[RESOLVED]KS9 / Noise in reflection on GPU rendering

Started by Andrew_G, November 18, 2019, 02:50:36 AM

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Sorry to say that, but I`m slowly starting to regret that I upgraded to KS9. It had to be better, but I come from one bug to another...

This time I have some strange issues with noise on glass reflections. When rendering on CPU I have clean image with 80-100 samples. When I switch to GPU I cant clean it until 1500-2000. I`m attaching images with comparision. The result is I have to stay with CPU rendering...

What's worse I hit another bug rendering this scene. When I set it up I rendered it using CPU - just for testing results. I switched for GPU and found all this noise. But when I unclick GPU button and returned to CPU rendering, my scene started to render very very slow. When I hit render button preparation of the scene takes about minute or longer (it definetely should take about few seconds) and rendering is super slow... Restarting KS, computer didnt helped at all. Unfortunately I cant attach this scene, as it is under NDA.

Not to mention that there is difference in refraction between CPU and GPU and some color differences. Turning denoise on GPU makes thing only worse. The image look awful with this option turned on.

Jesper Mosegaard

Hi Andrew

GPU and CPU sample count cannot be compared. Even with an equal-time comparison there are definitely situations where the CPU "wins" and vice versa. I'm considering giving the sample count on the GPU a different name. Camples and Gamples perhaps? ;)

There are definitely situations where denoise does not produce a nice image, especially for very low sample count or high noise - that is why it can be turned on or off as an image style. Given more samples it should converge to the right result though. In many cases though, you get an almost magic fix for hard to remove noise.

It has been our goal that GPU and CPU produce the same appearance given enough time to converge. So if you have materials that look different in color intensity or refraction please share them with us - or indicate which material type and texture setup. You could perhaps drag the material to a simpler geometry that can be shared?

We will try and replicate the slow scene rendering - this is not something we have seen before. Is the slowdown also in the view-port, or is this "limited" to rendering through the rendering dialog?

Trust that we are working hard to fix many of the initial issues that are found in GPU rendering in the 9.1 release.

Regards. Jesper


Hi Jesper,

Thank you for your anwser.

I get that samples in cpu and in gpu modes are not the same thing. What I`m saying is that in this case it looks like a bug, that engine cant smooth plain reflections even with high number of samples. I have this issue on almost every scene with glass and water materials. Also, I`m attaching another comparision of how it looks with 32 CPU samples and 1500 GPU samples. In this case scenario gpu looks just useless...

The fact that IOR looks different in this two image makes the whole thing worse. I know that I may look crazy, but there is a little difference in bottle color as well.

Yes, I know that such differences may occur, but on promo materials it looked way better. One click, and whole scene went way faster without such alterations... If this helps I`m attaching this scene.

I cant recreate this CPU rendering issue that I mentioned before. In this scene everything looks fine. The slow CPU rendering was only in rendering dialog. I suspect that this bug was quite simple. I set samples to 128 but in fact it was rendering way higher samples number - sorry, that I cant say more.


Hi Andrew,

We are looking at the issues you reported with GPU Mode, regarding the noise and the different appearance of refractions. Thank you for supplying the KeyShot scene. This is very helpful to troubleshoot (and hopefully address) the issues.



Hi Andrew,

We have fixed the reported issues.
Attached is a GPU rendering at 1500 samples.

The fix for the specular noise on transparent materials introduces more noise in refractions though, as can be seen for the "tube fitting" part.

These fixes will be part of the KeyShot 9.1 update early 2020.