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Basic compositing question

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Richard Smith
Basic compositing question
on Oct 18, 2011 at 9:45:08 am

I have a footage on top of which I need to place another footage, of a rendered 3D object, so that it appears it is part of the scenery. The 3D object is rendered in a separate 3D application then exported as a image sequence.

The problem is, when I render it the images also contain the black rectangle behind the object (the rectangle is the background of the 3D program). If I put the images on top of the 1st footage, the black background would also appear. What should I do for the BG to disappear?


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Kevin Camp
Re: Basic compositing question
on Oct 18, 2011 at 2:27:43 pm

the 3d object should be rendered with an alpha channel, thus making the black areas transparent.

the alpha channel could be rendered as a separate file then applied in ae, or as a part of the same file depending on the file type of the image sequence -- many file types can contain an alpha channel, like tif, tga, png, psd, so if you rendered to any of those you could have the 3d software include the alpha.

however if it took a while to render the 3d object, it may be faster to render just the 3d object's alpha channel as a separate sequence (to avoid a re-render of the whole object with alpha), but it depends a bit on how long the initial 3d render took...

in ae, if you have a separate alpha pass, you'll bring both the rgb render and the alpha render into the same comp. then there are a variety of ways to apply the alpha... in this case a fairly straight forward way would be to add an effect like set matte to the rgb portion, choosing the alpha layer as the layer to take the matte from and 'luminance' as what it should use as the matte. then you can hide the alpha layer.

if you see black fringing around the 3d layer add the remove color matting effect after the set matte effect.

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Richard Smith
Re: Basic compositing question
on Oct 18, 2011 at 5:18:46 pm

But aren't tif, tga, png lossy image formats? I would prefer to not lose quality.


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Michael Szalapski
Re: Basic compositing question
on Oct 18, 2011 at 7:19:10 pm

No; they are not lossy.
Well, some of them can be, but they aren't lossy when at highest quality settings.

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.


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Kevin Camp
Re: Basic compositing question
on Oct 19, 2011 at 8:08:49 pm

there are other image types that support alpha too, you didn't mention the file type that you used, so i was naming a few, but your original file type may support alpha too.

if it doesn't, and you'd like to keep using that format, you'll just need to include a separate alpha pass. it's not too difficult to use this workflow. it's actually very common.

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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