Alpha Premultiply problem

[shushens]

Hey guys, it's me again.

I am trying to composite this car onto a backplate of my choosing. I also have a pass for AO just between the car and the ground. I placed the car in a white constant shaded partition so only the AO is visible on white ground. And there is the beauty pass of the car, of course, where there is just the car with its alpha.

When I add the backplate as background, and the car as foreground and then merged that with the ground AO in multiply mode, in the final result, I am seeing a slight white outline around the car. Naturally, the alpha of the beauty pass is premultiplied. So, I re-rendered with pre-multiply off. But the result persists. So I change to "no clipping" and still it persists. I tried Dithering on and off, and it has no effect on this problem. So, what am I supposed to do to get rid of this problem? I do not want to bypass it in compositing level, I want my passes to get rendered problem-free.

I am using Fusion to comp it, but in Photoshop, same thing would happen, I am sure.

[shushens]

Here is a screenshot:

[schyzomaniac]

Could you post one frame from each of your passes? That way it's easier to tell what's going on.

cheers,
Daniel

[shushens]

Thanks for your response

I realized I did something stupid. I added the alpha of the beauty EXR to the effect mask which caused the problem. I unplugged it and it's practically invisible now.

However, I would love to know a little more about this. Do I really need to render without premultiply? What about dithering and clipping? I know the basic theory, but I am unaware of the industry practices.

[Rork]

I highly recommend a book called "The Art and Science of Digital Compositing" by Rob Brinkmann.

It will extensively explain stuff like this, so it becomes more clear to you. Also CMIVFX have some terrific training for Fusion and compositing in general.
And Gnomon has a very good one on basic compositing as well, it's for Shake but all stuff explained is valid for any composite app.

rob

[shushens]

Thanks for the suggestion, Rob! However, it's not the generalized perspective I was intending to look into, but more like XSI specific things. I did face this problem before in other softwares and disabling premultiply does solve it there. Here, however, something is happening a little differently and Fusion is giving me almost the desired result (not 100% if you look with a magnifying glass) regardless I have premultiply turned on or off. Photoshop is showing quite a bit of variation with the very same passes. Sometimes not premultiplying is showing white borders and premultiplying is not. And I tried all 3 kinds of clipping and could not see any difference anywhere. The user guide does not elaborate either.

So it's not actually the theory of premultiplying I am talking about, it's the pipeline issues between XSI and Fusion/Photoshop.

[Rork]

Alpha (pre)mulitplication shouldn't be an issue in any comp app. Most of them just have a checkbox to turn it on/off.

But after re-reading your post try this:

If you want just the contact AO of the car, remove the AO shader from the partition and add an override instead. Now add the 'primary rays visibility' to the partition and turn it off. Result of this is contact AO, and no matte for the car as the geometry is not seen(hidden) for the render.

rob

edit: Also using just the back-/foreground 'slots' of the 'merge' node in Fusion should be more than enough. No need to use the mask slot unless you're trying to mask out some part of a layer.