Dare I say - premult woes

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  09 September 2013
Dare I say - premult woes

I have some premultiplied render elements, the usual. Rendered from max using Vray against a black background. Exposure control enabled within the Vray physical camera (I've tried without too). I've used a multichannel exr with 1.0 gamma.

So I have three multichannel exr's FG, BG & Environment.

I can comp the beauty passes and they look great - no problems. I'm trying to comp the render elements for each .exr but I'm getting some slight black fringing.


Should be using 'Alpha masking', or any 'masks' within the Merge nodes? Toggling alpha masking and masks does affect the edges but I've not resolved the issue.

Should I check 'Premultiplied' in the Read node for my exr's? Doesn't seem to affect the result.

Anyone know of a good manufacturer of padded wall convering that's quite hard wearing?
  09 September 2013
Guessing that you are using Nuke, a proper way to do this would be to add an unpremult right after the read node that reads in your passes and setting it to divide all channels by rgba.alpha (if thats the channel where your alpha resides of course). You can then shuffle out the individual passes, add them back together and after all corrections are made premult again by the original rgba.alpha (you may need to copy it in from your read node before premulting to make sure it's not being modified).

Hope that helps!

  09 September 2013
Sweet - that did the trick.

I used shuffle copy to grab the original alpha for the premult node. Thanks man.

Last edited by MisterS : 09 September 2013 at 10:34 PM.
  09 September 2013
As a little experiment - I pushed the grading to the extreme on the reflection element. The black object in the image was originally chrome. This black and the yellow objects were rendered together in one pass and I've used a Vray multimatte element (Blue Channel) as a mask.

Attached you can see the jaggies that only occur within this pass. There are no problems where this is merged against the background.

Is this the nature of how things work when doing extreme grades or should I approach this differently?

I've tried using both the original MultuMatte mask and the unpremult version.
Attached Images
File Type: jpg edges.jpg (14.4 KB, 13 views)
  09 September 2013
I'm not a master compositor myself so this is anecdote but I've seen lots of the comp teams at various studios doing little things like blurring the alpha one or two pixels and doing a levels to peel it in. I know Nuke is pretty advanced so maybe it has an option already for this.

I'd say to the extreme something like this is normal, and maybe if the multimatte objects behind it were green or red it would be easier to blend when in the extreme. But this is just guesses.
  09 September 2013
That's a good idea. This is just a test scene - a bit of a warm up but I'll keep that in mind in future.

  11 November 2013
Color correction needs to be done on unpremultiplied images otherwise you get edge issues like this. In nuke you can use the standard unpremult/premult nodes or each color correction node has a built in option for premultiplication.
  11 November 2013
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