Matte shadow

[cledus_]

Hi all!

Is there a easy way to do this. I added a white material to the ground plane and did a render. It came out nice and white (no_matte.png). But if I use a VRayMtlWrapper and add the white material to the base material slot I get a very different result (matte.png). I will attach an image of my settings in the wrapper.

I would be so happy if someone could help me!

[Donald2B]

It's because you have the Matte option checked, therefore the object doesn't render but shows the background color instead.

Taken from the VRay help file:
Matte surface - makes the material appear as a matte material, which shows the background, instead of the base material, when viewed directly. Note that the base material is still used for things like GI, caustics, reflections etc.

[cledus_]

But the background color is white. So it doesen´t show the background. The problem is that it receives GI. If I uncheck receive GI I see the background but then the shadows get all messed up. What settings is to be used if I wan't the image with the matte material to look like the one without the matte, but I wan't to catch the shadows and the alpha?

[Donald2B]

Can you post your file?

I can take a look to see what's going on. Something is not right, just not sure what.

[cledus_]

Here is the file: test.zip

Thanks for taking the time!

[Donald2B]

Okay, there are a couple of problems here, one I am not sure how to correct in one pass.

The material used on the cylinders needs to have the ground plan visible to render correctly, otherwise you get some incorrect shading on the edges.
The Fix:
- remove matte material and assign regular material
- uncheck visible to camera

This will allow the cylinders to render correctly when compared to the original full render.

The other problem is then the shadows. Here is where I am stumped. You can create another ground object to be just a shadow catcher, but the shadows will be a little darker than the original image. I would typically render the shadows in a separate pass so I have full control over them in post.
Hack Fix:
- Adjust the alpha contribution of the shadows to less than one (Brightness number in the direct light rollout in Matte properties). This will adjust the intensity of the shadows created in the alpha. Then compare that with the original render and make the appropriate adjustments.

[cledus_]

My problem is, in this particular case I can't do post on the final render. That's why I just wan't it to work directly . The problem isn't the darkness of the shadow.The problem is that the plane that catches the shadow doesn't catch just the shadow, it gets gray allover and that affects the alpha channel.

[svetlozar.draganov]

Matte objects catches only shadows - the alpha is not what you are expecting because it also catches the shadows from VrayDomeLight.
VrayDomeLight is simulated in the scene by many small VrayLights and all these lights cast shadows over the ground-object and these shadows are cached by the Matte-object.
You have to either disable VrayDomeLight or disable its shadows or affect diffuse in order to get the result you are looking after.

[cledus_]

Yes that's it. Thank you both, now I know how it works!