This invention relates to the transmission of pictorial image data. More particularly, it is concerned with the progressive transmission and reconstruction of coded images in which an approximate image is reconstructed based upon partial information and details are added as additional information becomes available.
The progressive transmission and reconstruction of coded images allows an approximate image based upon partially received information to be constructed to which additional details are added as additional information becomes available. This procedure has various applications in the field of image communications, such as for interactive picture retrieving, variable-rate video conferencing, and the quick display of freeze-frame image transmission. One relatively simple scheme proposed by Knowlton U.S. Pat. No. 4,222,076 issued Sept. 9, 1980, deals with spatial domain data for the progressive transmission of gray-scale pictures. This approach has the advantages of simplicity in implementation and no coding distortion in the final reconstructed image. However, due to the nature of successive picture subdivision introduced by this method, the number of accumulated bits of information increases exponentially with each interation.
Other schemes that deal with transform domain data have been described in articles by Takikawa "Fast Progressive Reconstruction of A Transformed Image," IEEE Trans. Inform. Theory, vol. IT-30, pp. 111-117, January 1984 and Ngan "Image Display Techniques Using the Cosine Transform," IEEE Trans. Acoust., Speech, Signal Processing, vol. ASSP-32, No. 1, pp. 173-177, February 1984. Transform image coding is well known for its compression efficiency. Its nature renders it also suitable for efficient progressive transmission and reconstruction since low frequency transform coefficients contain most of the energy of image signals. Thus, a small subset of the transform coefficients is good enough for reconstructing a rough version of the whole image, while the remainder of the transform coefficients allow the receiver to add details to the initially reconstructed picture as they are received. In one such scheme the transform coefficients of each block of image data are considered as arranged in a square lattice and are sent and received in a zig-zag pattern in order from the large through the small variance values. This scheme is described in an article of Tescher and Cox "An Adaptive Transform Coding Algorithm," ICC Conference Records, pp. 47.20-24, 1976. Although the zig-zag technique provides better compression efficiency than other proposed transform domain schemes, it is desirable to further improve the efficiency with which image data can be transmitted, particularly during the first few iterations.