| 摘要 |
A DCT based lossy compresion method for compressing a digitised image composed of a matrix of image samples to provide a compressed image which satisfies a predefined bit budget. The digitised image is first sub-divided into blocks (e.g. of size 8X8 pixels). A discrete cosine transform (DCT) comprising a set of DCT coefficients is then derived for each block. A quantisation table is selected from a set of quantisation tables and, using the selected table, the coefficients of each DCT are quantised. A zero-value index, corresponding to the average number of zero value quantised DCT coefficients per DCT, is determined. A predicted zero-value index is calculated using said predefined bit budget and a quantisation table selected from said set of tables using the determined index and the predicted index. Using that selected table, the unquantised coefficients of the DCTs are requantised and the requantised coefficients compressed using run-length encoding and Huffman encoding. |
