[Bf-committers] Color pipeline todo: alpha

[Bartek Skorupa (priv)]

IMHO all the confusion comes from the fact that we actually don't refer to ALPHA, but to other three channels, i.e. to colors.
We use all those terms to say if the colors have or haven't been altered by the alpha channel.
I don't know every single piece of software in the world, but I think that terms "Straight" and "Premultiplied" are the ones that most of the users know and are used to.
I'm not a native English speaker, so maybe the way I understand certain terms is totally wrong, but when I think about this stuff I'd vote for having "Straight" and "Premultiplied" as the naming convention.
Those two words seem logical to me.
"The colors are straight" - means that they are straight. They are unchanged. I can use them as they are and when compositing, I have to multiply them by alpha.
"The colors are premultiplied" means that they have already been multiplied (premultiplied, i.e multiplied in advance) by alpha so I don't have to, or I should rather say I mustn't multiply them by alpha again.

The confusion results from using the words "straight" or "premultiplied" to describe the alpha, when in fact they describe colors.
"Straight Alpha" or "Premultiplied alpha" seem wrong. "Straight colors" or "Premultiplied colors" seem right.

"Unpremultiply" sounds to me like saying "unpush" instead of "pull".

Bartek Skorupa

www.bartekskorupa.com

On 20 gru 2012, at 21:01, Troy Sobotka <troy.sobotka at gmail.com> wrote:

> On Thu, Dec 20, 2012 at 11:52 AM, Brecht Van Lommel
> <brechtvanlommel at pandora.be> wrote:
>> I like the names associated and unassociated because they don't
>> describe how you arrived at the color, just the way it is stored.
> 
> Agree 110%. It also eliminates the rather erroneous assumption that
> unassociated and associated alpha are purely as simple as a divide or
> multiply away. A do-undo sequence on an associated alpha scene
> referred image with >1.0 values would likely destroy the data without
> some workarounds.
> 
> That still leaves our "Color Unpremultiply" order of events term a
> little... awkward.
> 
> Needless to say, perhaps you or Sergey could agree upon and set the
> baseline and then we can work to get the terms consistent throughout
> the UI.
> 
> With respect,
> TJS
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