High efficiency coding method for still natural images mingled with bi-level images

A bi-level image mingled still image coding method comprising:

• dividing an original still color image or grayscale image into a plurality of pixel blocks (n.times.m pixels), pixels of each block being detected as a bi-level image area or a pixel block containing a bi-level image area, each pixel of said detected block being rendered into a bi-level form by the use of a threshold value (Sa) to produce as a bi-level coded output, the distribution of pixel values in each of said pixel blocks being calculated so that a block in which said pixel values have centered on one or two specified values is detected as said bi-level image area or pixel block containing said bi-level image area;
• replacing such coded pixel value of each pixel block of said original image by a specified calculated value based on said each pixel value to produce a calculated value coded output obtained by coding said replaced calculated value, a mean value (Sh) of said pixel values in said each pixel block or a mean value (Sl) of said pixel values in said each pixel block being used as said calculated value depending on whether said pixel values in said each pixel block are greater or smaller than said threshold value (Sa);
• producing a difference between said coded pixel value of each pixel block of said original image and said calculated value coded output as a natural coded output;
• and
• obtaining a combined output by synthesizing said bi-level coded output, said calculated value coded output and said natural image coded output as the coded output of said original still color image or grayscale image.

A bi-level image mingled still image coding method comprising:

• dividing an original still color image or grayscale image into a plurality of pixel blocks (n.times.m pixels), pixels of each block being detected as a bi-level image area or a pixel block containing a bi-level image area, each pixel of said detected block being rendered into a bi-level form by the use of a threshold value (Sa) to produce as a bi-level coded output;
• replacing such coded pixel value of each pixel block of said original image by a specified calculated value based on said each pixel value to produce a calculated value coded output obtained by coding said replaced calculated value, a mean value (Sh) of said pixel values in said each pixel block or a mean value (Sl) of said pixel values in said each pixel block being used as said calculated value depending on whether said pixel values in said each pixel block are greater or smaller than said threshold value (Sa);
• producing a difference between said coded pixel value of each pixel block of said original image and said calculated value coded output as a natural coded output;
• and
• obtaining a combined output by synthesizing said bi-level coded output, said calculated value coded output and said natural image coded output as the coded output of said original still color image or grayscale image, in case of said natural image coding, an unnecessary high-frequency component being removed from a pixel value obtained by subtracting from the corresponding pixel of said original image a pixel value replaced by a pixel at an edge area of a natural image according to tone information, thereby preventing said pixel at said edge area from becoming blunt.