Matte objects being affected by GI ?? And appearing in the Alpha

[pablo_rohtau]

How is vray doing a matte object?
In my renders it seems the alpha is wrong, the objects set as matte in the ROP are catching the secondary lighting from the GI and they also have alpha.
I was expecting not being affected by the lighting, not appearing in the alpha and just culling other objects.
I have tried adding the objects as Force Matte in the ROP, Mantra style. And also adding the Vray properties to the object and setting them to be matte, I played with all the setting there and I always got the matte object affected by the GI and appearing in the Alpha.

As a side note, I discovered that to render volumes correctly with matte objects I have to render them as Volume Geometry set in the Volume Grid shader

Thanks

[david.anastacio]

How is vray doing a matte object?
In my renders it seems the alpha is wrong, the objects set as matte in the ROP are catching the secondary lighting from the GI and they also have alpha.
I was expecting not being affected by the lighting, not appearing in the alpha and just culling other objects.
I have tried adding the objects as Force Matte in the ROP, Mantra style. And also adding the Vray properties to the object and setting them to be matte, I played with all the setting there and I always got the matte object affected by the GI and appearing in the Alpha.

As a side note, I discovered that to render volumes correctly with matte objects I have to render them as Volume Geometry set in the Volume Grid shader

Thanks

Hey pablo_rohtau,

Matte objects should be affected by everything, that's normal, it only affects camera rays.
As an example, when you have a mountain and a terrain and you decide to render the mountain separate, when you set the mountain as matte on the terrain render and vice-versa for the mountain render, and you don't want the mountain to stop affecting the terrain when it's set to matte.

Hard to say why you still have alpha on your matte, the matte works correctly here, the matte object even has reflections in the shader and nothings shows up in the camera rays:



You could try doing a matte through object properties to see if works better for you. In addition to set it to matte and alpha -1, you also need to tell it to don't affect reflection gi, etc.



Best,
David.

[pablo_rohtau]

Thanks David.
I found the problem.
Someone enable Aerial Perspective in this scene, and by default it affects primary rays, which seems to affect everything, I assume Aerial Perspective is kind of a postprocess that is run after the shading process, so it doesn't know anything about a matte object.
So this is solved.
Now I just to figure out how to render the shadow casted on the ground from my smoke elements as a separate layer.
Keep learning vray.
Thanks guys!

[david.anastacio]

Hey again,

That's actually really odd, we ran into that issue like half a year ago, and reported it. If I remember it got solved, are you using an old build? Is it V-Ray 4 or 5?

Now I just to figure out how to render the shadow casted on the ground from my smoke elements as a separate layer

Smoke elements as phantom, and use a shadow element to extract the shadow of your ground. Alternatively you can set your ground as matte with object properties and use the"matte surface", "shadows" and "affect alpha" options, this way your shadows will be in the alpha of you matte.
Also, if your ground is casting shadows on itself, and you don't want those, just disable the "cast shadows" for your ground (object properties, visibility options).

Best,
David.

[pablo_rohtau]

Hi David.
Im using vray 5.00.50.

Yeah my first thouth was using smokes elements as phantom, like in Mantra, but didn't work. Probably I didn't set the shadow elemenet correctly. By shadow element I assume you are talking about a shadow channel in the AOVs.
But I has more success manually setting the matte in the objects properties and then create a matteShadow AOV, and that worked, is also handy I can have all the shadows casted by my elements onto the mattes in a separate channel in the EXR, is easier for compers.
Thanks!

[david.anastacio]

Just tested in our build here. No issues with matte objects and aerial perspective. uffff

[pablo_rohtau]

Thanks for looking at it David.
Good to know that at least is a known issue.
Now we know what the problem is and how to avoid it.
Thanks for your help!