[PD] PD GEM Alpha for Two Videos

Mike Pascarella mike at pascarellamultimedia.com
Sat Feb 25 04:18:22 CET 2006


Hi all,

I'm new to the list, but have been using Pd for a while.  Recently, I  
have been trying to use GEM to control the opacity of two videos.  I  
have found some threads in the archive discussing how to set it up,  
but have run into a strange problem.  If I have only one video  
active, I can change its opacity.  Once I introduce the second video,  
I lose the ability to change either video's opacity (actually, they  
both become 100% opaque).  Here's how I set it up:

Video 1:

gemhead 50
colorRGB (using a slider to adjust the fourth alpha channel)
alpha
pix_film
pix_texture
rectangle

Video 2:

gemhead 51
colorRGB (using a slider to adjust the fourth alpha channel)
alpha
pix_film
pix_texture
rectangle

I do have a work-around, but I'm baffled as to why it works  
(obviously, I need to learn more).  If I add the following to my pd  
document, I can then control the opacity of both videos:

gemhead 50
square (or any shape)

Since I don't want the new object to be visible, I changed it to the  
following:

gemhead 50
colorRGB 1 1 1 0
alpha
square

It works, but I don't know why.  Can anyone shed some light on what's  
going on here?  I'm using Miller's OSX compile 0.39-2 and the latest  
compiled version of GEM (G4 version).  System is a 1Ghz Ti-Book with  
768  Mb RAM running OSX 10.4.3. Thanks.

Mike





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