So I've been trying to render a scene in karma, using materialx shaders with maps from substance, and I'm having a weird issue where in Karma CPU and Houdini GL, part of my model is dark compared to the rest of it, however in Karma XPU, it renders correctly (although the overall quality of the render is lesser so using XPU isn't ideal). Is this a common issue, I'm unsure how to start troubleshooting it otherwise? The dark patch is not on the textures at all and the geometries vertex normals look fine. Please ignore the different color background, the windows photo edit tool got rid of the transparancy when I circled it because I couldn't be bothered to open photoshop haha

I've circled the most noticable dark patch here:




Here is the XPU render for comparison:

It seems the depth of field enabled, maybe turning it off then do the depth-of-field effect in post no?

When I render via CPU, I normally put higher Primary sample's values compare to XPU.
So, perhaps increasing Primary and Secondary samples may solve your problem?
(e.g. inserting doubled XPU's Primary Sample's value into CPU's Primary Samples to see noise is disappeared, then trying to reduce it.)

You may check this great tutorial about reducing the sampling noise as well
https://www.youtube.com/watch?v=dgK5i0cex1Q [www.youtube.com]