Mattes leaving a fine black line around the edge
by theboykidney
on
Sep 10, 2007 at 5:37:07 pm
Howdy folks,
Ive often seen it, but not until now to need to find the solution...
Ive got Mattes for a 3D render to pull out some objects etc. but when i use the matte, it doesnt do a thorough job, it leave a black line around the edge.
Re: Mattes leaving a fine black line around the edge by Steve Roberts on Sep 10, 2007 at 5:46:57 pm
It sounds as if the alpha (actually the RGB, but who's counting) of the renders was premultiplied. Can you re-interpret the footage as "premultiplied", choosing black as the color?
File>interpret footage>main.
Or are you trying to key an image that has no alpha?
Re: Mattes leaving a fine black line around the edge by GregNeumayer on Sep 10, 2007 at 6:48:41 pm
When you render your 3D, make sure you render with a "straight" alpha, not a premultiplied one. Then, of course, when you import your matte, interpret it as a "straight" alpha.
In short, premultiplied alphas define the semi-transparent areas (your anti-aliased edges) by determining what your object would look like semi-transparent over a black background*. Straight alpha channels, which are optimal, are purely defined by a level of transparency, without "premultiplying" it to any specific color.
*The color used to premultiply the semi-transparent areas is usually adjustable to any color you want, but often defaults to black.
If you 3D software has no ability to render a straight alpha, you'll need to try to get it to at least use a color most similar to the background you're compositing it to in AE. For example, use blue if rendering a plane to be put in a blue sky.
Most 3D apps nowadays will render a straight alpha. For Maya, there's a checkbox buried in the render settings called "premultiply" or something that needs to be unchecked (if I remember correctly).
Check your AE manual for more info on straight/premultiplied alpha channels.
Hope it helps.
-Greg
Re: Mattes leaving a fine black line around the edge by moldyboot on Sep 10, 2007 at 8:09:48 pm
if you have the 3d object footage rendered with an alpha channel, then setting the interpret footage settings to premultiplied should work, as mentioned before...
but, if you have the 3d footage and a matte as separate renders, and are setting the matte as the trackmatte for the 3d fill footage, precomp the matte and fill footage (after setting the fill footage to use the matte as a trackmatte). then apply the effect remove color matting, this should remove the fringe... if the fringe was not black, choose the color that the 3d object was rendered on.
in the future, if you can render as straight or unmatted in your 3d app and also include the alpha in the file, you can simplify this workflow... be aware though that not all 3d apps correctly render as straight with alpha... my somewhat old version of c4d gives me a white fringe around the object, and i actually think it is easier to remove the black fringe in the interpret footage settings.
