Lossless compression on PNG

[proyb6]

After compression from PNG to JP2, does it produce 100% exact details as PNG? From what I have tried to compare lossless JP2 produces more artificial (Affinity Photo with Contrast Negate filter - see those orange) while Lossless JPEG produces less artificial.

[boxerab]

It should be 100% identical. Can you post a link to a PNG image whose JP2 compressed version doesn't match ? Keep in mind that if there is an ICC profile in the PNG, then it will not be stored in the JP2 file, so this might explain visual differences.

[proyb6]

My test isn't standard and may be flaw, how do you compare both images in your process?

[boxerab]

I use Graphics Magick:

http://www.graphicsmagick.org/

gm compare FOO.png BAR.png -metric PSNR

You should get 1.#J for all values of PSNR if the files are truly equal.

[proyb6]

Do you mean .png and j2k? gm compare FOO.png BAR.j2k -metric PSNR

[comstock]

If you apply the same ICC profile found in your source image to your output JP2 and overlay them in Photoshop or, I assume, GIMP, the pixel values should be identical, provided you used losselss JP2 compression.

Here is blog post on how to compare images: http://bit.ly/2EPOroZ

[boxerab]

@proyb6 graphics magick doesn't have the best support for jpeg 2000, as it uses the obsolete Jasper library. The best approach in my experience is to do a round-trip encode/decode: take a PNG, encode to J2K, then decode yourself back to PNG or TIF, and compare with original using Graphics Magick.

Why don't you link to the PNG file ? That would be the easiest way of sorting this out.

[proyb6]

@boxerab I have deleted my test sometime ago and will retest tomorrow since it's quite last in Asia. Will take note of your advice. Thanks!