PDA

View Full Version : alpha channel with volumetric light problem



burvis
05-07-2008, 07:55 AM
have a volumetric light in the center of a ball with holes in it, so the rays shoot out... but I don't get the alpha of the rays from the volumetric light? (see atched images)

Is there a trick to obtain this? I tried all I could think of, but I'm stumpped

Any help, greatly appreciated:)

Chris

Surrealist.
05-07-2008, 09:40 AM
Flare to Alpha is found as a plugin in the Processing tab of the Effects menu under Add Image Filter. Video here. (ftp://ftp.newtek.com/multimedia/movies/w3dw/Volumetric.mov)

burvis
05-07-2008, 11:17 AM
thanks, I found that plug in, but it renders the alpha with some level of grey scale over the whole image, where it should only be where the volume light is showing the rays?

clagman
05-07-2008, 11:40 AM
Backgrounds are included in the alpha when you use flare2alpha if you don't use the unmultiplied alpha option (in the output tab of render globals).

burvis
05-07-2008, 11:48 AM
I tried both the selections there, but I am only looking in the image viewer ans selecting alpha only to see... with either selection the alpha looks like the sample image show... there is only blue backddrop on?

burvis
05-07-2008, 11:53 AM
just tried it with black BG and that seems good, so it was adding whatever the BG was to the alpha... normally LW renderes the backdrop as transparent right?

Surrealist.
05-07-2008, 12:08 PM
Yes, unusually regardless of what is in the BG but apparently not with flare to alpha as above. I did not know that by the way. Great Tip clagman. :) So I guess you will have to render it out to see the alpha correctly then. The Selection is at the bottom. Unpremultiplied Alpha. Like turning off a multiply filter for the alpha and the BG.

clagman
05-07-2008, 12:19 PM
burvis, try rendering out to 32 bit TIFF. That is how I work it and you'll find the background color renders as black no matter how you setup the environment (I think layout will even mention that it will "disable the background" if you choose unpremultiplied alpha). I like using 32 bit TIFF files because you don't have to futz with the separate alpha channel in AE. Also if you batch LZW compress them they are about the same size as JPEG at max quality.

Surrealist, I'm glad I could actually contribute something for once ;o)

Surrealist.
05-07-2008, 09:40 PM
Indeed!

And I forgot to mention about the 32 bit regarding the BG. :)