Noise when image is saved as PNG

[Alex_M]

Has anyone noticed that images saved as PNG's seem to have noise in areas that are practically noiseless?

Check the screenshot below. I've boosted contrast to better see the noise. The surface in VFB has zero noise even at 1600% zoom. Same when the image is saved as 8-bit TIF but for some reason PNG adds a noise pattern. This noise is there even if I render with ridiculous settings like min/max subdivs 4/24 and Noise threshold 0.001. I'm curious to know why the visual difference when both TIF and PNG are lossless formats.

[radoslav.platikanov]

Looks like dithering to me. You can see the effect of it if you render a gradient - you'll have banding artifacts in the VFB but not in the PNG.

[Alex_M]

Thanks for the reply. Why is there no dithering in the TIF file? And which is more correct and better to save as - TIF or PNG output?

[radoslav.platikanov]

Is this saved in 3ds Max? If so, Max itself handles the output, V-Ray directly writes only raw .vrimg and .exr files in this case.
V-Ray doesn't do any dithering when converting to 8-bit colors currently. I suppose this might change in the future.

As for the best format - it shouldn't really matter, the differences here are likely caused by Max, not the formats. In this case I'd prefer the dithered one, banding looks worse than random noise. Another option is to save to a 16-bit format and convert to 8-bit with another software.

[Alex_M]

Yes, this was rendered in Max 2018.4 and saved from the VFB. Maybe Max does something and adds dithering in the PNG. Anyways, thanks for the explanation on the characteristics of the formats. I think I'll continue to save as PNG as it is smaller size and this fine noise may prevent banding from happening when saving as 8-bit PNG as you mentioned.