JPEG2000 and Image Compression

[goatslayer]

Hi,

Does anyone know if setting the compression of an image to a PSNR, i.e. compress the image to 70 PSNR whether it is lossy compression?

I have seen contradictory write-ups on this. To my mind a lossless image has it's own PSNR. If it is set to 70 PSNR then it could potentially have to lose data to reach that PSNR.

I could be wrong, might have misunderstood PSNR.

Any help is appreciated.

Thanks,

rs

[olivthill2]

I agree with you.

In addition:

- When you don't use integer coefficients for your wavelet transformations, you are losing information, see http://en.wikipedia.org/wiki/JPEG_2000

- PSNR is an average value for a picture. You can have a higher PSNR for a color than for another color, or for a region of interest than for the rest of the picture.

- PSNR depends on the number of bits per pixel for a color. Usually a color is coded with a number between 0 and 255, so that's 8 bits per pixel and per color. But you can have higher "rates", and the PSNR will be different. See http://www.dsprelated.com/groups/imagedsp/show/192.php

[goatslayer]

Thanks olivthill2. Very useful.

I know a group of indivudals who have listed their compression as lossless but to 70db PSNR. I will have to find a way to break it to them that they have been using lossy compression.

[olivthill2]

Maybe, you can test with a very big black and white picture, having just one pixel in blue, and you would check if the blue pixel and its 8 neighbors are still the same.