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View Full Version : Problem/Bug with Reconstruction Filter and Compositing Layers?



evolross
10-18-2011, 12:40 AM
I'm working on a visual effects shot where I'm integrating a CG object into a plate. I've setup a still from the plate in the background and also in the foreground with a foreground alpha.

The problem I'm getting is when I adjust the reconstruction filter to try to better match the plate, the reconstruction filter affects the background and foreground compositing layers. So if I set the render to Mitchell (Soft), the render softens the composited VFX plate as well (it also does this when using the Soft Filter checkbox). This is makes is impossible then to better match the plate with a reconstruction filter as the plate is being adjusted by the filter also.

Is there some kind of switch or setting to prevent this? I always thought the filters were for the 3D content being generated by LW. I've attached an example of what I'm talking about. If you wanted to soften everything, you would use an Image Filter right? This seems like a bug to me.

Sensei
10-18-2011, 02:36 AM
No.
Background image is integrated with 3d stuff prior anti-aliasing stage. Environment and backdrop even earlier, at ray-tracing.
It has good reason to be this way- otherwise there would be black glow where background image is connecting with 3d stuff, because AA would assume it's surrounded by black color.

Chrusion
10-18-2011, 10:06 PM
So, wouldn't the solution be to render to a 32-bit format, like PNG, TIF, or PSD, etc. and composite the render with the original plates, using the 3D render's alpha to cut out the 3D render plate so the BG plate shows thru?

Same if you use a FG plate... since you need an alpha to cut out where the 3D objects are to appear behind it, you can take the same FG plate with its existing alpha and composite it back over the 3D rendering in your compositor of choice (AE, Fusion, Nuke, etc), thus preserving the quality of both FG and BG plates. Or disable the FG plate in the 3D render and composite it over the 3D render in post, which is composited over the BG plate.

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evolross
10-19-2011, 12:32 AM
It has good reason to be this way- otherwise there would be black glow where background image is connecting with 3d stuff, because AA would assume it's surrounded by black color.
Couldn't a method be designed to unpremultiply the black out (or whatever is in the environment) and then multiply it against your background layer, without filtering the background layer? Similar to how a compositing package would with renders that have straight alpha channels? And I can't think of any reason why the foreground layer would need to be filtered, unless it's to match the filtered background layer.


So, wouldn't the solution be to render to a 32-bit format, like PNG, TIF, or PSD, etc. and composite the render with the original plates, using the 3D render's alpha to cut out the 3D render plate so the BG plate shows thru?
Yes, but the point is to avoid this long run-around for each test render. I use LW's compositing layers while I work to quickly match the softness, lighting, texture, surfaces, etc. when doing VFX work. I don't want to switch to my compositing app after each test render.

evolross
10-25-2011, 09:38 PM
Couldn't a method be designed to unpremultiply the black out (or whatever is in the environment) and then multiply it against your background layer, without filtering the background layer? Similar to how a compositing package would with renders that have straight alpha channels? And I can't think of any reason why the foreground layer would need to be filtered, unless it's to match the filtered background layer.

I think I said this wrong... couldn't a straight render be done internally (similar to setting Alpha Format to Unpremultiply Alpha under Output settings) and then have this render, using its alpha, be composited onto the compositing layer without any effect to the compositing layer... similar to a how a compositing program would work? This way it's basically doing a slapcomp for you, which to my understanding is how it should work. Whatever it needs to do with the backdrop layer is fine as long as the render is not premultiplied by what's in the backdrop.