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shake lets you unmultiply your alpha channel from your images, letting you do colour work on that and keeps really nice edges on colour corrected stuff
does anyone want to share a technique they use to do the same in combustion [img]images/forum/smilies/icon_confused.gif[/img]
I’m going to use a little bit of necromancy and bump this topic. Is there no way to divide two layers in combustion under the compound RGB Operator?
Correct me if Im wrong but I though Combustion handled pre-multed alphas differently then Shake and there was really no need to un-mult them?
Only on certain operations such as footage and color correction.
It’s true you don’t always have to be switching back and forth like in shake, however, I want to be able to do some pre-comping of render passes and then apply the set matte operator later. This is possible if I render out unmultiplied images but you can’t always count receiving on that.
in combustion, you simply go to the footage level and enable or disable premultiplied alpha.
//gD
the sapphire math-ops have a divide node.
beside that, there’s really not much need to jump in-and-out of pre-mult like in shake. usually c* picks the right thing, if not, as gary wrote already, you set it at the footage level.best,
raykhemmerli wrote:the sapphire math-ops have a divide node.
beside that, there’s really not much need to jump in-and-out of pre-mult like in shake. usually c* picks the right thing, if not, as gary wrote already, you set it at the footage level.rayk
so as rayk pointed out, you can use the built in way to identify premultiplied alpha at the footage level or go out and spend $400-1500 for a plugin to do the same thing as a node in the pipeline.
//gD
combustion definitely has some problems with premultiplied footage.
This is an easily replicable bug:
1)Open any footage with semi-transparent alpha into a new 2D comp.
2)Set the footage’s aplha type as it should be
3) Render this comp to TIFFs without “premultiply” checked. In theory, the result should be the same as original.
4) Load your freshly rendered footage into c*. You will see that semi-transparent areas are darker, they somehow got premultiplied.If you do this in AE, your rendered footage is the same as original.
Also, when working with some premult’d footage, you can’t get rid of the black edges.
P.S.: Just now when trying to recreate this problem, I found out that it doesn’t occur with TGA sequences but it does with TIFFs.
When checking images in Photoshop, I found out that both TIFFS and TGAS were rendered correctly, but TIFFS imported into combustion didn’t behave as they should.
SO this probably is a problem of c’s TIFF reader.hi there,
i can’t confirm this (but don’t want to deny it either).
do you mind and send me a pic which displays the trouble.
rayk dot hemmerling at gmx dot de (no spaces)
what application was the original image created in? settings there?two things come to my mind at this point:
when re-importing the rendered pic into combustion, were the footage settings recognized correctly? -i.e. “un-premultiplied”.
you mentioned photoshop. as far as i recall, photoshop pre-multplies sometimes on white instead of black as most other applications.best,
raykOr do it for free with Knoll Unmult
Quote:Knoll Unmult recreates the opacity or alpha channel of a layer based on luminance also called brightness of the lens flare. Unmult assumes that the lens flare is rendered against black. The image is also adjusted to compensate for the new alpha channel.-Eric
Hello. I´m saul and be new in this forum.
sorry for my language level!
when you work with combustion you can define if the footage is pre-mult or un-mult i footage control. ok
the problen is when you work with composite or set matte operator if you need compound with your original matte later.
the ”composite” and ”set matte” operator pre-mult always??? I think soif you have to compose a pre-mult image with external matte image, it uses the ”merge” operator for to multiply the matte invert whit the background and the other ”merge” operator for to add wiht the pre-multipled image.
if you´ve internal matte channel, it uses ”compound rgb arithmetic” operator for to convert in external matte image.
uufff! finish!!!
somebody understand me with my level of language???
😀Hi, Saul!
I understand to you…I just wondering if this is beause I´m spanish too! 😀 😀
I try send to you the private messenger and I can´t do it 😥I live in Sevilla,…where do you live?
See you later. Falele
There is a free plugin for AE that is compatible with C* somewhere, if I remember correctly it was from Red Giant I think. But you can try to use the Channel > Alpha Arithmetic with Operator in “Set” and Operand at 100%, then use Set Matte to put your alpha back to where it was.
cheers,
diogoYou mean the one i linked to 4 posts up dg? 🙄
-Eric
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