VOP shadow colour problem

[seandunderdale]

Hi guys,

Im having trouble with getting a shadow to render correctly using just a dome light. I have a bright sun in the HDRI to give me direct shadows.

When I render the car shadow with the car vis off, and the floor visible, I get blue in the ambient areas, and warm sun tones where you would expect. Also the GI and contact shadows look solid and filled out.

However, when I render with a VOP on the floor set to "matte surface" "-1 alpha" "Shadows on" "affect alpha" on too, the shadow saves out without any of the colour the beauty had, and also looks washed out and doesnt look anywhere as dark.

Any thoughts?

[vlado]

Which V-Ray version are you using?

Best regards,
Vlado

[seandunderdale]

Hi Vlado,

We are on 3.5 but its been present on other versions.

[vlado]

Ok, good. Then I will need a scene if possible as things seem to work fine on my end.

Best regards,
Vlado

[seandunderdale]

Ive made a test scene that creates the result on my machine. Ive also saved out another jpg to show the results of the render layers in Nuke.

From what I can tell, the VOP alpha shadow has less density than the beauty, and its also harder to get a result as smooth as the beauty in terms of grain.

Vray_Test.zip

[Zdravko.Keremidchiev]

In you scene the brightens difference between mate and RGB is not so great as the images you shared in you first post. Here is the comparison:
The tint inconsistency is due to the fact that Nuke can't read our colored Alpha, it sees it grayscaled.
Therefore, could you please share the scene from your original scene so we could investigate further.

[Morbid Angel]

no you need to use a merge expression, such as this

rgb_alpha.zip

[seandunderdale]

Thanks Morbid Angel. We tried the script at the studio with a clean background and a VOP shadow render and still couldnt re-create the beauty.

We can rebuild a new scene to share if there are any options you feel would be worth pursuing but we are at a loss for how to get a floating shadow layer with correct density and ambient shadow colour contribution. We also find it renders very slow and with a lot of grain, even on a simple scene with a plane and a sphere.

[vlado]

The most accurate way to do it is described here (the part about compositing basics, you can skip the rest):
https://docs.chaosgroup.com/display/...positingBasics

Best regards,
Vlado

[seandunderdale]

Thanks for the responses guys. I do wonder if we've gotten wires crossed somewhere along the way though.

Our intention is the use the objectProperties to create a "floating" shadow with embedded alpha AND colour. Settings for this are matteSurface, -1 alpha, shadows, affect alpha. This is so when we deliver to client we can supply car element, shadow element (which doesnt need a blending mode) and background.

The problem we have is that trying to make a shadow this way with a domelight makes greyscale images, as well as them being long to render and very grainy.

Ive created a Maya scene using 2017 and Vray 3.52. We have also created a Nuke script showing the issue, and trying to create a fix using Morbid Angel's fix.

If we are using it wrong, or its not meant to work the way we are trying then let me know. Im happy to find workarounds. We just now want to find out why the shadow created this way doesn't match our beauty at all.

Just to add, during our tests we've found this problem to be related to using and HDRI for lighting, but using Sun & Sky seems to make a shadow that contains ambient / sky colour, and has transparency.

Heres the link to the zip

[seandunderdale]

Just following up to see if anyone has any further info on this. If the download or supplied files don't work or aren't sufficient, let me know.