JP2001128182A - Image encoding method and computer-readable recording medium storing image encoding program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform reversible encoding on image data indicating multi-valued images at a high compression factor. SOLUTION: In the image encoding method in which image data are encoded by predicting the picture element value of a noticed picture element X in the image data from the picture element values of a plurality of reference picture elements A-E which are different from the noticed picture element X, the picture elements A-E which are not directly adjoined to the noticed picture element X or indirectly adjoined to the picture element X through other reference picture elements are provided at separate positions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image encoding method for compressing image data, and more particularly to an image encoding method for image data representing a multi-valued image. Further, the present invention relates to a computer-readable recording medium storing an image encoding program for compressing image data by a computer that executes processing according to the program.

[0002]

2. Description of the Related Art It is generally known that image data representing a full-color image has a very large amount of data. For example, 300 SPI (Spot per inch), A4 size 4 colors full color (Red, Green, Blue 8 bits each color, total 24 bits)
Image has a data amount of about 24 MB. Therefore, when storing and transmitting image data, compression processing is often performed.

Conventionally, as a method of compressing image data, for example, JPEG (Joint Photographic Expert Gr
Oup), a lossy coding called a baseline process is widely known. In this JPEG baseline method, as shown in FIG.
Divided into × 8 pixel blocks, DCT for each block
(Discrete Cosine Transform)
DCT transforms DCT data using a quantization matrix.
Quantization is performed for each coefficient to compress high-frequency components, and finally, quantized data is entropy-coded (Huffman-coded) and output.

However, in the JPEG baseline method,
Since the DCT transform is two times of matrix operation of 8 × 8 pixels and the quantization is division of elements of the matrix, processing one pixel of one color requires 16 multiplications and 14 additions. ,
One more division is required. Therefore, 300S
Since the entire image data of the PI is about 8 million pixels and 4 colors, a total of 522 million multiplications, 448 million additions, and 32 million times are used. Is required. In addition to this, there is Huffman coding, image reading, and other overhead. Even with a state-of-the-art workstation, it takes more than one minute to compress four full-color image data of 300 SPI, A4 size. Spend time. Further, the compression by the JPEG baseline method is lossy compression, and there is a disadvantage that an image before encoding cannot be restored even if any encoding is performed.

[0005] For these reasons, in recent years, various predictive coding methods have been proposed as a reversible compression method capable of performing compression with a relatively small amount of calculation. For example, JP-A-9
Japanese Patent Application Laid-Open No. 2007-774 discloses a method in which a plurality of pixel values around a target pixel are used as prediction values, and lossless encoding is performed using a prediction value with the smallest prediction error among them. More specifically, as shown in FIG. 13, the value of the target pixel P is predicted from the value of the pixel X or Y immediately above it based on different parameters, and the difference between each predicted value and the target pixel value (prediction) Error) is calculated, one prediction value is selected based on the magnitude of the prediction error and a predetermined priority, and the value of the pixel of interest P is encoded based on the selected prediction value, so that the image is efficiently processed. A system that can compress data is disclosed.

For example, Japanese Patent Application Laid-Open No. 9-224253 discloses a computer graphic (Computer Graph).
ics; CG) and page description language (Page Description Langu)
An image suitable for reversibly compressing image data generated by an age (PDL) processing system is disclosed. For details, see FIG.
, The value of the pixel of interest X is predicted by different prediction methods from the values of the pixels a to d such as immediately to the left and immediately above,
Calculate the prediction error between each predicted value and the pixel value of interest,
It is determined whether or not the prediction error is “0”, and if the prediction error is “0”, the value of the pixel of interest X is encoded based on the prediction value corresponding to the prediction error. There is disclosed an image capable of realizing a high compression ratio for an image having a high predictive accuracy.

Further, for example, Japanese Patent Application Laid-Open No. H11-234683
Japanese Patent Application Laid-Open Publication No. H11-163873 discloses an image encoding method that performs high-speed lossless encoding with a high compression ratio on image data generated by a CG or PDL processing system. According to this image encoding method, the following processing is performed for each line of image data to be compressed. First, as shown in FIG.
The value of the target pixel X is predicted from the values of the pixels a, b, c, and d immediately before, immediately above, at the upper left, and at the upper right by the first prediction method, and if the predicted value matches the value of the target pixel X, A code representing the prediction method 1 is output. When the predicted value of the first prediction method does not match the value of the target pixel X, the predicted values of the second and subsequent prediction methods are obtained for the first time, and these are compared with the value of the target pixel X. Then, if there is a matching predicted value, a code indicating the corresponding prediction method is output, and if none of them match, a prediction error is output. This not only realizes a high compression rate for an image having a high predictive accuracy, such as a CG image, but also enables high-speed compression encoding.

[0008]

However, in the above-described conventional lossless image coding method using predictive coding, in each case, the value of a target pixel is predicted from its surroundings, more specifically, from the values of pixels adjacent to the target pixel. Therefore, a high compression ratio may not be obtained depending on image data to be subjected to compression processing.

Examination of image data that cannot be efficiently compressed by the conventional image encoding method has revealed that most of these image data represent shaded images. The shaded image here is a multi-tone image,
That is, a process for converting an image having a large number of bits per pixel into an image having a lower gradation, that is, an image having a small number of bits per pixel, has been performed.

Such a shaded image is often used on a personal computer. For example, in Windows (Windows; registered trademark of Microsoft Corporation) widely used as an operating system for personal computers, one pixel is composed of R, G, and B.
In order to convert a full-color image represented by a total of 24 bits of 8 bits for each color into a limited color image in which the number of bits is smaller, for example, one pixel is represented only by 8 bits, a shading process is sometimes performed. This is often performed, for example, when there is a limit on the memory capacity that can be used for displaying images.

However, unlike a full-color image, a halftone image cannot directly express a halftone color. This is, for example, 24 pixels per pixel for a full-color image.
what was possible 2 ²⁴ gradation for a bit is because only 8bit next 2 ⁸ gradations per pixel by shading treatment can not be expressed. For this reason, in a shaded image, a plurality of continuous pixels are regarded as one block, so that a halftone color is simulated.

From this, the pixel structure of the shaded image is
There is a high probability that the same pixel value will appear in each of the vertical, horizontal, and oblique fixed periods. For example, in the above-described Windows, as shown in FIG. 15, since the shading process is performed in units of blocks of 8 pixels in length and width, the same pixel value appears at positions separated by 8 pixels, and the period of the mesh is In many cases, the number of pixels is 8 in both the main scanning direction and the sub-scanning direction. In addition, since the eight-pixel cycle network includes four sub-screens with four pixel cycles, the probability that the same pixel value appears every four pixels is high.

Therefore, regarding image data representing a shaded image, especially when the image before shading has a fixed halftone, the pixel values of adjacent pixels are not continuous. Even if a compression process is performed using a reversible image encoding method based on encoding, there is a high possibility that a high compression rate cannot be obtained.

Therefore, the present invention has been made in view of the above situation,
An object of the present invention is to provide an image encoding method that enables lossless encoding with a high compression ratio to image data representing a multi-valued image. Another object of the present invention is to provide a computer-readable recording medium that stores an image encoding program that enables lossless encoding with high compression ratio for image data representing a multi-valued image.

[0015]

An image encoding method according to a first aspect of the present invention predicts a pixel value of a target pixel in image data from a pixel value of a reference pixel different from the target pixel. In an image encoding method for encoding, a plurality of reference pixels which are not directly adjacent to the target pixel or indirectly adjacent to the reference pixel via another reference pixel are provided at positions apart from each other.

According to the image encoding method of the present invention, a plurality of reference pixels are provided at positions not directly adjacent to the target pixel or indirectly via another reference pixel. Even in the case of image data having a pixel structure in which the pixel values of the pixels are not continuous and the same pixel value appears at regular intervals, the target pixel can be predicted based on each reference pixel. Encoding can be performed.

According to a second aspect of the present invention, there is provided an image encoding method for predicting and encoding a pixel value of a target pixel in image data from a pixel value of a reference pixel different from the target pixel. In the above, when the image data has the same image data in a block unit consisting of a predetermined number of pixels corresponding to a predetermined limited color color, the reference pixel and the target pixel the size of the block from the target pixel The distance is set at a distance.

According to the image encoding method of the present invention, the image data to be subjected to the compression processing corresponds to a predetermined limited color, for example, a shaded image used on a personal computer. In the case of representing, the reference pixel is set at a position apart from the target pixel by a distance in units of the size of a block composed of a predetermined number of pixels, for example, the pixel values of adjacent pixels are not continuous, Moreover, even in the case of image data representing a shaded image having a pixel structure in which the same pixel value appears at regular intervals, the target pixel can be predicted based on the reference pixel, so that efficient encoding can be performed. become.

Further, a computer-readable recording medium storing the image encoding program according to claim 3 is:
An image encoding program for causing a computer that executes processing according to a program to predict a pixel value of a target pixel in image data from a pixel value of a reference pixel different from the target pixel and perform an encoding process. In the encoding process, a plurality of reference pixels that are not indirectly adjacent to the target pixel directly or indirectly via another reference pixel are provided at a position apart from each other, and an image encoding program is stored. is there.

According to the third aspect of the present invention, when the computer executes the image encoding program stored in the recording medium, the reference pixel is directly connected to the target pixel or via another reference pixel during the encoding process. Since a plurality of pixels are provided at positions that are not indirectly adjacent to each other, for example, even if the image data has a pixel structure in which the pixel values of adjacent pixels are not continuous and the same pixel value appears at regular intervals, each reference pixel , The pixel of interest can be predicted, and efficient coding processing can be performed.

Further, a computer-readable recording medium storing the image encoding program according to claim 4 is:
An image encoding program for causing a computer that executes processing according to a program to predict a pixel value of a target pixel in image data from a pixel value of a reference pixel different from the target pixel and perform an encoding process. In the case where the image data has the same image data in a block unit composed of a predetermined number of pixels corresponding to a predetermined limited color, the reference pixel is set in units of the size of the block from the target pixel. It stores an image encoding program characterized by being set at a distance.

If the computer executes the image encoding program stored in the recording medium according to the fourth aspect of the present invention, the image data to be compressed corresponds to a predetermined limited color in the encoding process. In this case, for example, in the case of representing a shaded image used on a personal computer, the reference pixel is set at a position away from the target pixel by a unit of the size of a block composed of a predetermined number of pixels. Therefore, for example, even if the image data represents a shaded image having a pixel structure in which the pixel values of adjacent pixels are not continuous and the same pixel value appears at regular intervals, the pixel of interest based on the reference pixel Can be predicted, and efficient coding can be performed.

[0023]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an image encoding method and an image encoding program according to the present invention.

The image encoding method according to the present invention includes, for example,
It is realized as a program executed by a computer system or a subsystem thereof. Specifically, it can be realized as an image coding application executed by a computer system, or as a support program (library) used by a host application. Therefore, in the image encoding method according to the present invention, the image encoding program may be stored in a computer-readable recording medium (specifically, a CD-ROM or the like) that executes processing according to the program. . However, the present invention is not limited to this. Even if circuit components are discretely assembled to constitute an image encoding device or a part thereof,
Needless to say, it may be configured as a part of C (special purpose IC) or a dedicated board.

[First Embodiment] FIG. 1 is a schematic diagram for explaining features of a first embodiment of an image encoding method according to the present invention, and FIGS. 6 is a flowchart showing an example of the procedure of FIG.

As shown in FIG. 1, in the image encoding method according to the present embodiment, in addition to the pixel A immediately before the pixel X of interest to be encoded, the image B directly above, the pixel C at the upper left and the pixel D at the upper right, the pixel A feature is that the pixel value of the pixel E eight pixels before X is also used as a prediction value as it is. That is, in the present embodiment, the prediction encoding for the target pixel X is performed using five types of predictors based on the pixel values of the pixels A to E, respectively.

The procedure of the predictive encoding may be performed by using a known technique. As an example, the procedure may be performed according to the procedure shown in FIGS. The procedure shown in FIGS. 2 to 4 is substantially the same as that disclosed in Japanese Patent Application Laid-Open No. H11-234683. More specifically, as shown in FIG. 2, for example, in the predictive coding, after a predetermined initial setting (step 11, hereinafter, step is abbreviated as S), the coding process is completed for all the pixels of the image data to be processed. It is determined whether or not (S12). If the encoding process for the last pixel has been completed, the process proceeds to an end routine. If there is an unprocessed pixel, the following process is repeated until the process is completed for all pixels. That is, first, pixels for one line of the image are read (S13), and encoding processing (compression processing) is performed on the one line as described later (S14). When the encoding process for one line is completed, after performing a predetermined post-process (S15), the same process is repeated for the next and subsequent lines (S13 to S1).
5).

The compression process (S14 described above) for one line of pixels may be performed, for example, as shown in FIG. First, the target pixel pointer p is set at the head of the line (S21). Then, the value of the pointer p is compared with the address of the last pixel of the line, and it is determined whether or not the processing for the one line is completed (S22). If the processing for one line is not completed, the following processing is repeated until the processing for one line is completed. That is, prediction is performed on the pixel of interest based on the first prediction method, and a first predicted value is obtained (S23). This prediction uses the pixel values around the pixel of interest or the result of computing those values. For example, as shown in FIG. 1, one of the immediately preceding pixel A, the image immediately above, the upper left pixel C, the upper right pixel D, and the pixel E eight pixels before is used for the target pixel X. Note that which pixel value is used may be fixed for each predictor or may be changed adaptively. Further, a predictor that performs prediction based on a plurality of pixel values may be used. When the first prediction value is obtained, it is determined whether or not the first prediction value matches the pixel value of the target pixel (S24). If they match, a code indicating the first prediction method is output (S25). On the other hand, if they do not match, the second and subsequent predictions are performed as described below (S26). afterwards,
The target pixel pointer p is advanced by one pixel (S27), and the same processing is repeated for the next and subsequent pixels (S22 to S2).
7).

The second prediction and subsequent processing (S26 described above) may be performed, for example, as shown in FIG. First, prediction is performed based on the second prediction method and subsequent prediction methods to obtain a plurality of predicted values (S31).
This prediction is performed in the same manner as in the case of obtaining the first predicted value, but using different parameters (pixel values) from those in the case of the first predicted value. When the second and subsequent predicted values are obtained, it is determined whether or not there is one that matches the pixel value of the pixel of interest (S32). If there is one that matches, a code indicating the matching prediction method is output. (S33). The same can be applied to the case where another predictor such as the third predictor is added. On the other hand, if there is no match between these predictors, an error between the pixel value of the previous pixel and the pixel value of the target pixel is calculated (S34), and the error is encoded and output (S3).
5). Thereafter, when the encoding of one pixel is completed, the pointer p indicating the pixel value of interest is advanced, and the process returns to the beginning of the loop (FIG. 3).
S22).

By performing predictive encoding for all pixels of image data to be processed in such a procedure, as long as the predicted value of the first prediction method matches the pixel value of the pixel of interest, the other prediction methods are used. Since no prediction is performed, the processing is speeded up accordingly.

In the image coding method according to the present embodiment, when predictive coding of each pixel is performed, as described above, the pixel A immediately before the pixel X of interest, the image B immediately above, the pixel C above left, and the pixel D above right are used. In addition, the pixel value of the pixel E eight pixels before the target pixel X is also set as the reference pixel. That is, as is clear from FIG. 1, on a plane formed in each of the main scanning and sub-scanning directions in the image data, that is, on the XY plane in the drawing, the pixel directly adjacent to the target pixel or another reference pixel is displayed. A reference pixel is provided at a position separated by a predetermined distance from a group of reference pixels that are indirectly adjacent to the reference pixel.

Therefore, according to the image encoding method of the present embodiment, for example, image data having a pixel structure in which the pixel values of adjacent pixels are not continuous and the same pixel value appears at regular intervals. However, since the target pixel X can be predicted based on each of the reference pixels A to E, efficient lossless encoding can be performed. In other words, even with such image data, the target pixel X is not predicted from only the pixels A to D adjacent to the target pixel X as in the related art, so that a high compression rate can be obtained.

In addition, for image data that has been subjected to a shading process of converting one pixel into a limited color image represented by only 8 bits, there is a high probability that the same pixel value appears at regular intervals. By setting the reference pixel E at a position away from the target pixel X by a certain distance (eight pixels), the target pixel X can be predicted based on the reference pixel E, and efficient lossless encoding can be performed. become. In particular, for example, in Windows, halftone processing is performed in units of blocks of 8 pixels in length and width, and the same pixel value appears at a position separated by 8 pixels. By setting beforehand, a higher compression ratio than before can be obtained. That is, the image encoding method according to the present embodiment is very suitable for use with image data corresponding to a shaded image used on a personal computer.

In this embodiment, each of the reference pixels A to E
Is used as an example of the predicted value, but a value obtained by adding a predetermined constant to these values may be used. In particular, when the pixel value changes gradually, it is preferable to add a predetermined constant. It is particularly effective to use "1", "-1" or the like as the constant in this case.

[Second Embodiment] Next, a second embodiment of the present invention will be described.
An embodiment will be described. FIG. 5 is a schematic diagram for explaining the features of the image encoding method according to the second embodiment of the present invention, and FIG. 6 is a schematic diagram for explaining the features of the modification.

As shown in FIG. 5, in the image encoding method according to the present embodiment, in addition to the pixel A immediately before the pixel X of interest to be coded, the image B immediately above, the pixel C at the upper left and the pixel D at the upper right, the pixel The feature is that the pixel value of the pixel F four pixels before X is also used as the prediction value as it is.

Therefore, in the image encoding method of the present embodiment, similarly to the case of the above-described first embodiment, since the reference pixels separated by a predetermined distance on the same line as the pixel of interest are provided, for example, Even in the case of image data having a pixel structure in which the pixel values of pixels to be executed are not continuous and the same pixel value appears at regular intervals, lossless encoding can be performed more efficiently than in the past.

Also, for example, in Windows, in the case of a shaded image, the same pixel value appears every four pixels since a half screen of eight pixels vertically and horizontally consists of four sub-screens of four pixels. The probability is high, and by setting the reference pixel F at a position separated from the target pixel X by a certain distance (for four pixels), the target pixel X can be predicted based on the reference pixel F, which is more efficient than in the past. Good lossless encoding can be performed.

Moreover, the image encoding method of the present embodiment
Compared with the case of the above-described first embodiment, since it is only necessary to refer to a narrow range such as four pixels before instead of eight pixels before, there is also an advantage that a memory capacity used for performing predictive coding can be reduced. is there. Also, when the image density changes relatively sharply, particularly when gradation occurs in the main scanning direction or when many vertical lines are drawn in the image, the reference pixel F Is advantageous in that the compression ratio is further increased as compared with the case of the first embodiment.

In this embodiment, the case where the reference pixel F is set four pixels before is described. However, as shown in FIG.
The pixel value of the reference pixel E eight pixels before the target pixel X may be used together. Further, the target pixel X
It is also conceivable to set a pixel preceding by an integer multiple of 4 as a reference pixel. Even in these cases, it is needless to say that lossless encoding can be performed more efficiently than in the conventional case, as in the case of the above-described first or second embodiment.

[Third Embodiment] Next, a third embodiment of the present invention will be described.
An embodiment will be described. FIG. 7 is a schematic diagram for explaining the features of the third embodiment of the image encoding method according to the present invention, and FIG. 8 is a schematic diagram for explaining the features of the modification.

As shown in FIG. 7, in the image coding method according to the present embodiment, the pixel value of the pixel G eight lines before the pixel X of interest is added to the pixel value of the pixel E eight pixels before the pixel X of interest. Is also characterized in that it is used as it is as a predicted value. That is, the pixel A immediately before the pixel X of interest, the image B directly above, and the pixel C
In addition to the pixel D and the upper right pixel, the pixel value of the pixel E eight pixels before the pixel of interest X and the pixel value of the pixel G eight lines before the pixel of interest X are used for prediction.

Therefore, in the image encoding method of this embodiment, even if the image data has a pixel structure in which the pixel values of adjacent pixels are not continuous and the same pixel value appears at regular intervals. Thus, lossless encoding with higher efficiency than before can be performed.

In Windows, for example, since the halftone processing is performed in blocks of 8 pixels in length and width, the same pixel value appears at positions separated by 8 pixels, so that the reference pixels E and G Are set at positions separated by a fixed distance (8 pixels or 8 lines) from the target pixel X, the target pixel X can be predicted on the basis of the reference pixel E or the reference pixel G. Efficient lossless encoding can be performed.

In particular, according to the image encoding method of the present embodiment, when the image density changes relatively sharply in the sub-scanning direction, specifically when gradation occurs in the sub-scanning direction, In the case where many horizontal lines are drawn, for example, by setting the reference pixel G eight lines earlier,
Alternatively, there is an advantage that the compression ratio is higher than in the case of the second embodiment.

In this embodiment, the case where the reference pixel G is set eight lines before is described. However, the reference pixel H may be set four lines before the target pixel X as shown in FIG. However, even in this case, it is needless to say that lossless encoding can be performed more efficiently than in the prior art, as in the case of the present embodiment. Moreover, when the reference pixel H is set four lines before, compared to the case of the present embodiment, a narrow range such as four lines before rather than eight lines may be referred to, so that it is used in performing predictive coding. The memory capacity can be significantly reduced. The reference pixel is not limited to eight or four lines before the target pixel X, but may be a pixel on a line that is an integral multiple of four as a reference pixel.

[Fourth Embodiment] Next, a fourth embodiment of the present invention will be described.
An embodiment will be described. FIG. 9 is a schematic diagram for explaining features of the image encoding method according to the fourth embodiment of the present invention, and FIG. 10 is a diagram showing an example of an image structure processed by the image encoding method. FIG.

As shown in FIG. 9, the image coding method according to the present embodiment is characterized in that the pixel value of the pixel I two pixels before on the two lines of the target pixel X is directly used as the predicted value. That is, the pixel A immediately before the pixel X of interest, the image B immediately above,
In addition to the upper left pixel C and the upper right pixel D, the pixel value of the pixel I two pixels before two pixels on the line of interest X is used for prediction. As a result, the line connecting each of the reference pixels A to D and I and the target pixel X has a fixed area (3 lines × 3) on the XY plane in the drawing.
(For four pixels).

In the image encoding method according to the present embodiment,
For example, even in the case of image data having a pixel structure in which pixel values of adjacent pixels are not continuous and the same pixel value appears at regular intervals, lossless encoding can be performed more efficiently than in the past. .

In particular, the image encoding method of this embodiment is different from the first to third embodiments described above in that the method is used when the processing angle of the corresponding shaded image is not 0 degree. Will be effective. For example, instead of performing the shading process along the main scanning direction and the sub-scanning direction, the shading process may be performed at a certain angle. For example, as shown in FIG. 10, when the shading process is performed at an angle of 45 degrees, there is a high possibility that the same pixel value appears at regular intervals in the diagonal 45 degrees direction. Therefore, the reference pixel I is set at a certain distance (two lines and
By setting the pixel at a position farther away from the pixel, the reference pixel I
, The target pixel X can be predicted, and lossless encoding can be performed more efficiently than before.

In this embodiment, for the sake of simplicity, a small shading period is used for illustration. However, if the shading period is large, it is sufficient to increase the number of lines and pixels. . If the angle of the halftone is not 45 degrees, the number of lines and the number of pixels from the target pixel may be different, and the pixel value of a pixel separated by a distance matching the halftone period may be used for prediction. That is, the position of the reference pixel I is
What is necessary is just to set arbitrarily according to the period, angle, etc. of the shading process.

[Fifth Embodiment] Next, a fifth embodiment of the present invention will be described.
An embodiment will be described. FIG. 11 is a schematic diagram illustrating the features of the fifth embodiment of the image encoding method according to the present invention.

As shown in the figure, the image encoding method according to the present embodiment differs from the image encoding method described in the first embodiment with the conventional image encoding method according to the image data to be processed. There is a characteristic in that they are properly used. Specifically, in the present embodiment, the image data to be processed is 8b per pixel.
Whether it corresponds to the limited color image of it or 1
As shown in FIG. 11A, the pixel value of the pixel E eight pixels before the pixel of interest X is also used for prediction, depending on whether the pixel corresponds to a full-color image of 24 bits per pixel, or FIG.
As shown in FIG. 1B, pixels A to D adjacent to the pixel X of interest
Whether only the prediction is used or not is switched.

This is because image data corresponding to a full-color image of 24 bits per pixel has not been subjected to shading processing in many cases, so that the network data described in the first to fourth embodiments is used. Without performing pattern prediction, the target pixel X
This is because efficient lossless encoding can be performed based only on the pixels A to D adjacent to the image data, and quick lossless encoding can be realized by not performing halftone pattern prediction.

Whether the image data to be processed corresponds to the limited color or the full color may be determined by recognizing the number of bits per pixel in the image data to be processed. However, this determination may be made based on whether or not the image data to be processed represents a shaded image. Whether or not the image is a halftone image can be easily determined based on a halftone dot period, for example, whether or not the same pixel is frequently generated every eight pixels.

As shown in FIGS. 11 (a) and 11 (b), it is determined whether or not to perform the halftone pattern prediction by using different preset predictors (an 8-bit image predictor and a 24-bit image predictor). It may be prepared by switching which predictor to use in accordance with the above-described determination result for the image data to be processed.

As described above, in the image encoding method according to the present embodiment, if the image data to be processed corresponds to the limited color, the predictive encoding is performed in the same manner as in the first embodiment. Therefore, lossless encoding can be performed more efficiently than before, but if the image data to be processed is full-color compatible, predictive encoding is performed in the same manner as in the past, so that quick lossless encoding is possible. . That is, according to the image encoding method of the present embodiment, versatility and flexibility for the image data to be processed are enhanced, and suitable lossless encoding is realized for both the limited-color-compatible data and the full-color-compatible data. it can.

In this embodiment, lossless encoding is performed as described in the first embodiment when image data to be processed corresponds to a limited color color. It goes without saying that lossless encoding may be performed as described in the second to fourth embodiments, and even in such a case, it is not possible to secure the versatility and flexibility as described above. Needless to say.

[0059]

As described above, according to the image encoding method described in claim 1 of the present invention and the computer-readable recording medium storing the image encoding program described in claim 3, Since a plurality of pixels are provided at positions that are not directly adjacent to the target pixel or indirectly via another reference pixel, for example, the pixel values of adjacent pixels are not continuous, and the same pixel value appears at regular intervals. Even with image data having such a pixel structure, the target pixel can be predicted based on each reference pixel, and efficient encoding can be performed. Therefore, the image data representing the multi-valued image is converted into, for example, a limited color color (shading processing or the like).
Is performed, the pixel of interest is not predicted only from pixels adjacent to the pixel of interest as in the related art, so that a high compression rate can be obtained.

According to the image encoding method of the present invention and the computer-readable recording medium storing the image encoding program of the present invention, image data to be subjected to compression processing Is a color corresponding to a limited color (for example, a color representing a shaded image), the reference pixel is set at a position away from the target pixel by a unit of the size of a block of a predetermined number of pixels. For example, even for image data representing a shaded image having a pixel structure in which the pixel values of adjacent pixels are not continuous and the same pixel value appears at regular intervals, the target pixel is predicted based on the reference pixel. And efficient coding can be performed. Therefore, even if the image data representing the multi-valued image is subjected to, for example, conversion to a limited color (shading processing or the like), only the pixel adjacent to the pixel of interest is used as in the related art. Since the target pixel is not predicted, a high compression ratio can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating features of a first embodiment of an image encoding method according to the present invention.

FIG. 2 is a flowchart illustrating an example of a procedure of an image encoding method.

FIG. 3 is a flowchart showing in detail a part of the procedure of the image encoding method of FIG. 2;

4 is a flowchart showing a part of the procedure of the image encoding method of FIG. 3 in further detail.

FIG. 5 is a schematic diagram for explaining features of the image encoding method according to the second embodiment of the present invention.

FIG. 6 is a schematic diagram illustrating a feature of a modification of the second embodiment of the present invention.

FIG. 7 is a schematic diagram illustrating features of a third embodiment of the image encoding method according to the present invention.

FIG. 8 is a schematic diagram for explaining features of a modification of the third embodiment of the present invention.

FIG. 9 is a schematic diagram for explaining features of a fourth embodiment of the image encoding method according to the present invention.

FIG. 10 is an explanatory diagram illustrating an example of an image structure processed in a fourth embodiment of the present invention.

FIG. 11 is a schematic diagram for explaining features of a fifth embodiment of the image encoding method according to the present invention, and FIG.
Is a diagram showing feature points in the case of corresponding to a limited color color,
FIG. 4B is a diagram showing characteristic points in a case where the image corresponds to full color.

FIG. 12 is an explanatory diagram showing an example of an image structure in a JPEG baseline image coding method.

13A and 13B are schematic diagrams (part 1) for explaining feature points in an example of a conventional image encoding method, in which FIG. 13A illustrates a state in which an immediately preceding pixel is referred to, and FIG. FIG. 7C is a diagram illustrating a state in which reference is made, and FIG. 7C is a diagram illustrating characteristic points in a case where both the immediately preceding pixel and the immediately above pixel are referred to.

FIG. 14 is a schematic diagram (part 2) for explaining feature points in an example of a conventional image encoding method.

FIG. 15 is an explanatory diagram illustrating an example of an image structure in a case where shading processing is performed in blocks of eight pixels in length and width.

[Explanation of symbols]

A, B, C, D, E, F, G, H, I... Reference pixels, X.
Attention pixel

Claims (4)

[Claims] 1. An image encoding method for predicting and encoding a pixel value of a pixel of interest in image data from a pixel value of a reference pixel different from the pixel of interest, comprising: An image encoding method, wherein a plurality of reference pixels that are not indirectly adjacent to each other via a pixel are provided at positions apart from each other. 2. An image encoding method for predicting and encoding a pixel value of a pixel of interest in image data from a pixel value of a reference pixel different from the pixel of interest, wherein the image data has a predetermined limited color color. Correspondingly, when the same image data is included in a block unit consisting of a predetermined number of pixels, the reference pixel is set at a position apart from the pixel of interest by a distance in units of the size of the block. Image encoding method. 3. An image code for causing a computer, which executes processing according to a program, to predict a pixel value of a target pixel in image data from a pixel value of a reference pixel different from the target pixel and to perform an encoding process. An image encoding program characterized by providing a plurality of reference pixels that are not directly adjacent to the target pixel or indirectly indirectly via another reference pixel at positions apart from each other in the encoding process. A computer-readable recording medium storing a computer. 4. An image code for causing a computer, which executes processing according to a program, to predict a pixel value of a target pixel in image data from a pixel value of a reference pixel different from the target pixel and perform an encoding process. Conversion program, when the image data has the same image data in a block unit consisting of a predetermined number of pixels corresponding to a predetermined limited color, when the reference pixel from the target pixel of the block A computer-readable recording medium storing an image encoding program, wherein the recording medium is set at a position apart by a distance in units of size.