Light Freckles on Transparent Materials

jennieelectrix · July 02, 2019, 01:20:30 AM

I've been using KeyShot for a few months now and seem to have worked out most of my issues, but this one I can't seem to get around.
I'm trying to render a product which has a grey transparent window on the front (important because other products I've rendered in the same scene that didn't have the window, didn't have this problem).
When I do the result is lots of light freckles all over (see screenshots, one is with caustics switched on and one is without).
I've tried everything I can think of with this, I've been in contact with support who suggested I increase the samples to 128 (they were previously at 40) however I'm now 30 minutes into rendering this simple 36 frame animation and it is on 1% progress. I have around 10 - 15 of these to render full 360 animations for, so I don't have the time to sit and wait for it to render at this rate!
Any suggestions that won't mean waiting days for one scene to render would be massively appreciated with this.

DriesV · July 02, 2019, 02:24:06 AM

Hi Jennie,

Are you using Custom Control as the quality mode for rendering? I have a suspicion that you are.

These "light freckles" (I love that term, btw... Sounds better than "splotches") can occur when using Custom Control to render transparent surfaces. They are especially noticeable with transparent parts, covering diffuse surfaces in close proximity, as appears to be the case in your images.

Increasing Samples will not do much for solving the issue.
The only way to avoid these freckles with Custom Control is to disable 'Global Illumination Cache' (the third checkbox in the Custom Control settings).
This will get rid of the spots, but might introduce more fine grain noise in diffuse surfaces below the transparent cover. You can increase the Samples and/or Global Illumination quality to reduce the noise. Be careful when adjusting the Global Illumination slider! Increasing this value beyond 2 will cause ballooning render times.

Alternatively, you can also render with the Maximum Samples or Time quality modes. Those modes will never show the freckle artifacts with transparent surfaces.

Let me know how this works for you.

Dries

DriesV · July 02, 2019, 02:27:14 AM

Also, 128 samples is a lot when using Custom Control.
In most scenarios, 8 to 16 samples would suffice. You can always increase the material samples of e.g. rough metals or glass separately. That will be much more efficient to render.

Dries

DriesV · July 02, 2019, 03:54:55 AM

Would you be able to share the scene? There are a few things I would like to test.

You can send data securely via our WeTransfer. Send to dries-at-luxion-dot-com.

Dries

jennieelectrix · July 02, 2019, 04:38:17 AM

Hi Dries,

Thanks for getting back to me. I've tried what you said (using Maximum Samples instead of Custom) and set it to 16, however even though the light freckles are gone, it's now quite grainy (see below).

I'll try and get the scene over to you via WeTransfer now.

Thanks,
Jennie

DriesV · July 02, 2019, 04:41:47 AM

Hi Jennie,

Have you tried https://keyshot.wetransfer.com/?
You should be able to upload 20 GB. Make sure to enter dries-at-luxion-dot-com as recipient.

Dries

DriesV · July 02, 2019, 04:45:33 AM

Quote from: jennieelectrix on July 02, 2019, 04:38:17 AM
Hi Dries,

Thanks for getting back to me. I've tried what you said (using Maximum Samples instead of Custom) and set it to 16, however even though the light freckles are gone, it's now quite grainy (see below).

I'll try and get the scene over to you via WeTransfer now.

Thanks,
Jennie

With Maximum Samples, you need significantly more samples than with Custom Control. The rendering algorithms are quite different for those two modes.

Custom Control with Global Illumination Cache unchecked should be less grainy though.

Dries

mattjgerard · July 02, 2019, 06:52:53 AM

Just to tag on to this as I'm very curious, is there a table that shows when we would be best to use the different rendering modes? When would it be better to use Custom Control vs Max Samples, etc? It seems that it depends on the materials in the scene, might be a good reference for chasing down these sort of gremlins. I never would think to change the render mode in order to troubleshoot.

jennieelectrix · July 02, 2019, 07:03:46 AM

Quote from: mattjgerard on July 02, 2019, 06:52:53 AM
Just to tag on to this as I'm very curious, is there a table that shows when we would be best to use the different rendering modes? When would it be better to use Custom Control vs Max Samples, etc? It seems that it depends on the materials in the scene, might be a good reference for chasing down these sort of gremlins. I never would think to change the render mode in order to troubleshoot.

This is a really good idea actually - especially for people like me that don't know what they're doing!