Alpha Compositing - Alpha Blending

Alpha Blending

Alpha blending is the process of combining a translucent foreground color with a background color, thereby producing a new blended color. The degree of the foreground color's translucency may range from completely transparent to completely opaque. If the foreground color is completely transparent, the blended color will be the background color. Conversely, if it is completely opaque, the blended color will be the foreground color. Of course, the translucency can range between these extremes, in which case the blended color is computed as a weighted average of the foreground and background colors.

Alpha blending is a convex combination of two colors allowing for transparency effects in computer graphics. The value of alpha in the color code ranges from 0.0 to 1.0, where 0.0 represents a fully transparent color, and 1.0 represents a fully opaque color. This alpha value also corresponds to the ratio of "SRC over DST" in Porter and Duff equations.

The value of the resulting color is given by:

\left\{ \begin{array}{l} out_A = src_A + dst_A (1 - src_A) \\ out_{RGB} = \bigl( src_{RGB} src_A + dst_{RGB} dst_A \left( 1 - src_A \right) \bigr) \div out_A \\ out_A = 0 \Rightarrow out_{RGB} = 0 \end{array} \right.

If the destination background is opaque, then, and if you enter it to the upper equation:

\left\{ \begin{array}{l} out_A = 1 \\ out_{RGB} = src_{RGB} src_A + dst_{RGB} (1 - src_A) \end{array} \right.

The alpha component may be used to blend to red, green and blue components equally, as in 32-bit RGBA, or, alternatively, there may be three alpha values specified corresponding to each of the primary colors for spectral color filtering.

Note that the RGB color may be premultiplied, hence saving the additional multiplication before RGB in the equation above. This can be a considerable saving in processing time given that images are often made up of millions of pixels.

Read more about this topic:  Alpha Compositing

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