[BUG] 8Bit PNG output Gamma issue

[Zom-B]

Hey Guys, I just did a recent test with EXR vs PNG output.

Doing that I recognized that Indigos PNG Output seems is different to EXR @ gamma 0.454545454.. => 8bit

Click for bigger view!

It seems that Indigo does gamma correction of the image after converting to 8bit!
Doing so in Photoshop I get a 99.5% similar result to the PNG output!

Isn't it correct and "lossless" to do so in the 32bit image and convert then to 8bit?!

[StompinTom]

Hey Guys, I just did a recent test with EXR vs PNG output.

Doing that I recognized that Indigos PNG Output seems is different to EXR @ gamma 0.454545454.. => 8bit

PNG-vs-EXR.gif

It seems that Indigo does gamma correction of the image after converting to 8bit!
Doing so in Photoshop I get a 99.5% similar result to the PNG output!

Isn't it correct and "lossless" to do so in the 32bit image and convert then to 8bit?!

Could that perhaps be the source of some of the banding problems we've experienced as well?

[Zom-B]

Could that perhaps be the source of some of the banding problems we've experienced as well?

Something that could be checked quite easy
Any Thread here about this, maybe with some example file to test?

[FakeShamus]

here are some tests I made to show the banding I've been dealing with. just flat sky with camera tonemapping (color is super-exaggerated, of course, to highlight the problem).

I tested converting the gamma of the tonemapped exr in photoshop both before and after downgrading the file to 8-bit rgb and you get similar banding either way. (2nd image)

the main thing that seems to be missing in Indigo is a dithering function. If you convert to 8 bit from 32 in photoshop, using dithering, it eliminates the problem (3rd image). you get a slightly rougher grain structure if you convert to 8bit first and then do the gamma correction, but still, no banding.