JP2878600B2 - Stepwise multi-tone image encoding device and decoding device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stepwise multi-tone image encoding apparatus and decoding apparatus for reproducing and displaying a multi-tone image step by step.

[0002]

[Prior art]

<Background 1> As a new image communication service, soft copy communication represented by an image database search, an audio graphic conference, and the like is receiving attention.
Soft copy communication is a method of displaying information using a display in front of the user, making it easy to process information such as reproduction, deletion, and editing, and a hierarchical representation for the purpose of quickly grasping the entire image. Also suitable for. Therefore, a hierarchical transmission method suitable for progressive build-up, in which a schematic image is transmitted at an early stage and thereafter the image quality is gradually improved, has been standardized. International Recommendation ITU-TT. In 81, the input image is divided into 8 × 8 pixel blocks, and each block is subjected to an orthogonal transform called DCT to obtain 8 × 8 pixels.
After the pixel data of 8 is converted into 8 × 8 DCT coefficients,
The 64 DCT coefficients are divided into a plurality of groups (bands), only the first band is encoded for all blocks in the image in the first scan, and the remaining bands are encoded one by one in subsequent scans. Method (spectral selecti
on) and all coefficients of the DCT, first the MSB (Most
Significant Bit) side multiple bits are encoded for all blocks in the image, and the remaining LSB (Leas)
There is a method (successive approximation) in which bits on the side of (t Significant Bit) are encoded at 1 bit / scan. <Background 2> In addition, the binary image coding method (ITU-T
T. In 82), the multi-tone image is decomposed into bit planes, each bit plane is encoded as a binary image, and transmitted in units of bit planes, so that multi-tone data can be hierarchically transmitted. <Background 3> In an image processing apparatus such as an image scanner or a facsimile apparatus, as a method for converting an input multi-tone image into a binary image, for example, Journal of Image Electronics Society of Japan, Vol. 10, No. 5, (1981) ) Is used. Further, as an application thereof, in order to display an original multi-tone image on a display having a limited tone display capability, the difference between the number of tones of the original image and the number of display tones of the display is interpolated by the above-described systematic dither method. Is also possible. Specifically, when displaying 8-bit gray scale data (256 gray scales) on a display of 4 bit gray scales (16 gray scales), the systematic dither method using a 4 × 4 dither matrix is used to perform pseudo-simulation. 256 gradations can be displayed.

[0003]

A multi-tone image such as a photograph is transmitted by a method suitable for progressive build-up with the aim of grasping the entire image at an early stage by the method as described in Background 1 above. In such a case, when the display to be displayed has a gradation display capability of about 8 or 16 gradations, such as a low-cost LCD, a contour called a pseudo contour which does not exist in the original image exists in the final image, which causes image deterioration. Also, in the method such as the background 2, a pseudo contour exists in the final image, the correlation between the pixels of the bit plane is poor, the entropy is large, and the image compression effect cannot be expected. Further, even if the method of the background 3 is used to remove the false contour, the method of the background 1 causes distortion and noise due to the presence of high-frequency components that are not present in the original image, and the method of the background 2 is more effective than the above. Further, the correlation between the pixels of the bit plane is poor, and an image compression effect by encoding cannot be expected due to an increase in entropy. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a stepwise multi-tone image encoding device and a decoding device which solve the above problems.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, a stepwise multi-tone image encoding apparatus according to the present invention is an encoding apparatus for encoding a multi-tone image in which one pixel is represented by multi-values. First converting means for converting an original multi-tone image into simple multi-tone data which can be displayed by truncating bits, and converting the original multi-tone image into dither multi-tone data which can be displayed by a dither method. Second conversion means, third conversion means for converting the grayscale number data converted by the first and second means into gray code data, and gray code data converted by the third conversion means Are sequentially encoded in bit plane units from the highest bit plane to the lowest bit plane of the simple multi-tone data, and then sequentially bit-coded from the highest bit plane to the lowest bit plane of the dither multi-tone data. Those comprising encoding means for stepwise encoded in over down units. Further, the stepwise multi-tone image encoding device of the present invention converts the original multi-tone image into simple multi-tone number data that can be displayed by truncating bits.
Conversion means, and second conversion means for converting the original multi-tone image into dither multi-tone data that can be displayed by the dither method,
Third conversion means for converting the gray scale number data converted by the first and second means into gray code data, and gray code data converted by the third conversion means, From the combination of the highest-order bit plane and the highest-order bit plane of the dither multi-gradation data to the combination of the lowest-order bit plane of the simple multi-gradation data and the lowest-order bit plane of the dither multi-gradation data sequentially and sequentially Encoding means for performing the encoding. Further, in the encoding means for encoding gray code data in bit plane units in each of the above-described stepwise multi-tone image encoding devices, the dither multi-gradation encoded data is converted into simple multi-tone data for each bit plane. The difference is encoded.

Further, a stepwise multi-tone image decoding apparatus according to the present invention comprises a decoding step of decoding the coded data coded in bit plane units in the stepwise multi-tone image coding apparatus into gray code data. And a reverse conversion means for reversely converting the decoded gray code data into gradation number data.

[0006]

After converting an original multi-tone image into a number of tones according to the display capability of the display, the tone data is converted into a gray code instead of directly converting the tone data into a bit plane data block. . FIG. 3 illustrates such a situation. The Z axis indicates the number of gradations, and the XY plane indicates the bit plane. The conversion from the block A to the number of gradations according to the display capability of the display is performed by LSB (Leas
Block B is generated by truncating the lower few bits from (t Significant Bit). After that, the conversion is performed using the Gray code, and the block C is generated. FIG. 4 shows a table for converting the gradation number data into a gray code. This means that each bit value when a natural number is expressed in a natural binary form is converted from MSB (Most Significant Bit) to (b ₀ , b ₁ ,..., B
_n-1 ), which is expressed in Gray code (g ₀ , g ₁ , ....,
g _n-1 ), there is the following relationship between g _i and b _i . g _i = b _i (i = 0) g _i = b _i XOR b _i-1 (i = 1,2, ..., n-1) XOR Exclusive OR This means that the Hamming distance between adjacent codes is In a code that is 1 and changes slowly, such as a photo, the entropy is small because the bit plane using the Gray code has a strong correlation between adjacent pixels.
The image compression effect by encoding is enhanced. By sequentially transmitting the coded data in bit plane units starting from the MSB, the number of gray scales on the receiving side is 1 bit, 2 bits, 3 bits,. . . That is, the image quality is improved sequentially in two tones, four tones, and eight tones. In addition, when the gradation display capability of the display reaches the maximum, the pseudo gradation number is added and the final gradation number is added by successively transmitting the bit data in the bit plane unit which is dither-converted to the original multi-gradation image sequentially from the MSB. Specifically, Leq = (L−1) n ² +1 (1) Leq: equivalent number of gradations L: display gradation display capability n: dither matrix size The gradation number is displayed. When the bit plane subjected to the dither conversion is input on the receiving side, the bit plane is replaced with the already displayed simple multi-tone bit plane. The order is as follows: simple multi-gradation bit plane MSB, dither multi-gradation bit plane MSB, simple multi-gradation bit plane (MSB-1), dither multi-gradation bit plane (MSB-1),. . . It is also possible to transmit in the order of. Further, since the amount of change between the simple multi-tone bit plane and the dither multi-tone bit plane at a certain level is relatively smaller than that between the planes, encoding of the difference information can reduce encoded data. .

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing one embodiment. FIG. 1 is a block diagram showing an embodiment of a communication system using a stepwise multi-tone image encoding device and a decoding device according to the present invention. In the present embodiment, a communication system using the stepwise multi-tone image encoding device and the decoding device of the present invention includes, on the transmission side, a memory in which an original multi-tone image input by a scanner or a camera is stored. 1
1, a simple multi-gradation processing unit 12 that receives gradation number data of the display capability of the display from the storage memory 11 and performs processing to reduce gradation bits by simple processing such as truncation; A dither multi-gradation processing unit 13 that receives the gradation number data of the display capability of the display as input and performs processing for reducing gradation bits by dithering processing
A gray code conversion unit 14 for converting a normal bit stream of image data output from the simple multi-tone processing 12 and the dither multi-tone processing 13 into a gray code bit stream; A memory 15 for storing the converted multi-gradation processing data for each bit plane, a memory 16 for storing the dither multi-gradation processing data converted by the gray code conversion unit 14 for each bit plane, Encoding unit 17 for encoding a bit plane obtained by the processing and a bit plane obtained by the dither multi-tone processing (the encoding is performed by a conventional binary image encoding method, for example, M
The H, MR, MMR method, the JBIG method and the like can be considered. On the receiving side, a decoding unit 18 corresponding to the encoding unit and a memory 19 for storing gray-coded simple multi-tone data for each bit plane
A memory 20 for storing gray-coded dither multi-gradation data for each bit plane, a gray code inverse converter 21 for converting each gray-coded bit stream into a normal bit stream, The display unit 22 is configured to display gradation data based on a bit stream. The transmission side and the reception side are connected by a communication path 23.

FIG. 2 is a diagram showing the processing contents of the dither multi-gradation processing section 13. FIG. 2A is a graph showing the number of gradations of an arbitrary horizontal scanning line in an original multi-tone image. (B) shows a graph in which the input gradation number data of the display is used as a reference gradation, and a periodic noise called dither is superimposed on the reference gradation data. This dither may be a pattern having regularity in a cycle of a known Bayer or the like, or may be a random pattern. Input original multi-gradation data is compared with the reference tone at first _{(b), g n-1} <I ≦ g n (2) I: original multi-gradation data g _n: gradation in the n gradation In the section [g _n−1 , g _n ] of data, the pixel is compared with the dither pattern of (b) in pixel units. If I <D, I = n−1 (3) If I ≧ D, I = n D: dither The original multi-tone data is reduced to the number of tones corresponding to the display tone capacity of the display by the processing of pattern data. (C) shows the result of the dither multi-tone processing.

Next, a stepwise display method using the present invention is described assuming that the number of gradations of the original image is 256 gradations (8 bits), the display capacity of the display is 8 gradations (3 bits), and the dither matrix size is 4 × 4 pixels. explain. On the transmission side, the low-order 5 bits of the 256-tone (8-bit) data stored in the original multi-tone image storage memory 11 are truncated by the simple multi-tone processing unit 12 and sent to the gray code conversion unit. Further, the dither multi-gradation processing unit 13 calculates the above equation (2),
The data is reduced to 3 bits by the processing of the expression (3) and sent to the Gray code conversion unit 14. The gray code conversion unit 14 converts the natural binary 3-bit stream data into a gray code 3
Convert to bitstream. The bit stream data is stored in bit plane memory, and the simple 8-gradation data is stored in the bit plane memory 15 and the dither 8-gradation data is stored in the bit plane memory 16. Thereafter, the data is coded and transmitted from the highest bit, that is, bit 2 to bit 0, in bit plane units. At this time, there are two types of transmission patterns. The first pattern is simple data bit 2
→ Simple data bit 1 → Simple data bit 0 followed by dither data bit 2 → Dither data bit 1 → Dither data bit 0 in order. (Hereinafter referred to as transmission pattern 1). The second pattern is to transmit in the order of simple data bit 2 → dither data bit 2 → simple data bit 1 → dither data bit 1 → simple data bit 0 → dither data bit 0. (Hereinafter referred to as transmission pattern 2). Here, the bit plane of the simple data and the bit plane of the dither data of the same bit have locally different portions due to the effect of dithering. Since both the pattern 1 and the transmission pattern 2 transmit the bit plane of the simple data and then transmit the bit plane of the dither data, the bit plane of the dither data is not encoded and transmitted as a whole, but the difference from the simple data is calculated. The encoding is expected to reduce the code amount.

On the receiving side, the coded data in bit plane units received according to the transmission pattern 1 or the transmission pattern 2 is decoded into gray code data by the decoding unit 18, the simple data is stored in the memory 19, and the dither data is stored in the memory 19. Then, the gray plane data is accumulated in the bit plane 20, and then the gray plane is converted by the gray code inverse conversion unit 21 into natural binary data for each pixel. 22 for display. After displaying the simple data for a certain bit plane, the display of the dither data is rewritten for each bit plane when there is data for all the bit planes, and when the dither data is difference data from the simple data, the simple data is displayed. The dither data can be displayed by the exclusive OR of the difference data.

[0011]

According to the present invention, progressive build-up display of a multi-tone image such as a photograph for the purpose of quickly grasping the entire image can be realized without deteriorating the image.

Further, the number of gradations of the original image is reduced according to the gradation display capability of the display on the decoding device side, and bit planes corresponding to the number of gradations are used as bit planes by simple bit reduction and pseudo by dithering. By preparing bit planes with gradations and encoding them sequentially from the highest bit, progressive build-up display with more subdivisions and more gradations is possible, and each bit plane is gray code This makes it possible to increase the compression ratio of the image encoded data.

Further, since the amount of change between the simple multi-tone bit plane and the dither multi-tone bit plane at a certain level is relatively smaller than that between the planes, the difference information is coded to obtain the coded data. Reduction can be expected.

[Brief description of the drawings]

FIG. 1 shows a block diagram of an embodiment of a communication system using a stepwise multi-tone image encoding device and a decoding device according to the present invention.

FIG. 2 is a diagram showing processing contents of dithering processing provided in one embodiment of the present invention.

FIG. 3 is an explanatory diagram showing an example in which a multi-tone image block is processed according to the number of display gradations.

FIG. 4 is a diagram showing a Gray code conversion table.

[Explanation of symbols]

 Reference Signs List 11 Original multi-tone image storage memory 12 Simple multi-tone processing unit 13 Dither multi-tone processing unit 14 Gray code conversion unit 15 Simple multi-tone data storage memory (transmission side) 16 Dither multi-tone data storage memory (transmission side) 17) Encoding unit 18 Decoding unit 19 Simple multi-gradation data storage memory (receiving side) 20 Dither multi-gradation data storage memory (receiving side) 21 Gray code inverse conversion unit 22 Display unit 23 Communication path

Claims (4)

(57) [Claims] 1. An encoding apparatus for encoding a multi-tone image representing one pixel in multi-values, wherein the encoding device converts the original multi-tone image into simple multi-tone number data that can be displayed by truncating bits. Conversion means for converting the original multi-tone image into dither multi-tone number data which can be displayed by the dither method; and converting the tone number data converted by the first and second means. Third converting means for converting the gray code data into gray code data, and converting the gray code data converted by the third converting means from the highest bit plane to the lowest bit plane of the simple multi-gradation data sequentially in bit plane units. Encoding means for successively encoding the data sequentially from the highest bit plane to the lowest bit plane of the dither multi-tone data in bit plane units. Tone image coding apparatus. 2. An encoding apparatus for encoding a multi-tone image representing one pixel in multi-values, wherein the encoding device converts the original multi-tone image into simple multi-tone number data that can be displayed by truncating bits. Conversion means for converting the original multi-tone image into dither multi-tone number data which can be displayed by the dither method; and converting the tone number data converted by the first and second means. Third conversion means for converting into gray code data, and the gray code data converted by the third conversion means into a highest-order bit plane of simple multi-gradation data and a highest-order bit plane of dither multi-gradation data. Coding means for sequentially and stepwise coding from the combination to the combination of the lowest bit plane of the simple multi-gradation data and the lowest bit plane of the dither multi-gradation data. Tone image coding apparatus. 3. An encoding means for encoding gray code data on a bit plane basis, wherein the dither multi-gradation encoded data encodes a difference from the simple multi-gradation data for each bit plane. Claim 1 or Claim 2
2. The stepwise multi-tone image encoding device according to item 1. 4. The stepwise multi-tone image encoding device according to claim 1, 2 or 3, decodes the encoded data encoded in bit plane units into multi-tone data. A stepwise multi-tone image decoding device, comprising: decoding means for decoding coded data coded on a bit-plane basis into gray code data; and gray code data decoded by the decoding means. A step-wise multi-tone image decoding device, comprising: an inverse conversion means for performing an inverse conversion into tone number data.