System for variable quantization in jpeg for compound documents

An image compression system capable of compressing text and pictures, comprising:

• a discrete cosine transformer capable of receiving an image in the form of a plurality of blocks of digital pixel data of the text and pictures, said discrete cosine transformer for applying a discrete cosine transform to said plurality of blocks of digital pixel data to provide a plurality of blocks of discrete cosine transform coefficients representative of the frequency of changes in said digital pixel data whereby a high frequency of changes indicates text and a low frequency of changes indicates pictures;
• a variable quantization subsystem operatively connected to said discrete cosine transformer responsive to each of said plurality of blocks of discrete cosine transform coefficients to determine the activity of said digital pixel data by computing an activity metric for each of said plurality of blocks of discrete cosine transform coefficients whereby a high frequency of changes has a high activity metric and a low frequency of changes has a low activity metric;
• quantization tables for providing a predetermined lossy quantization factor for each activity metric whereby a high activity metric has a low lossy quantization factor and a low activity metric has a high lossy quantization factor;
• a quantizer connected to said discrete cosine transformer, said variable quantization subsystem, and said quantization tables for applying said predetermined lossy quantization factor to each of said plurality of blocks of discrete cosine transform coefficients based on said activity metric for each of said plurality of blocks of discrete cosine transform coefficients to provide a plurality of blocks of quantized data;
• Huffman tables for providing a predetermined lossless entropy encoding factor for each of said predetermined of blocks of discrete cosine transform coefficients;
• an entropy coder connected to said Huffman tables and said quantizer to apply a lossless entropy encoding factor to each of said plurality of blocks of quantized data to output a plurality of blocks of compressed entropy coded data in a digital file with the lossy quantization factor and the lossless entropy encoding factor applied to each of said plurality of blocks of compressed entropy coded data.

An image compression system capable of compressing text and pictures, comprising:

• a discrete cosine transformer capable of receiving an image in the form of a plurality of blocks of digital pixel data of the text and pictures, said discrete cosine transformer for applying a discrete cosine transform to said plurality of blocks of digital pixel data to provide a plurality of blocks of discrete cosine transform coefficients representative of the frequency of changes in said digital pixel data whereby a high frequency of changes indicates text and a low frequency of changes indicates pictures;
• a variable quantization subsystem operatively connected to said discrete cosine transformer responsive to each of said plurality of blocks of discrete cosine transform coefficients including:
• subsystem quantization tables for providing a predetermined lossy quantization factor for predetermined of blocks of discrete cosine transform coefficients;
• a quantizer connected to said discrete cosine transformer and said subsystem quantization tables for applying a lossy quantization factor from said quantization tables to said each of said plurality of blocks of discrete cosine transform coefficients to provide a plurality of quantized data blocks;
• an activity computer connected to said quantizer for calculating an activity metric for each of said quantized data blocks with high frequency data having a high activity metric and low frequency data having a low activity metric;
• a scaling computer for determining predetermined relationships between said activity metric and quantization scale for said quantization table;
• quantization tables for providing a predetermined lossy quantization factor for each of said plurality of discrete cosine transform coefficients;
• a multiplying junction connected to said scale computer and said quantization tables for scaling said predetermined lossy quantization factor by calculated amounts and providing predetermined, scaled, lossy quantization factors;
• a quantizer connected to said discrete cosine transformer and said multiplying junction for applying said predetermined, scaled, lossy quantization factor to each of said plurality of blocks of discrete cosine transform coefficients to provide a plurality of blocks of quantized data;
• Huffman tables for providing a predetermined lossless entropy encoding factor for each of said predetermined of blocks of discrete cosine transform coefficients;
• an entropy coder connected to said Huffman tables and said quantizer to apply a lossless entropy encoding factor to each of said plurality of blocks of quantized data to output a plurality of blocks of compressed entropy coded data in a digital file with the lossy quantization factor and the lossless entropy encoding factor applied to each of said plurality of blocks of compressed entropy coded data.