtaining a threshold value, THRc(t), corresponding to the context C _{P } which is changed in each interpolated pixel based on its context; and (
d) comparing the predicted value with the threshold value of step (c), and determining the interpolated pixel as “(1” if the predicted value in step a) is greater than the threshold value in step (c), and setting the pixel value of the interpolated pixel as “ 0” if the predicted value is equal to or less than the threshold value. 7. The computer usable medium of
a) is calculated by summing the values of the neighboring pixels which are weighted in a way that the nearer pixel to an interpolated pixel, the larger its weight value. 8. The computer readable medium of
t) in said step (c) is obtained using the context value to decide whether each interpolated value is a “(1” or “0” is determined using one of some predefined values comprising 12 default values which are calculated from the equation use in step a). 9. The computer readable medium of
0, 0.0625, 0.1875, 0.25, 0.375, 0.4375, 0.5625, 0.625, 0.75 and 0.9375. 10. The computer usable medium of
c) are obtained by the steps of: (
c) 1 defining arrays h(c)(t) and THRc(t), where c is an index of the predicted values in step (a), and t is an index of proposed threshold values; (
c) 2 initializing the array h(c)(t) to zero, and receiving the original binary image and the reduced image; (
c) 3 determining the location of the interpolated pixel to be interpolated, according to a raster scanning, and calculating a context of the reference pixels; (
c) 4 initializing the threshold index t of proposed threshold values; (
c) 5 calculating interpolation value INP(P) of the interpolated pixel as in step (a); (
c) 6 comparing the current proposed threshold value THRc(t) with the calculated predicted value INP(P), and setting the pixel value of the interpolated pixel to “(1” if the interpolation value INPP) is greater than the proposed threshold value THRc(t), and setting the pixel value of the interpolated pixel to “(0” if the interpolation value INPP) is less than or equal to the proposed threshold value THRc(t); (
c) 7 comparing the pixel value of the interpolated pixel with the original pixel value, and increasing the value of the array h(c)(t) and the index of proposed threshold values t by “ 1” if there is a hit, where a hit occurs when the pixel value of the interpolated pixel is equal to the original pixel value; (
c) 8 checking whether the comparison of the step (c) 6 has been performed on all proposed threshold values, and returning to the step (c) 6 if the comparison has not been performed on all candidate values; (
c) 9 checking whether the interpolation has been performed on all pixels of the binary image, and returning to the step (c) 3 if the interpolation has not been performed on all pixels of the binary image; and (
c) 10 setting the proposed threshold value having the highest frequency of hits as the threshold value of the corresponding interpolation value, if the interpolation is performed on all pixels of the binary image. Description This is a continuation of U.S. patent application Ser. No. 08/979,852, filed Nov. 1. Field of the Invention The present invention relates to an interpolation method for a binary image, and more particularly, to an improved interpolation method in which a variable threshold value used for determining a pixel value to be generated by interpolation is determined according to a context (the state value of adjacent pixels). This invention has been adopted in ISO/IEC JTC1/SC29/WG11 N1903 (ISO/IEC 14496-2 Subpart 7 Committee Draft. 2. Description of the Related Art Recently, a function for processing shape information has been added to MPEG-4. The shape information, which refers to object information of the image, is expressed as a binary image. To code such a binary image, MPEG-4 adopts a content-based arithmetic encoder (CAE). For lossy shape coding a down sampling method and an upsampling method are performed in each shape macro block. A shape image is divided into shape blocks which have M×M block size. The down sampling refers to a method for reducing the binary image block according to a given conversion ratio. The reduced image block is transmitted together with the conversion ratio. Here, the conversion ratio is determined such that an error between the original binary image block and an binary image block restored later is within a predetermined range. The reduced image block obtained by the down sampling is coded by a context based arithmetic encoder (CAE) and then transmitted. The up sampling method is used to restore such a reduced image block. Up sampling is for restoring the reduced image block into the original binary image, by interpolation. In such an up sampling process, an effective interpolation method must not cause excessive blocking and smoothing effects in the restored binary image. To satisfy the above requirement, it is an object of the present invention to provide an improved interpolation method in which a context (state value of pixels adjacent to a pixel (interpolated pixel) generated by interpolation) is used for interpolation, thereby reducing blocking and smoothing effects. To achieve the above object, there is provided an interpolation method for a binary image, for restoring a reduced binary image, reduced from an original binary image by down sampling, into the original binary image, the method comprising the steps of: (a) calculating an interpolation value based on the pixel values (object pixel values) of the reduced image around an interpolated pixel; (b) calculating a context C The invention may be embodied in a general purpose digital computer that is running a program from a computer usable medium including but not limited to storage media such as magnetic storage media (e.g., ROM's, floppy disks, hard disks, etc.), optically readable media (e.g., CD-ROMs, DVDs, etc.), hybrid formats (magneto optical disks) and carrier waves (e.g., transmissions over the Internet). For instance, a part of the present invention can be computer usable medium having computer readable program code means embodied therein for processing by a machine for restoring a reduced binary image, reduced from an original binary image by down sampling, into the original binary image, the computer program being executable by the machine to perform the following steps: (a) calculating an interpolation value based on the pixel values of pixels (object pixels) around an interpolated pixel; (b) calculating a context C (c) obtaining a threshold value corresponding to the calculated context; and (d) comparing the interpolation value with the threshold value of the step (c), and setting the pixel value of the interpolated pixel as “1” if the interpolation value is greater than the threshold value, and setting the pixel value of the interpolated pixel as “0” if the interpolation value is equal to or less than the threshold value. The above object and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: FIG. 1A is a diagram illustrating coding and decoding methods for a binary image according to MPEG-4; FIG. 1B is a diagram illustrating a down sampling step shown in FIG. 1A; FIG. 1C is a diagram illustrating an up sampling step shown in FIG. 1A; FIGS. 2A through 2D are diagrams illustrating an interpolation method according to the present invention; and FIG. 3FIGS. 3 and 3A in combination is a flowchart illustrating a threshold value learning method according to the present invention; and FIGS. 4A through 4C show the effect of the interpolation method according to the present invention, in contrast to the conventional method. In FIG. 1A, coding and decoding methods for a binary image having object information according to the MPEG-4 includes a down sampling step In the down sampling step In the coding step The reduced image coded through the coding step FIG. 1B illustrates in detail the down sampling step In the down sampling step, if half or more pixels of each group of four pixels in a macro block are equal to “1”, the pixel value of the converted pixel becomes “1”. Here, pixels having value “1” represent a part of the picture containing an object, and pixels having value “0” represent a part of the picture without an object. The boundary between pixels having value “1” and pixels having value “0” represents the boundary of the object information. FIG. 1C illustrates in detail the up sampling step Referring to FIG. 1C, a conventional interpolation method will be described in detail. In FIG. 1C, pixels A, B, C and D represent object pixels, which are pixels concerned in the interpolation, and pixels P First, an interpolation value INP[P] is obtained by a linear interpolation method as follows, using the object pixels A, B, C and D:
where r and s are weights, and r is greater than s. That is, for each interpolated pixel, the larger weight is applied to the pixel which is the closest to each interpolated pixel. Next, the obtained interpolation value INP[P] and a threshold value THR are compared. Here, the threshold value THR is set to a value which is half of the largest possible interpolation value. If the interpolation value INP[P] is greater than the threshold value THR, the pixel value of that interpolated pixel becomes “1”. Otherwise, the pixel value of the interpolated pixel becomes “0”. The pixel values are assigned arbitrarily and by convention. Naturally, they can be switched. Here, only one threshold THR is applied while a plurality of interpolation values INP may be applied. Thus, if the interpolation value INP[P] and the threshold value THR are close to each other, it is unclear whether the pixel value of the interpolated pixel is correct or not. Accordingly, serious blocking or smoothing phenomenon is present in the restored binary image. To reduce the blocking or smoothing, a threshold value to be compared with the interpolation value INP[P] is adaptively determined according to the context (state values of the pixels (reference pixels) around the interpolated pixel), thereby reducing ambiguity in determination of the pixel value of the interpolated pixel. Also, proposed threshold values are determined using a learning method. An improved interpolation method according to the present invention will be described with reference to FIGS. 2A through 2D. In FIGS. 2A through 2D, pixels A, B, C, D, C First, the interpolation value of a pixel to be generated by interpolation is calculated. Here, the interpolation value is calculated by a bilinear interpolation method. However, the interpolation value may alternatively be calculated by using the average of the object pixels or other methods. The interpolation value is calculated using the bilinear interpolation method by the following equation (1):
where h and v of each term represent horizontal and vertical distances between the object pixel and the interpolated pixel included in the corresponding term. The interpolation value INP[P] is determined as a value between 0 and 1. The number of combinations of the object pixels is equal to 16, and the number of interpolation values INP[P] is 16. However, if duplicate values are not counted, the actual number of interpolation values is 12, which are all between 0 and 1. Accordingly, an integer t has the range of Then, the context (state value of the reference pixels) is calculated by the following equation (2): where P represents the position of the interpolated pixel (P indicates P FIGS. 2A through 2D show positions and indices of the reference pixels used for interpolating the interpolated pixels P The proposed threshold values THRc[t] are determined by a learning method, described below. Then, the interpolation value INP[P] and the threshold value THRc based on the context are compared to determine the pixel value of the interpolated pixel. If the interpolation value INP[P] is greater than the threshold value THRc, the pixel value of the interpolated pixel becomes “1”. Otherwise, the pixel value of the interpolated pixel becomes “0”. Next, a method for determining the threshold value THRc based on the context of the reference pixel will be described. The threshold value THRc is obtained by a learning method. The learning is performed by the step of comparing the original binary image with the restored binary image. In detail, referring to FIG. 3, first, arrays h[c][t] and THRc[t] are defined (step Here, the array h[c][t] holds numbers of hits, where a hit is the name given to when the restored pixel value is equal to the original pixel value. The hits are determined by applying all of the possible threshold values to each context. Here, c is an index of the interpolation values, and t is an index of the proposed threshold values. In the case of FIG. 2, since the number of interpolation values and the number of proposed threshold values are both equal to 12, the size of the array becomes h[11][11]. Also, the array THRc[t] stores the proposed threshold values. Here, t is an index of the proposed threshold values. Then, the array h[c][t] is initialized to zero (step After initializing the array h[c][t] to zero, the original binary image and the reduced image are input (step Then, the location of an initial interpolated pixel is determined. The interpolation is performed from the upper-left to the lower-right by a raster scanning (step Once the location of the interpolated pixel is determined, context is calculated by the above equation (2) (step The threshold index t is initialized (step Then, the interpolation value INP[P] of the interpolated pixel is calculated (step The current proposed threshold value from THRc[t] is compared with the calculated interpolation value INP[p] (step The pixel value P of the interpolated pixel is compared with the original pixel value (step It is checked whether the comparison of step If the condition of step If the condition of step If the interpolation has been performed on all pixels of the binary image, the proposed threshold value providing the highest frequency of hits is set as the threshold value of the corresponding interpolation value (step If the condition of step According to the interpolation method of the present invention, ambiguity of the restoration performed by using only the interpolation value obtained by the bilinear interpolation can be removed by using the context, thereby reducing restoration error in the restored binary image. The invention may be embodied in a general purpose digital computer that is running a program or program segments originating from a computer readable or usable medium, such medium including but not limited to magnetic storage media ((e.g., ROM's, floppy disks, hard disks, etc.), optically readable media (e.g., CD-ROMs, DVDs, etc.) and carrier waves (e.g., transmissions over the Internet). A functional program, code and code segments, used to implement the present invention can be derived by a skilled computer programmer from the description of the invention contained herein. FIGS. 4A through 4C show the effect of the interpolation method according to the present invention, in contrast to the conventional method. In detail, FIG. 4A shows the original binary image containing the MPEG-4 logo, FIG. 4B shows the result of the conventional interpolation described with reference to FIG. 1C, and FIG. 4C shows the result of the interpolation according to the present invention. As can be seen from FIGS. 4B and 4C, the blocking and smoothing effects are sharply reduced compared to the conventional method. As described above, in the interpolation method of the present invention, the ambiguity in the comparison between the interpolation value and the threshold value is removed by using the context (state value of the reference pixels around the interpolated pixel), thereby reducing the blocking and smoothing phenomena in the restored binary image. Patent Citations
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