Progressive block-based coding for image compression

Abstract

A method of image compression includes significance switching of DCT coefficients in block-based embedded DCT procedures. Bitwise digitized DCT coefficients are passed through successive significance sweeps of the whole image from the most significant down to the least significant coefficient bit planes. With each new sweep, newly significant coefficients may appear within a block, and block-masking is used to transmit the addresses of those newly significant coefficients. An off-mask may also be used. The invention further relates to a hardware or software-based image encoder.

1. A method of image compression comprising the steps of:
 (a) dividing an image to be compressed into a plurality of image blocks; 
 (b) carrying out a two-dimensional block transform on each block to produce a corresponding plurality of coefficient blocks; 
 (c) bitwise digitizing the coefficients within each coefficient block to define a plurality of bit planes for each coefficient block; 
 (d) defining a group of one or more consecutive bit planes starting with the most significant bit plane; 
 (e) selectively flagging those, by a coder device, coefficients which first become significant within the a group of one or more consecutive bit planes of corresponding coefficient blocks of image blocks resulting from a block transform, starting with the most significant bit plane; and  
 (f) transmitting, by the coder device, information representative of the positions of the said flagged coefficients and transmitting the bits within the a group of the said flagged coefficients; and, 
 (g) repeating steps (d) to (f) one or more times, with each new group starting with the most significant bit plane not previously dealt with and, at each repeated pass, also transmitting the bits within the current group of those coefficients which were previously flagged on an earlier pass. 
 
     
     
       2. A method as claimed in  claim 1  in which step (g) is carried out across the entire image to be compressed further comprising: 
 repeating the selectively flagging and the transmitting one or more times, each with a new group starting with a most significant bit plane not previously dealt with and, at each repeated pass, also transmitting bits within a current group of those coefficients which were previously flagged on an earlier pass. 
 
     
     
       3. A method as claimed in claim  1  2 in which step (g) the repeating is separately repeated performed for each image block. 
     
     
       4. A method as claimed in claim  1  2 in which the block transform is the a two-dimensional Discrete Cosine Transform. 
     
     
       5. A method as claimed in claim  1  2 in which the block transform is the a Lapped Orthogonal Transform. 
     
     
       6. A method as claimed in claim  1  2 in which the block transform is the a Fast Fourier Transform. 
     
     
       7. A method as claimed in claim  1  2 further including, at step (f) transmitting, by the coder device, mask information representative of a binary mask which defines the positions of the said selected flagged coefficients. 
     
     
       8. A method as claimed in  claim 7  in which the binary mask defines the positions of the selected said flagged  coefficients within each coefficient block in JPEG zig-zag order. 
     
     
       9. A method as claimed in  claim 7  in which the binary mask is associated with the a mask length code to define the mask end point. 
     
     
       10. A method as claimed in  claim 7  in which the binary mask is associated with a stop-code to define the a mask end point. 
     
     
       11. A method as claimed in  claim 7  in which transmitted mask information is an entropy-coded version of the mask. 
     
     
       12. A method as claimed in  claim 11  in which the transmitted mask information is an arithmetic coded version of the mask. 
     
     
       13. A method as claimed in  claim 11  in which the transmitted mask information is a Huffman coded version of the mask. 
     
     
       14. A method as claimed in  claim 7  in which the transmitted mask information is run length coded. 
     
     
       15. A method as claimed in claim  1  2 in which the a binary mask defines the positions of the selected said flagged coefficients within each coefficient block in JPEG zig-zag order, the binary mask is associated with a mask length code to define the a mask end point, and the mask length code defines the a mask end point zig-zag address. 
     
     
       16. A method as claimed in claim  1  2 in which the a binary mask defines the positions of the selected said flagged coefficients within each coefficient block in JPEG zig-zag order, the binary mask is associated with a mask length code to define the a mask end point, and the mask length code defines the a Manhattan distance from a DC term to the mask end point. 
     
     
       17. A method as claimed in claim  1  2 further including the step of transmitting information representative of a binary off-mask for defining the positions of coefficients whose bits are no longer required to be sent. 
     
     
       18. A coder for encoding images device, comprising the steps of:
 (a) means for dividing an image to be compressed into a plurality of image blocks; 
 (b) means for carrying out a two-dimensional block transform on each block to produce a corresponding plurality of coefficient blocks; 
 (c) means for bitwise digitizing the coefficients within each coefficient block to define a plurality of bit planes for each coefficient block; 
 (d) means for defining a group of one or more consecutive bit planes starting with the most significant bit plane; 
 (e) means fora mechanism configured to selectively flagging thoseflag coefficients which first become significant within the a group of one or more consecutive bit planes of corresponding coefficient blocks of image blocks resulting from a block transform, starting with the most significant bit plane; and  
 (f) means fora mechanism configured to transmittingtransmit information representative of the positions of the said flagged coefficients and transmitting the bits within the a group of the said flagged  coefficients; and, 
 (g) means for repeating steps (d) to (f) one or more times, with each new group starting with the most significant bit plane not previously dealt with, and means for transmitting, at each repeated pass, the bits within the current group of those coefficients which were previously flagged on an earlier pass. 
 
     
     
       19. A coder device as claimed in  claim 18  in which the means for transmitting information representative of the positions of the said selected coefficients mechanism configured to transmit comprise comprises a binary mask means mechanism. 
     
     
       20. A coder device as claimed in  claim 19  including means for transmitting a transmitting mechanism configured to transmit, as synchronized data streams, the coefficient bits and mask information. 
     
     
       21. A video coder/decoder comprising: 
 a coder and an associated decoder, wherein  
 (1) the coder encoding images and comprising the steps of comprises:
 (a) means for dividinga mechanism configured to divide an image to be compressed into a plurality of image blocks; 
 (b) means fora mechanism configured to carryingcarry out a two-dimensional block transform on each block to produce a corresponding plurality of coefficient blocks; 
 (c) means fora mechanism configured to bitwise digitizing thedigitize coefficients withwithin each coefficient block to define a plurality of bit planes for each coefficient block; 
 (d) means for defininga mechanism configured to define a group of one or more consecutive bit planes starting with thea most significant bit plane; 
 (e) means fora mechanism configured to selectively flaggingflag those coefficients which first become significant within the group; 
 (f) means fora mechanism configured to transmittingtransmit information representative of the positions of the said flagged coefficients and for transmitting the bits within the a group of the said flagged coefficients; and, 
 (g) means fora mechanism configured to repeating steps (d) to (f)repeat operation of the mechanism configured to define a group and the mechanism configured to transmit information one or more times, with each new group starting with thea most significant bit plane not previously dealt with, and means for transmitting, at each repeated pass, the bits within the a current group of those coefficients which were previously flagged on an earlier pass, and 
 
 (2) the decoder being arranged to maintain a running record, as transmission between the coder and the decoder proceeds, of the coefficients which are currently significant. 
 
     
     
       22. A method as claimed in claim 2 in which the repeating step is carried out across an entire image to be compressed.