Abstract

A method of compressing image data to achieve a desired compression ratio corresponding to a desired file size. A target compression ratio (the ratio of uncompressed image data to compressed image data) is specified. A subset of the image data is selected. A first quantization parameter is selected, the subset of the image data is compressed using that quantization parameter, and the resulting compression ratio is calculated. A second quantization parameter is then selected, the subset of the image data is compressed using the second quantization parameter, and the resulting compression ratio is calculated. A target quantization parameter corresponding to the target compression ratio is calculated by interpolating between the first quantization parameter and the second quantization parameter and the corresponding compression ratios. The target quantization parameter is applied to the entire set of image data to compress the image data at approximately the target compression ratio. As a result, the size of the file containing the compressed data is approximately the same as the target file size. The present invention allows rate control in less computation time than it takes to compress the image; the time to estimate the target quantization parameter can be done in less computation time than needed to compress the full image.