View Full Version : alpha channel for greenscreened elements

11-01-2004, 03:41 AM
we've got some greenscreened elements we've filmed and we've extracted alphas in Shake to map on simple planes in LW but we're not too happy with the result when rendering: We get black edges around the elements.
Anyone knows what's the best settings in a compositing app to get the perfect alpha for rendering in LW ?


11-01-2004, 04:08 AM
Hi Lorex,
there are two ways to help solve the problem:

a) render the Alpha out as non-multiplied (which will give you harsh edges in the RGB part of semi-transparent areas).

b) render out two passes, one with RGB only, the other with the Alpha mapped as the colour. Then merge those two to an RGBA file in your compositing tool.


Here is what happens in LW: If you apply the Alpha as the transparency, it will affect the RGB mapped image as well in the semi-transparent areas, resulting in a doubly applied Alpha in the those areas.

Actually, there is a third way: try the UnMult.ls script provided with LW8 on your image. Mind you, due to LScript limitations, this may crash with extremely high res images.