In <ef37f531.0309041713.10abdd41posting.google.com > Eric Pepke wrote:Is this related to the stuff in the docs that says stuff like "Cocoa and> This bit me last night; don't let it bite you.
> When preparing images for compositing, pre-multiply r, g, and b with
> 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
> 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 = 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.
Quartz expect bitmaps with alpha to have the color values premultiplied"?
matt neuburg, phd = [email]matttidbits.com[/email], [url]http://www.tidbits.com/matt[/url]
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