matte-shadow with dome light hdri

[OPEN_RANGE]

In the screen grab I have a simple plane and have set it's vray properties for matte-shadow as shown. Also shown is the resulting alpha channel from rendering. The scene is lit with a vray dome light using a PG HDRI Sky map. The alpah incorrectly contains a faint but noticeable amount of the entire plane (you can see that if you look closely). It should only show the Box and it's shadow.

If I light the scene with a vray sun the alpha is correct.

I can't figure what I have set wrong in the dome light with hdri that is causing the incorrect matte-shadow result.

Any help much appreciated.

[kosso_olli]

The result is to be expected with HDRI lighting, because it is casting soft shadow in all directions depending on the HDRI used.
The plane is too small, therefore I suggest the usage of VrayPlane from the V-Ray toolbar as ground plane. The problem should be gone then.

[OPEN_RANGE]

Thank you Oliver. I did not know about this issue with matte-shadow objects not working with hdri lighting. I did have the plane vray properties set to not receive or generate gi and also tried max object properties set to not cast or receive shadows. Made no difference.

What I showed was a rough test. What I am dealing with is a small building on a slope. So, I wanted to have the shadow cast onto a small plane that matched the slope of the existing grade around the proposed building and underneath (it is to be built on piers and so elevated above the ground a bit). I tried using a vray plane and also rotating it to be similar to the existing slope. That still results in the faint, unwanted, shadow across too much of the scene.

Too bad, possibly this could be a feature request? Update vray dome light with HDRI to work correctly with vray matte-shadow object properties?

[^Lele^]

Too bad, possibly this could be a feature request? Update vray dome light with HDRI to work correctly with vray matte-shadow object properties?

Well, it *is* working right.
If it cast no shadow, then it wouldn't be working.
You can exclude the shadow-matte plane from shadow casting from the dome light, and only the sun's shadow will be showing in the alpha.
See attached.

p.s.: doing so will however turn off the adaptive dome.
It could be a minor issue if it's an exterior, but it could mean setting up the scene with portals if you are rendering an interior.
If that is a deal breaker, simply set yourself up with a shadow pass scene: it'll render quickly, and the lower Dome speed won't matter.

[kosso_olli]

I did not know about this issue with matte-shadow objects not working with hdri lighting.

They do work just fine. As Lele pointed out, there might be another solution to what you are looking for.

[OPEN_RANGE]

Well, it *is* working right.
If it cast no shadow, then it wouldn't be working.
You can exclude the shadow-matte plane from shadow casting from the dome light, and only the sun's shadow will be showing in the alpha.

Thank you Lele. I only used the vray sun to check that I had the vray object matte shadow properties set correctly. I am lighting the exterior scene with only the dome light with hdri. That provides the best match for the lighting in the backplate photo.

Probably I need to use the shadow pass scene as you describe, but honestly not 100% sure how to do that. Rendering a vray shadow render element I think.

I can work around this by erasing out the unwanted shadow that appears on the ground plane in post, no problem. I was just asking cause I thought I might have something set incorrectly that was causing this and maybe I could learn something.

Still, it seems unfortunate that vray matte-shadow works as needed with vray sun/sky but not with dome/hdri.

Thanks again to you both!

[^Lele^]

I think i do not understand what you mean by "intended".
Is the HDRI one with a sun in it? Or is it an overcast one?
Regardless of kind, however, if it's a good match for the backplate you need to insert it in, the shadows as they are should look convincing, soft or not that they may be.
When looked at from inside the alpha, they generally look a lot stronger than they become once composited: did you try and put the composite together, and if you did, does it look wrong?

Feel free to hit me via email (it's in my signature) with more examples, and i'll try and help set you up.

[OPEN_RANGE]

Thanks for your generous offer Lele. I sent you an email.