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I noticed that there are a couple of requests for erasing half of the opacity of a channel.
Some scripts distort the resulting blue toward green. I addressed this by adjusting the tv_pixelmatrix to be .5 for the color channel being reduced.
This means reducing the blue by half would be "tv_pixelmatrix 1 0 0 0 0 0 1 0 0 0 0 0 .5 0 0 0 0 -0.5 1 0" - This seems to give a balanced color result in my tests with some rainbow gradients. If you push it about 5-6 times it totally eliminates the channel's presence (remember, half of half is not zero). Give it a shot, improve it, whatever!

Thanks,
David

Oh - I couldn't figure out how to remove 50% of the white without altering the color...maybe someone can help with that...

That's sooo nice, thanks!
Maybe you can try to attenuate the whole alpha then get back the alpha with another pixelmatric on colors alpha?
Also I'd stack multiple pixelmatric buttons to be sure to kill an alpha with one button, depending on people's need to attenuate or remove a color.

I have some related scripts here that can help other people : http://forum.tvpaint.com/viewtopic.php?f=11&t=9927

Knew you'd come through Nathan! Thanks for suggestions, I'll try more stuff today...

héhé, always here

hehe is héhé some french laughter?

Exactly : )
Btw my idea was not working : (

this is perfect! thanks a lot!