[PD] mask videos

chris clepper cgc at humboldtblvd.com
Fri Dec 17 16:42:04 CET 2004

On Dec 17, 2004, at 6:43 AM, Andres Cabrera wrote:

> Hi,
> What I've found when exporting from Final Cut Pro, is that the only 
> codec that keeps the alpha channel is "Animation" when using Millions 
> of colors+. Anyone found another one?

The 'None' codec should do this as well, but the data rate is really 
high.  All of the usual codecs and project settings in FCP are the 
video color-space, YUV/YCbCr , which does not have an alpha channel.  
The default color-space in GEM on OSX is YUV.

> If you want to mask according to a color of the image (like chroma 
> keying), you can use either [pix_coloralpha] or [pix_ chroma_key], 
> depending on what you need.

Two things about [pix_coloralpha]:  One, it does nothing on YUV data.  
Two, it just averages the R,G,B values, which really doesn't correspond 
to anything useful.  In other words, drastically different colors will 
give the same mask, and the only really useful values are full white or 
black (the only unique ones when averaging).

> Don't forget to put [alpha] somewhere in the chain to enable alpha 
> blending, and be careful of the rendering order (gemhead number), 
> because objects drawn earlier, even if in front will not blend 
> correctly.

Good suggestions.


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