Method of compressing digital images

A method for compressing a digital image comprising a matrix of elements, each element comprising a plurality of digital components of different types for representing a pixel, the method comprising:

• determining at least one energy measure of the digital image;
• estimating a gain factor as a function of the at least one energy measure by
• estimating a first number of bits required to encode the AC coefficients of all the blocks using the quantization tables scaled by a pre-set factor as a first function of the at least one energy measure,
• calculating a second number of bits required to encode the DC coefficients of all the blocks using the quantization tables scaled by the pre-set factor,
• estimating a basic compression factor provided by the quantization tables scaled by the pre-set factor according to the first number of bits and the second number of bits, and
• estimating the gain factor as a second function of the basic compression factor;
• splitting the digital image into a plurality of blocks, and calculating for each block a group of discrete cosine transform (DCT) coefficients for the digital components of different types, each group of DCT coefficients comprising at least one direct current (DC) coefficient and a plurality of alternating current (AC) coefficients;
• and
• quantizing the DCT coefficients for each block using a corresponding quantization table scaled by the gain factor for determining a target compression factor;
• wherein determining the at least one energy measure is performed in a pixel domain using the plurality of digital components.

A device for compressing a digital image comprising a matrix of elements, each element comprising a plurality of digital components of different types for representing a pixel, the device comprising:

• energy means for determining at least one energy measure of the digital image;
• estimation means for estimating a gain factor as a function of the at least one energy measure, the function being determined experimentally according to the target compression factor;
• discrete cosine transform (DCT) means for splitting the digital image into a plurality of blocks, and calculating for each block a group of coefficients for the digital components of different types, each group of DCT coefficients comprising at least one direct current (DC) coefficient and a plurality of alternating current (AC) coefficients;
• quantization means connected to said DCT means for quantizing the DCT coefficients for each block using a corresponding quantization table scaled by the gain factor for achieving a target compression factor;
• said energy means determining the at least one energy measure in a pixel domain using the plurality of digital components;
• wherein said estimation means is for
• estimating a first number of bits required to encode the AC coefficients of all the blocks using the quantization tables scaled by a pre-set factor as a first function of the at least one energy measure, the first function being determined experimentally according to the target compression factor,
• calculating a second number of bits required to encode the DC coefficients of all the blocks using the quantization tables scaled by the pre-set factor,
• estimating a basic compression factor provided by the quantization tables scaled by the pre-set factor according to the first number of bits and the second number of bits, and
• estimating the gain factor as a second function of the basic compression factor, the second function being determined experimentally according to the target compression factor.

A digital still camera comprising:

• an image acquisition unit for transmitting light corresponding to an image of scene;
• a sensor unit connected to said image acquisition unit for providing a digital image of scene, the digital image comprising a matrix of elements, each element comprising a plurality of digital components of different types for representing a pixel;
• and
• a control device for compressing the digital image and comprising
• a discrete cosine transform (DCT) circuit for splitting the digital image into a plurality of blocks, and calculating for each block a group of discrete cosine transform (DCT) coefficients for the digital components of different types, each group of DCT coefficients comprising at least one DC coefficient and a plurality of AC coefficients,
• a quantization circuit connected to said DCT circuit for quantizing the DCT coefficients for each block using a corresponding quantization table scaled by a gain factor for achieving a target compression factor,
• an energy circuit for determining at least one energy measure of the digital image, and
• a processor for estimating the gain factor as a function of the at least one energy measure, the function being determined experimentally according to the target compression factor,
• said processor
• estimating a first number of bits required to encode the AC coefficients of all the blocks using the quantization tables scaled by a pre-set factor as a first function of the at least one energy measure, the first function being determined experimentally according to the target compression factor;
• calculating a second number of bits required to encode the DC coefficients of all the blocks using quantization tables scaled by the pre-set factor;
• estimating a basic compression factor provided by the quantization tables scaled by the pro-set factor according to the first number of bits and the second number of bits;
• and
• estimating the gain factor as a second function of the basic compression factor, the second function being determined experimentally according to the target compression factor.