add option for exact alpha compositing with a background - #34
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The current implemention of alpha compositing is fast but approximating, by copying the foreground only in the regions where the alpha channel is greater than 0x80. This is fast because it resorts to memory region copying operations and avoids the weighted average of foreground and background color. Yet, if the current terminal background color is black and the image is mostly transparent or black, a black square is shown. This commit introduces an option -A to allow for exact alpha compositing with a solid background, which can be specified as either "white", "black", "green", a greylevel in the 0-255 interval or an RGB triplet.
This commit adds two backgrounds for transparent images, in addition to the solid one: the noise background is randomly grey, the checkerboard:size background alternates between light and dark squares of the given size (default 20).
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The current implemention of alpha compositing is fast but approximating, by copying the foreground only in the regions where the alpha channel is greater than 0x80. This is fast because it resorts to memory region copying operations and avoids the weighted average of foreground and background color. Yet, if the current terminal background color is black and the image is mostly transparent or black, a black square is shown.
This commit introduces an option -A to allow for exact alpha compositing with a solid background, which can be specified as either "white", "black", "green", a greylevel in the 0-255 interval or an RGB triplet.