Visible difference between GPU and CPU renders

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Headroom
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Visible difference between GPU and CPU renders

Post by Headroom » Sat Jul 13, 2019 10:47 am

I rendered this simple test scene to see how GPU Pathtracing would compare to CPU rendering, BIDIR MLT in this case.
My interest was mainly to see evaluate difference between dispersion and caustics.

Both images rendered for 40 minutes.the light source is a HDRI with many small light sources and the material of the twisted ring is the Lead Crystal from the online material DB with couchy B enabled.
The GPU renderer is nobly cleaner and the caustics are clearly visible. Nice!

However, there are dark areas mostly visible in the GPU render in in the left part of the image in the outer edges of the twisted torus. Those are not present in the CPU render. I checked the other CPU render modes as well (just not for 40 minutes) and these dark areas are not present in those render modes either..

Thoughts ?
Attachments
Twist_torus.Scene_circus40minbidirmlt.jpg
BIDIR MLT
Twist_torus.Scene_circus40minpathGPU.jpg
Path GPU

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contegufo
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Re: Visible difference between GPU and CPU renders

Post by contegufo » Sat Jul 13, 2019 7:49 pm

Hi

the second image is the best overall; which gpu did you use?
Mac Mini 2011 - 2,3 GHz Intel Core i5
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fused
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Re: Visible difference between GPU and CPU renders

Post by fused » Sat Jul 13, 2019 8:43 pm

By default the GPU pathtracer uses a path depth of 8. It can lead to darkeing in certain areas, especially with specular materials. You can turn up the depth and the dark spots should get lighter or disappear completely (but it will also render a little slower).

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Headroom
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Re: Visible difference between GPU and CPU renders

Post by Headroom » Sat Jul 13, 2019 11:53 pm

@fused
Thanks for the tip. increasing Path Depth does indeed eliminate these dark areas. it's just not a setting available in the Blendigo UI (or I simply have to look harder ):

@Contefugo:
All of these images have rendered for 40 minutes on a 2017 MacBook Pro with a Radeon Pro 560.
This is not usually how long you want to wait for a GPU render, but these serve as experiments for me to determine the best render set up for my situation. At this time I think I'll invest in a eGPU to complement my notebook.
Twist_torus.Scene_circus40minpathGPU_pathdepth16.jpg
GPU Path depth 16
Twist_torus.Scene_circus40minpathGPU_pathdepth24.jpg
GPU Path depth 24

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pixie
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Re: Visible difference between GPU and CPU renders

Post by pixie » Sun Jul 14, 2019 12:14 am

Well, the thing is, specular materials and caustics always tend to take a lot more to denoise then simpler materials.

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Zalevskiy
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Re: Visible difference between GPU and CPU renders

Post by Zalevskiy » Sun Jul 14, 2019 2:56 am

wow I just found out about this option, why not add to faq? to make new indigo users understand

popular render engines have video instructions, this will all help make friends with this wonderful render engine

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