Apparatus and methods for re-compression having a monotonic relationship between extent of compression and quality of compressed image

A computerized method for generating a quantization matrix usable for independent disjoint block-level recompression of an image, the image generated by independent disjoint block level compression of a precursor image using at least one first quantization matrix, the method comprising:

• providing a second quantization matrix;
• updating, using a processor, the second quantization matrix to provide a monotonic relationship between a compression extent and a compression quality when performing independent disjoint block-level recompression of the image using the second quantization matrix, including, for at least one (row, column) position, performing the following, giving rise to the quantization matrix:
• calculating a ratio between values located at the position in the first quantization matrix and in the second quantization matrix;
• calculating a rounding of the ratio;
• calculating a rounding error, being a difference between the ratio and the rounding;
• and
• if the rounding error fulfills an unacceptability criterion, decreasing, using the processor, the value at the position in the second quantization matrix.