background plate renders with solid alpha?

[ngrava]

Hey Folks,

I have a shot that I'm trying to render for a job that uses a background plate in the environment slot of the camera. There are refractions in the scene that need to render through to the background plate. First issue is that the plate is getting gamma applied to it. Second issue is that the plate is rendering solid in the alpha channel and knocking out any mattes from the objects.

Am I just going about this wrong? How do you guys normally render refractive objects that need to integrate into live action footage?

Thanks for any help!

[Morbid Angel]

Yes this is not the best method. It depends on your vray version and of course on the amount of refraction, but usually there is a way to enable affect channels in refraction (of the shader) to pass alpha down through the refraction. This will allow you to comp the refractive object over the plate in comp rather then in 3d.

For the distortion in refraction, if you want to go the comp rout you can simply fake it through distortion. But if you want to use 3d refraction, then refracting a plate is the only way, and passing the alpha should be also part of affect channels option in the shader.

The gamma correction issue can be solved if you open image plane from the camera and go to attributes/vray/add gamma, then it will render correctly.

[ngrava]

Thanks Dmitry,

Normally I wouldn't care as much about the refractions through glass and simple things like that but in this case it's a RealFlow sim. Also, I tried adding the Vray attributes to the Background plate image but it doesn't seem to do anything. I tried all sorts of values and gamma settings but no change to the image was visible. Any idea of a situation that I might have gotten myself into where these parameters don't work? Maybe something else could be overriding them?

Thanks again!

[vlado]

First issue is that the plate is getting gamma applied to it.

You can add the texture input gamma V-Ray attributes to the image plane and choose the correct color space for the image.

Second issue is that the plate is rendering solid in the alpha channel and knocking out any mattes from the objects.

What kind of file format is the image in?

Finally, which V-Ray version?

Best regards,
Vlado

[ngrava]

Hey Vlado,

You can add the texture input gamma V-Ray attributes to the image plane and choose the correct color space for the image.

Actually, I did apply the Vray properties to the image plane and changed the settings quite a but. However, I didn't see any change in the gamma of the image.

What kind of file format is the image in?

Open EXR.

Finally, which V-Ray version?

We just downloaded the latest version of Vray for Maya 2.4 earlier this month.

[Morbid Angel]

are you working in linear space altogether? are you using sRgb button in vray frame buffer to view things in gamma space? The image plane gamma should just work when you apply it, nothing needs to change. So there could be something else?

[ngrava]

Yeah, It's really funny. I just did a bunch of tests in an isolated scene to see if I could figure it out. Here's what I found: My Color mapping is set for "Linear Multiply" and the gamma is set for 2.2. (which I think is the default) So, in the Vray frame buffer, I have the SRGB button clicked. The background plate is now double gamma'd when I render it. I don't understand at all what's going on here but this is where things get really weird... If I put the Vray "Texture input gamma" properties on the image plane, there is no change to the image when I render again. I set the input gamma to 1 (as I would do in Max) and I had tried all sorts of color mapping settings until suddenly... It was lighter! So I figured, maybe it's backward or something. Sure enough, after some experimentation, I found that cranking the gamma up to 4.4 seems to render the closest to the original image gamma. I'm totally confounded as to why this is. Maybe this makes total sense to you guys but it seems counter-intuitive to me.

Anyway, one issue solved. Now I still need to figure out how to tell Vray to ignore the background plane in the alpha. Any ideas?

[Morbid Angel]

by setting in color mapping gamma to 2.2 and pressing sRgb in buffer you are double gamma correcting. What you need to do is check in color mapping - don't affect color - adaptation only.

What I said before about affect all channels didn't work for the alpha?

[ngrava]

by setting in color mapping gamma to 2.2 and pressing sRgb in buffer you are double gamma correcting. What you need to do is check in color mapping - don't affect color - adaptation only.

What I said before about affect all channels didn't work for the alpha?

Well, that's not really the issue I'm having. The Background (ImagePlane) is rendering solid in the alpha as if it was an object in it's own right. This should not be the case. So, even if I get the refractive objects to render in the alpha, the alpha channel is all white due to the Image plane blocking it all out.

[ngrava]

After mush searching I've discovered that it's simply not possible. Thanks to Maya, the image plane is seen as an object and will block out the alpha. People will tell you over and over on these forums that you should never render with the image plane visible. This is true for the most part but not in the case of refractions through to the background. In that case, you are simply out of luck. Better luck next life time where they figure out that they should at least give you the option of controlling the alpha contribution of the image plane.

In 3DSMax you simply put the footage into the background and your transparent objects render through to it with refration but the background doesn't render in the alpha.

I'm not happy. I also can't believe that people don't run into this all the time. Say you need to integrate a splash of water into a background plate? Or a creature made of glass walking down a street in a live action plate? How would you do this?

[Morbid Angel]

well I don't see it being an issue. Just dont use image plane then. Put your plate image sequence into vray env background as refreactive override set the mapping to screen that's it .

[Svetlozar Draganov]

After mush searching I've discovered that it's simply not possible. Thanks to Maya, the image plane is seen as an object and will block out the alpha. People will tell you over and over on these forums that you should never render with the image plane visible. This is true for the most part but not in the case of refractions through to the background. In that case, you are simply out of luck. Better luck next life time where they figure out that they should at least give you the option of controlling the alpha contribution of the image plane.

In 3DSMax you simply put the footage into the background and your transparent objects render through to it with refration but the background doesn't render in the alpha.

I'm not happy. I also can't believe that people don't run into this all the time. Say you need to integrate a splash of water into a background plate? Or a creature made of glass walking down a street in a live action plate? How would you do this?

There is Alpha Contribution option which will do exactly what you want - it is located in the Vray Object Properties. Set its value to -1 and the object won't be visible in the alpha channel but it will be visible in the refraction: