View Full Version : No output Alpha Channel for Point Lights?

12-07-2003, 12:34 AM
I am not new to using alpha channels in LW. I use them all the time to composite animated characters onto live video with AVID and FCP. However, I could not pull an alpha channel for a point light. Basically it is a scene of a flying light bit, with light lens flare, to composite over footage later. I tried to use Render Alpha Fader (as mentioned in another post), but it only created what looked like a bad bitmap image and not the smooth glowing edges that I need. I am attaching a frame for reference. Thank you in advance for your solutions.

12-07-2003, 02:45 AM
Lens flare is a post-process filter so there is no alpha created. There is a plug-in called Flare2Alpha specifically for this, in the Effects palette, but I've never used it - you can also just render a separate pass of the lens flare and use it for the Alpha.

12-07-2003, 03:27 AM
That plug-in worked great! Thanks for the tip! I'm still not sure what you were referring to by redering a separate pass to get an alpha channel? My current problem is solved nonetheless.
Thanks Toby!

12-07-2003, 12:43 PM
Glad I could help!

what I meant was to render an additional black&white lens flare and use that image as your alpha.

12-07-2003, 02:00 PM
O.K., maybe I was a little too early to accept what Flare2Alpha provided. Upon close examination, I found that it kept a black (render background color) halo around the matte. The solution may be to render on a red (the same color as the point light) background, BUT, . . . Toby's suggestion inspired me to search the 'net for alternatives and I found one better! Plus, it's free!! It's called ReverseAlpha and I found it on Flay at:

It pulls a terrific alpha channel, with no halo. All this and I'll show one DV frame of a B.A.B.E. FORCE agent pushing the buttons on her wristcom and have a light appear (as the Brotherhood creeps up the stairs). Sheesh!

Thanks to Ariel Chai for the plug-in and thanks to Toby for making me think outside the box! (Damn those blinders)

12-08-2003, 10:03 AM
if you are comping in AFX you need to apply the "remove color matte" filter and you should probably switch the transfer mode to screen or similar. That's all