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durden
09-27-2004, 02:11 PM
I have been trying to export a straignt alpha but can't seem to do it in LW 8? Is it even possible. I even tried fader alpha mode. I am animating a nice smooth curve that is being composited in after effects. The animation need to be clean without premultiplication. Any help in this area would be great. Thanks

Silkrooster
09-27-2004, 10:05 PM
Well I had to do some test renders, but I found out to export just an alpha you need to render the scene by pressing F10 instead of F9. Rendering a single image does not generate the straight alpha file. Make sure in the render options you mark save alpha and give it a name and what format you want. By changing the alphas in the scene editor or in the object properties panel you can disable any alpha that will interfere.
Silk

BeeVee
09-28-2004, 06:29 AM
If you are using a format that can hold an alpha within it, for instance PNG32, then the alpha will be saved with your image if you use F9.

B

Silkrooster
09-28-2004, 07:41 PM
When he said straight alpha I assumed he meant the alpha with out the image.
Silk

toby
09-28-2004, 09:10 PM
Did anyone mention the 'Save Alpha' checkbox in render options? (only works with F10 btw)

oops -yes you did

durden
09-28-2004, 10:08 PM
Ok I found what I am looking for it is called spriteedger and it is in the imageprocessing window you have to add it as a image filter....This is of coarse another LW work around, but it works and quite well. The feature should be built in, as an option in the alpha saving menu. Ok guys you all need to learn what a straight and premultiplied alpha channel is because your gonna get bit in the arse if you don't. An alpha rendered straight not premultiplied is:

With a straight alpha channel, a footage item keeps the transparency information in a separate channel (the alpha channel only), not in any of the visible color channels. This kind of alpha channel is also known as unmatted alpha. With a straight alpha channel, the effects of transparency are not visible until the image is displayed in an application that supports straight alpha.

(the lowdown when you composite the footage into another compositing program it doesn't have a light stroke around the image and imports clean, better and best when you are doing lens flares)

With a premultiplied alpha channel, a footage item keeps the transparency information in the alpha channel and also keeps the same information in the visible RGB channels, which are modified, or multiplied, with a background color. A premultiplied alpha channel is known as matted alpha with a background color. The colors of semitransparent areas, such as feathered edges, are shifted toward the background color in proportion to their degree of transparency.

(the lowdown is that it looks good in the render but when you composite the image over footage you see a light stroke which is pulled from the background color that was available in the scene when it rendered. It blends it with the edge pixels of your 3d image with the background color, in the end when you composite it it has an outline around the edges) :)

toby
09-29-2004, 02:39 PM
What does this Spriteedger plug-in do that the Alpha saver doesn't? As far as I know the Alpha saver saves a b&w image of the Alpha.

durden
09-29-2004, 03:35 PM
alpha saver where's that?

jds580s
09-29-2004, 05:37 PM
For some visuals of what he is talking about check here, about 1/2 way down the page

Premult vs. Straight Alpha (http://onthez.com/www4/tutorials.php?path=tutorials/other/&tut=alpha.html)

I think when toby was talking about alpha saver he was referring to saving an independent grayscale image with alpha info from the render options menu. This is another good way to get the proper straight alpha channel as you have all the RGB info independent from the A.

With RGB+A you need to know ahead of time if the A is premult or straight, and what the matte color will be.

durden
09-29-2004, 06:01 PM
I think when toby was talking about alpha saver he was referring to saving an independent grayscale image with alpha info from the render options menu. This is another good way to get the proper straight alpha channel as you have all the RGB info independent from the A.

With RGB+A you need to know ahead of time if the A is premult or straight, and what the matte color will be.

Although, just because the alpha channel is independent doesn't mean its better all it means is that it is seperate no?

Silkrooster
09-29-2004, 07:44 PM
From what I can tell between spriteedger and saving alpha only. Is the alpha is reversed and spriteedger is 32bit CMYK while alpha only is 8bit grayscale.
Silk

toby
09-29-2004, 09:29 PM
Although, just because the alpha channel is independent doesn't mean its better all it means is that it is seperate no?

Well it certainly means that it's not pre-multiplied - isn't that what you wanted?