matte object problem - reflect and shadow

[vlado]

This feature is now implemented in the last service pack 2.30 as a checkbox in the vray properties, right? It doesn't work for me, or I am doing something wrong. Take a look at the pictures: The first one shows the old behavior, with the reflection of the ground plane. The second one shows the ground plane with "Matte for refl/refr" ticked. I expect ONLY the shadows to appear in the reflection of the sphere. However, the ground plane still shows up in the reflection. I attached the scene file for Max 2011, what I am doing wrong?

Nothing is wrong, this is the expected result - V-Ray does automatic projection mapping of the background on the matte object for the reflections. In the typical case, you would have a photograph as the background, and it is expected this photograph to appear "mapped" onto the matte objects.

There is also another problem: Let's say I have a car which is lit by an HDRI. The HDRI casts a shadow of the car onto the ground plane, and I would like to have that shadow in the reflection of the car. Because we render all our stuff against a black background for compositing reasons, I can not turn on the environment background. When I do this, the ground plane renders black in the reflection of the car. Is there a workaround for this? I would like to avoid baking the shadows...

So how do you expect the shadow to appear in the reflections? Black shadow on a black background is still black... you can of course change the color of the shadows on the matte objects to white, for example, but how would you use that? Or do you want the background to be black for the camera rays, but to still see the HDRI in the reflections so that the shadow can be seen against it?

Best regards,
Vlado

[kosso_olli]

First of all, sorry for my last post. Sounds a bit rude I know, but it was getting late yesterday...

Or do you want the background to be black for the camera rays, but to still see the HDRI in the reflections so that the shadow can be seen against it?

Vlado, that's exactly what I was hoping for when I heard that this feature is coming. You see, my demand for this is because of this: Most of the time I have to render cars, just like many other users of Vray. Let's say I want to put a car into a saltlake environment, with a really bright sun at the sky. This sun in the HDRI will cast a shadow from the car onto the ground. From some angles, you would see that shadow being reflected in the doors, side skirts etc. of the car. My workflow until now was to bake that shadow as a black and white mask, and then put that in the opacity slot of a black Vray material. This faked the shadow in the reflections. Of course, when you changed position of the light, you have to bake shadows again... I would like to avoid this step, as it is somewhat time consuming. Now I tried the matte for refl/refr option in the Vray properties. As I found out, this only works as long as you make sure the HDRI you are using is also put into the environment map slot in Max. Unfortunately I cant do this, as I need to render against black background for several reasons. If I am rendering against black background, the reflection from the matte in my objects gets black, because Vray samples against the background, not against the HDRI in the invisible dome light.

Also, there is a little problem with the fact that Vray does automatic projection of the background image onto the matte. This may be great for certain cases, but for me it is making some trouble: Often, at least for me, the background image or backplate is quite different from the HDRI (other lighting condition, different camera position/angle). If I put the backplate into the environment slot, you can see a hard edge in the reflection of objects where Vray blends into the HDRI. In the past, I was used to do some manual camera projection of the backplate onto a plane, which gives me the freedom to seamlessly blend between the HDRI and the backplate using some gradients or hand painted masks, so no transitions are visible in the reflection of objects. In short: Is it possible that Vray could sample against the HDRI in a domelight when using matte refl/refr, not the environment color/map?

I hope that this long description makes any sense, if not, please tell me and I will try to explain it with some scene files.

Regards

Oliver

[vlado]

If I put the backplate into the environment slot, you can see a hard edge in the reflection of objects where Vray blends into the HDRI.

You could use a circular opacity map on the base material (or a distance blend or something) to make the mapped camera projection blend with the reflected environment.

In short: Is it possible that Vray could sample against the HDRI in a domelight when using matte refl/refr, not the environment color/map?

For a dome light, no (there may be more than one dome lights in the scene). There are two options here that we can implement. One is to use the reflection environment for secondary mattes, the other is to add a fourth environment slot specifically for secondary mattes. Or perhaps to add a check-box to always use black environment for camera ray, but the regular environment for secondary matte rays. Or maybe something else?

Best regards,
Vlado

[kosso_olli]

I think the checkbox might be worth a try.

[vlado]

Ok then, will see what will come out of it

Best regards,
Vlado

[trixian]

Just Curious..... When would one use more than one domelight in a scene?

[vlado]

I've seen people use one dome light for diffuse illumination and one for reflections.

Best regards,
Vlado

[Dariusz Makowski (Dadal)]

Uhh that shadow for secondary rays looks a bit bad... on a side note is there a way to render bigger than 2k textures for dome lights?

[vlado]

on a side note is there a way to render bigger than 2k textures for dome lights?

Why wouldn't you be able to render bigger than 2k textures?

Best regards,
Vlado

[kosso_olli]

I think he refers to the resolution parameter of the assigned texture in the dome light. The max. value you can assign is 2048, although you can use a script command to override that.

[vlado]

You can use MaxScript to override it, but there is rarely any need to do that, even if the actual resolution of the HDRI is much bigger. The resolution in the dome light itself only affects the importance sampling and you rarely have that much detail in the actual lighting.

Best regards,
Vlado

[Dariusz Makowski (Dadal)]

Im more worried about reflection sharpness than lighting for the moment

[vlado]

Im more worried about reflection sharpness than lighting for the moment

In that case why would you need to adjust the resolution parameter at all? The HDR image will appear with its native resolution in reflections anyways. [Clear reflections at any rate; for glossy reflections there is a small hack that needs to be done.]

Best regards,
Vlado

[kosso_olli]

The HDR image will appear with its native resolution in reflections anyways.

I'm not sure on this. In the past I've had numerous scenes which gave me very pixelated reflections when the resolution was set to 512. This is also valid if the HDR is seen in a refraction. Type in the max. value in the resolution spinner and everything is fine.

[vlado]

I'm not sure on this. In the past I've had numerous scenes which gave me very pixelated reflections when the resolution was set to 512. This is also valid if the HDR is seen in a refraction. Type in the max. value in the resolution spinner and everything is fine.

See the comment in the [brackets].

Best regards,
Vlado