Refraction Alpha Problem

[pe_tor]

I've been having a problem properly rendering objects that are refractive with a proper alpha Channel.

The attached image shows the problem. The object (car headlamp lens) is in front of another object but because the object is refractive the alpha channel is not being blocked by the object behind.

This is happening even when there are many objects behind the lens and can get worse as we have multiple lenses in the scene.

I've tried messing around with transparency levels, raytrace depths and secondary biases and nothing changes it. I've even tried different types of image sampling to get around it without luck. We've been using nightly builds of vray but the issue still seems to be there. We never had this problem in max with vray but since we've been switching to maya we have had this problem.

Any help would be appreciated

[tashko.zashev]

Hi,

You have set the refraction to affect the alpha, so this result in the alpha channel is expected.
From our help,

this will cause the material to transmit the alpha of the refracted objects, instead of displaying an opaque alpha.

In 3DS max will work the same way.

[pe_tor]

That is true, for below where there are no other objects.

However you can see that the transparent object is in front of something that is not transparent. That object is blocking the alpha for most of my lens object but not all of it. If I where to comp this image over another image of a sky or something the sky would be able to come through from behind the blocking object. That should not happen.

I would expect that if a a transparent object was infront of something that is not transparent the alpha for that area should be opaque and in the image I am showing above it is not and that is the problem.

[vlado]

Because of the way refraction works, it may happen that a refraction ray goes through total internal reflection and goes out not away from the camera but towards it, not hitting any object - and in that case the alpha will be black. You can try to increase the max. refraction depth, but on the whole, I think this works as expected.

Best regards,
Vlado

[pe_tor]

My first post was rendred with maxing out the transparency levels and ray depth.. as is the following example.

I was hoping to show a more general example without getting into specifics but here is a very specific example of some of the problems we've been having.





The images show the beauty image and the alpha. There is an outer lens, an inner lens, reflectors behind the inner lens, the rest of the car behind the inner lens and a cove behind the rest of the car. It does not seem like a lens surrounded by other objects should affect the alpha in such away.

But still the inner bummpy lens shows up in the alpha, this does not seem correct and we have not had these kinds of issues with 3dsmax and vray. This is one of the more extreme versions but we still have problems with headlight edges showing up in the alpha that we would not expect.

[vlado]

Hmm, ok - can you attach the scene from the first post so that I can look into it? If there is a difference between 3ds Max and Maya, we would definitely want to fix this.

Best regards,
Vlado

[pe_tor]

maya_headlight_problem_vlado.zip

Here is the scene from the first post, it also has a sphere in it where you can see the alpha strangeness close to the vertex positions. Thank you for looking into this.

[pe_tor]

I just wanted to make sure you saw the file and that it works for your testing purposes. Let me know if you need anything else. Thank you Vlado Vray is great!

[CAPTURE_MM]

Hi,
i share this problem. Any solution yet?
Greetings

[vlado]

I'm still scratching my head on that one... what V-Ray does is correct, but apparently it's not quite what you want and I have to figure out a way to do it.

Best regards,
Vlado

[CAPTURE_MM]

We also deal with cad-data. maybe it is how maya handles the normals or something... so maybe it is a maya thing.
i never had this problem with selfmade models. hm... i'll do a test.

[vlado]

We also deal with cad-data. maybe it is how maya handles the normals or something... so maybe it is a maya thing.
i never had this problem with selfmade models. hm... i'll do a test.

I don't think it's the normals in this case, but it doesn't hurt to check

Best regards,
Vlado

[CAPTURE_MM]

it does not seem to be a normals or cad-data problem... sorry...
how does vray handle it? after one refractive material was hit, vray of course changes the ray direction dependent on the surface normal and ior. and for the alpha it additionaly shoots one ray straight forward through the refractive object, irgnoring the normals and ior?
something like that?