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+Porter, Thomas and Duff, Tom, "Compositing Digital Images", Computer Graphics Vol 18, Num 3, July 1984
+
+Compositing Digital Images
+Thomas Porter
+Tom Duff
+Computer Graphics Project
+Lucasfilm Ltd.
+
+
+Perter and Duff's classic paper "Compositing Digital Images" lays the
+foundation for digital compositing today. By providing an "alpha channel,"
+images of aribitary shapes — and images with soft edges or sub-pixel coverage
+imformation — can be overlayed digitally, allowing separate objects to be
+rasterized separately without a loss in quality.
+
+Pixels in digital images are usually represented as 3-tuples containing
+(red component, green component, blue component). Nominally these values are in
+the [0-1] range. In the Porter-Duff paper, pixels are stored as (R,G,B,\alpha)
+4-tuples, where alpha is the fractional coverage of each pixel. If the image
+only covers half of a given pixel, for example, its alpha value would be 0.5.
+
+To improve compositing performance, albeit at a possible loss of precision in
+some implementations, the red, green and blue channels are premultiplied by the
+alpha channel. This also simplifies the resulting arithmetic by having the
+colour channels and alpha channels use the same compositing equations.
+
+Several binary compositing operations are defined:
+- over
+- in
+- out
+- atop
+- xor
+- plus
+
+The paper further provides some additional operations for implementing fades and
+dissolves, as well as for changing the opacity of individual elements in a
+scene.
+
+The method outlined in this paper is still the standard system for compositing
+and is implemented almost exactly by modern graphics APIs such as OpenGL. It is
+all but garunteed that this is the method we will be using for compositing
+document elements in our project.
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