Image compression method

Abstract: The present invention has a purpose to provide an image compression method for transmitting more image information. This image compression method compresses a gray scale original image with a plurality of pixels comprising steps of: i) diminishing in size the gray scale image step by step so that a plurality of diminished images are generated, by the following steps; a) dividing an image to be diminished into a plurality of partial areas with substantially equal size to one another; b) calculating an mean pixel value of pixels in each partial area so that each mean pixel value is defined as a representative pixel value of each the partial area; and c) substituting the pixels of each the partial area by the one pixel with the representative pixel value; ii) enlarging in size the minimal diminished image of diminished images step by step so that a plurality of enlarged images are generated, by adding one pixel between adjacent two pixels of an image to be enlarged, the one pixel to be added having a pixel value of an average pixel value of the adjacent pixels; iii) comparing corresponding pixel values of the diminished image and enlarged image with equal size to each other so that a plurality of compared image are generated, each pixel value of the compared image being binary indicating which pixel value of the images is bigger than the other; iv) calculating an error coefficient for each enlarged image representing a total difference between the pixel value of enlarged image and the diminished image so that a plurality of error coefficients are generated; and v) representing the gray scale original image by the diminished images, compared images and error coefficients.

Description: The present invention relates to an image compression method according to the preamble part of claim 1. Such an image compression method is already known from Optical Engineering, vol. 26, no. 7, July 1987, pages 581-589, K.H. Tzou: "Progressive image transmission: a review and comparison of techniques." One known image compressing method is DCT. This method is, however, very complicated and limited to the transmission of only a few frames per second although special purpose LSI is used. It is the object of the present invention to solve the conventional problems and to provide an image compression method for transmitting more image information. This object is solved by the subject matter of claim 1. Figure 1 is a conceptual diagram showing image compression relating to the present invention. Hereinafter, an embodiment of the present invention is described with reference to the attached drawings. In Figure 1, an original image IM 0 of gray scale is diminished into 1/2 step by step by a ratio of length: the gray scale images IM 2 (diminution ratio is 1/2), IM 4 (diminution ratio is 1/4). IM 8 (diminution ratio is 1/8) and IM 16 (diminution ratio 1/16) are generated. These gray scale images are generated by representing 2 X 2 convolution by a pixel of its mean density. These diminished images are described as "phasic diminished image" hereafter. The number of times of phase of diminution is selected suitably. For example. if IM 0 has 128 X 128 pixels. then IM 16 has 8 X 8 pixels which is enough to be small. Then it is a final stage. An image of the final stage is described as "The minimum image" hereafter. Next. starting from the minimum image, error elements in restoring to the original state are generated in order. First, IM' 8 is generated by enlarging with interpolation into twice as large as IM 16 . ( Such an image is described as "phasic enlarged image". ) Enlargement with interpolation is performed by using well-known method such as that recognizing the mean value of adjoining two pixels as the middle pixel. Here. the density of each pixel of IM' 8 is subtracted from that of the pixel of IM 8 with respect to the corresponding pixel. When the subtracted result is positive, the output is defined as "1". On the other hand, when the subtracted result is negative or 0, the output is defined as "0". This binary output shows the direction of an error differential (positive or negative) of a phasic enlarged image and a phasic diminished image. The image Δ (IM 8 ) given the binary data to each pixel is "phasic binary image" hereafter. A phasic binary image of each enlarged phase after Δ (IM 8 ) is shown as Δ (IM 4 ), Δ (IM 2 ) and Δ (IM 0 ) in Figure 1. At the same time generating phasic binary image, a coefficient Δ8 representing error of a phasic enlarged image corresponding to a phasic diminished image is calculated. This coefficient represents an error and the mean squared error etc., can be adopted. A good result can be obtained using a squared value of the mean value of square root of an error. A coefficients generating each phasic binary image after coefficient Δ8 are shown by Δ4 and Δ2 in Figure 1. At the same time that this coefficient is calculated, an image multiplied a phasic binary image on the coefficient and image added a phasic diminished image are generated. An image generated in such a way in order on each phasic enlargement is called "phasic weighted error image": they are shown by IM'' 8' IM ''4 and IM ''2 in Figure 1. It is possible to suitably select the number of the times of phasic enlargement, however, in the case the sizes of the images on sending and receiving are the same, it will be a final stage because they are the same size as the original image, as phasic expanding image IM' 8 has 16 X 16 pixels, IM' 4 has 32 X 32 pixels, IM' 2 has 64 X 64 pixels and IM' 0 has 128 x 128 pixels. The present invention is able to send a lot of image information because it sends diminished gray scale images. It is also able to obtain almost the same image as an original image because error coefficients on an intermediate stage in enlarging the gray scale image into the original size. As mentioned above, according to the present invention, it is possible to send more image information by compressing high speed image information of TV telecommunication in gray scale.

Claims: An image compression method for compressing a gray scale original image consisting of a plurality of pixels comprising steps of: i) diminishing in size said gray scale original image step by step up to a minimal diminished image, so that a plurality of diminished images are generated by the following steps: a) dividing an image to be diminished into a plurality of partial areas with substantially equal size to one another; b) calculating a mean pixel value of pixels in each partial area, so that each said mean pixel value is defined as a representative value of each said partial area; and c) substituting said pixels of each said partial area by one pixel having said representative value; ii) enlarging in size said diminished images step by step starting from the minimal diminished image, so that a plurality of enlarged images is generated, by interposing one pixel between two adjacent pixels of an image to be enlarged, said one pixel to be interposed having an average value of said adjacent pixels; iii) comparing corresponding pixel values of said diminished image and enlarged image of equal size to each other, so that a plurality of compared images is generated; characterized by iv) calculating one error coefficient for each enlarged image, said error coefficient representing a total of the differences between the pixel values of said enlarged image and the pixel values of the diminished image of equal size, so that a plurality of error coefficients is generated; and v) representing said gray scale original image by said minimal diminished image, said compared images and said error coefficients; and wherein each pixel value of said compared image is binary and indicates which one of said corresponding pixel values is bigger than the other. An image compression method according to claim 1, wherein said error coefficient is calculated for each enlarged image by averaging the square root of said differences between the pixel values of said enlarged image and the pixel values of the diminished image of equal size.

Inventors: SHOU GUOLIANG, TAKATORI SUNAO, YAMAMOTO MAKOTO, SHOU, GUOLIANG, TAKATORI, SUNAO, YAMAMOTO, MAKOTO
Applicants: SHARP KK, YOZAN INC, SHARP KABUSHIKI KAISHA, YOZAN INC.
Application year: 1993
Main classification: H04N7
IPC codes: H04N7, H04N1, H03M7