REGARDING IMAGE COMPRESSION .

I would like to know the image compression .. these are the my basic doubts which may be very basic .. but i can not make clear to it.. so make me clear ..

1) how to calculate compression ratio? output image size/input image size .. if so how to be the compression ratio.. in some websites they have given minimum o to maximum 1.. in some websites they have given like 24:1 .. justification regarding this

2) whether .jpg image can be used to compression input .. since it is already compressed by JPEG sytem can we get significant parametrics like PSNR and ENTROY

Answer 1

[Walter Roberson]

1) The 24:1 figure is just the inverse of the ratio. For example, if the output size is 1/3 the size of the input, 0.3333, then the inverse is 3:1, which corresponds to the idea that every 3 bits of input were (on average) compressed to 1 bit of output.

2) Every non-empty input file can be compressed by use of a suitable algorithm -- an algorithm that might only have gains for that one particular file. The question then becomes not whether .jpg files can be used as compression input (they can): the question becomes whether you can find an algorithm that compresses the subset of .jpg files that are interesting and useful for your particular purpose.