Alpha blending

[Eric Pepke]

This bit me last night; don't let it bite you.

When preparing images for compositing, pre-multiply r, g, and b with alpha.

There are two means of blending commonly used on the Mac, interpolated
and pre-multiplied. (Well, there's also a third, but it isn't used much.)

With interpolated blending, a source color (c_s) painted over a colored
destination (c_d) with source alpha (a_s) gives a final color (c_f):

c_f = a_s * c_s + (1 - a_s) * c_d

In premultiplied blending, it's

c_f = c_s + (1 - a_s) * c_d

Which means that c_s must be premultiplied with c_a beforehand
relative to the c_s in the interpolated case.

It's obvious why Apple chose this, because making c_s bigger than
that lets you put specular highlights on glass. But, like many things
that are obvious, it isn't necessarily obvious when you're working on
something else.

[matt neuburg]

In <ef37f531.0309041713.10abdd41@posting.google.com > Eric Pepke wrote:

> This bit me last night; don't let it bite you.
>
> When preparing images for compositing, pre-multiply r, g, and b with
> alpha.
>
> There are two means of blending commonly used on the Mac, interpolated
> and pre-multiplied. (Well, there's also a third, but it isn't used
> much.)
>
> With interpolated blending, a source color (c_s) painted over a
> colored destination (c_d) with source alpha (a_s) gives a final color (
> c_f):
>
> c_f = a_s * c_s + (1 - a_s) * c_d
>
> In premultiplied blending, it's
>
> c_f = c_s + (1 - a_s) * c_d
>
> Which means that c_s must be premultiplied with c_a beforehand
> relative to the c_s in the interpolated case.
>
> It's obvious why Apple chose this, because making c_s bigger than
> that lets you put specular highlights on glass. But, like many things
> that are obvious, it isn't necessarily obvious when you're working on
> something else.

Is this related to the stuff in the docs that says stuff like "Cocoa and
Quartz expect bitmaps with alpha to have the color values premultiplied"?
m.

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[Eric Pepke]

matt neuburg <matt@tidbits.com> wrote in message news:<20030905074827438-0700@news.la.sbcglobal.net>...

> Is this related to the stuff in the docs that says stuff like "Cocoa and
> Quartz expect bitmaps with alpha to have the color values premultiplied"?
> m.

Yes. But the only reference I've seen is in [NSBitmapImageRep
getBitmapDataPlanes], which I only found afterward.