Un-premultplying images in Combustion.

[PIL]

Hi all. I have worked always with Flame and Shake and the clasic premultiply alpha channel problem when you color correct doesn,t exist in Flame, because you always work with separate clips and key-in / matte layers, nor in Shake, because you have a lot of different tools to premultiply-unpremultiply when you want.
Mi question is: what about Combustion? If you open an image with the alpha cannel premultiplied (ej: a cg image), how can you un-premultiply to do a color correction without problems?

Thank,s a lot.

[Anonymous]

you just have to go in fotage control and select color + alpha and check off the premultiply with!
Or you can select color as well without any alpha channel then, using a channel operator, add your favourite alpha matte!

Hope this help

Francesco Paglia
Combustion artist

[kuban]

You mean there is a premultiplacation problem with flame* or not? If you select in your surface menu, “blending mode->add” you will have no problems with clips premultiplied over black.
But if rendered over other colors, there are solutions with logic ops.

[Anonymous]

Why do you have to change the blending mode in ADD?
I don’t understand!
You don’t have to have problem in C* and flame as well.
If you make an image already premultiplied also in flame you’ll see the black edge!
If you render the front unstraight matted you will see a poor image that will be good using its alpha!

[kuban]

>>> Chek
For example if you have got a rendered image with alpha in 3D softwares like softimage, and if the image is rendered over black. You then have black edge problems(or if the objects are transparent, you have black surface problems). So in that kind of situations, you have to set the blending mode to “add”.

[Anonymous]

In C* you can solve that problem enabling the premultiply with black option.
You can’t do the same in Flame, the only way to solve the problem is to check between the Softimage render setting something like:
preserve color or
render unstraight matte or
premultiply color with matte or
something like that!
The problem is not in your compositing software but in the way you are rendering the 3D sequence!

[kuban]

But also when you render an imported 3D object within action, it will premultiply the object with backgorund color. So if you want to composite the action rendered object later. You will have the same black edging problems.

And lets think of another situation, you have 2 transparent images in action, and one of them is crossing over the another one. You render the result & matte of the layers. Later you want to composite this, in another action setup, without loading the original 2 layers. Then you will need the “add” feature again, if you rendered the sequence over black.

Or some particle effects you created in action, which the particles have some color effect, like a flame. Particles change their color over time. Then you have hundreds of transparent layers with different colors. The above problem will appear again. So “add” mode is also usefull in flame*

[Anonymous]

If you have one of the latest Flame you can output the matte as well, better if you use a batch!
Don’t mind about the crossing between 2 layers because the matte doesn’t interact with the color front!
You’ll have a new color made by the sum of the previous ones![/quote]

[Author]

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