Matte Object with Shadows / Alpha channel is denoised differently from RGB channel - V-Ray 5.2.3

[Bowen Studios]

Hello,

I am struggling to find a solution for rendering shadows on top of footage while keeping the shadow quality consistent from rendered geometry to the alpha channel.

I've attached two images to show the difference between checking the "Denoise Alpha" box for the VRayDenoiser render element. The images are screenshots from the VFB, but the same results show up if the RGB channel is saved as a PNG and overlayed in post.

Image 1 shows the result of the shadows when the box is checked. The shadows on the rendered geometry are sharp, but the same shadow in the alpha channel becomes very blurry and detail is completely lost when it is denoised.



Image 2 shows the result when the box is unchecked. The shadows are sharp but noisy, leading to an obvious quality difference between the shadow on the rendered geometry and the shadow in the alpha channel.



Image 3 shows the VRay Properties for the shadow catcher object. It is a matte object, affect alpha is checked, shadows checked, affect alpha checked.



The denoiser settings are set to "mild", I'm rendering with BF/LC. Minimum subdivs: 1, Max: 100, noise threshold 0.01, Sharp Quadratic Image filter, max Transp. levels: 20.

Is there any way to get the shadow to be denoised exactly the same as the RGB channel? I like the sharp shadows on the geometry, but I don't like how the shadow gets completely blurred when it gets denoised.

Thanks!

[caypiranha]

try to increase the Subdivs multiplier on the shadow catching Objects. Maybe there is a more elegant solution (then based on different render settings) but it solves it purpose definitely - also without gaining too much additional rendertime

btw: are you using Vrayplane?

[Bowen Studios]

try to increase the Subdivs multiplier on the shadow catching Objects. Maybe there is a more elegant solution (then based on different render settings) but it solves it purpose definitely - also without gaining too much additional rendertime

btw: are you using Vrayplane?

Thanks for the suggestion!

I first tried increasing the multiplier to 5, no change. Then I tried 20, still no change. I don't think that parameter is having an effect.

FWIW, the Alpha channel shows the rendered geometry as pure white, whereas the shadow falling on the shadow catcher shows up in the Alpha channel as off-white (regardless of denoiser settings):


And to answer your question, no, the shadow catcher is not a VrayPlane - it's a mesh object.

Thanks again! I also hope there is an elegant solution.

[seandunderdale]

Rendering CPU or GPU?

[Bowen Studios]

Rendering CPU or GPU?

This is all CPU rendering. Have not tried GPU, but would be fruitless because this project cannot be rendered with GPU.

[hermit.crab]

I first tried increasing the multiplier to 5, no change. Then I tried 20, still no change. I don't think that parameter is having an effect.

Odd. It should help.

FWIW, the Alpha channel shows the rendered geometry as pure white, whereas the shadow falling on the shadow catcher shows up in the Alpha channel as off-white (regardless of denoiser settings):

That's normal. Otherwise, there wouldn't be any blending with the background.

I guess you can try lowering the Strength/Denoiser Radius, so as to retain the fine details. Also, increasing the overall resolution of the image and the image sampler settings would certainly help.