问题
Seems like it's not as simple as RGB1*A1 + RGB2*A2...how are values clipped? Weighted? Etc.
And is this a context-dependent question? Are there different algorithms, that produce different results? Or one standard implementation?
I'm particularly interested in OpenGL-specific answers, but context from other environments is useful too.
回答1:
I don't know about OpenGL, but one pixel of opacity A is usually drawn on another pixel like so:
result.r = background.r * (1 - A) + foreground.r * A
result.g = background.g * (1 - A) + foreground.g * A
result.b = background.b * (1 - A) + foreground.b * A
Repeat this operation for multiple pixels.
回答2:
The above answer works if the image isn't premultiplied alpha. However if you use that type of blending with a premultiplied alpha image, there will be a black border.
Premultiplied Alpha:
When the image is created, the color values are multiplied by the alpha channel. Take a look at this one pixel example:
Pixel: r = 1, g = 0, b = 0, a = 0.5
When it's saved, the rgb vales will be multiplied by the alpha value giving:
Pixel: r = 0.5, g = 0, b = 0, a = 0.5
To blend this kind of image you need to use the following formula:
result.r = background.r * (1 - A) + foreground.r
result.g = background.g * (1 - A) + foreground.g
result.b = background.b * (1 - A) + foreground.b
Non-premultiplied Alpha
In this example, the alpha channel is completely separate to the color channels.
Pixel: r = 1, g = 0, b = 0, a = 0.5
When it's saved:
Pixel: r = 1, g = 0, b = 0, a = 0.5
It's the same. In this case the answer provided by minitech is correct.
More details can be found here: Premultiplied alpha
来源：https://stackoverflow.com/questions/10768836/how-does-alpha-blending-work-mathematically-pixel-by-pixel