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authorMattias Andrée <maandree@kth.se>2020-04-28 20:46:56 +0200
committerMattias Andrée <maandree@kth.se>2020-04-28 20:46:56 +0200
commit18a1da68841e1f22cf1a942c3e7f7deb160f7ed0 (patch)
tree9d6c5f63be4774b6c2077ec8fc096d40719f9aa9 /algorithms/dual-alpha_blending
parentFix blending function error for non-premultiplied surfaces (diff)
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Document alpha blending algorithm
Signed-off-by: Mattias Andrée <maandree@kth.se>
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+Some variable names:
+
+ C_out = some colour channel in the output
+ C_bg = some colour channel in the background
+ C_fg = some colour channel in the foreground
+
+ A_out = the alpha channel in the output
+ A_bg = the alpha channel in the background
+ A_fg = the alpha channel in the foreground
+
+
+The normal alpha blending algorithm is used render
+a translucent object over another translucent object.
+The alpha blending algorithm is:
+
+ C_out⋅A_out = C_bg⋅A_bg⋅(1 − A_fg) + C_fg⋅A_fg
+
+ A_out = A_bg⋅(1 − A_fg) + A_fg
+
+
+libskrift expands this algorithm to allow the alpha
+channel of the foreground object to be applied unto
+the background, replacing the alpha channel,
+effectively cutting out part of the background and
+the adding the foreground, and it can do this partially,
+so that even if the background is opaque, the result
+can be translucent where the text is. This requires
+a new parameter for the foreground object (but not
+the background object): opacity (we will call this
+variable O). The new alpha blending algorithm
+(“dual-alpha blending”) is:
+
+ C_out⋅A_out = C_bg⋅A_bg⋅O⋅(1 − A_fg⋅O) + C_fg⋅A_fg⋅O
+
+ A_out = A_bg⋅O⋅(1 − A_fg⋅O) + A_fg⋅O
+
+This algorithm works as the normal alpha blending
+algorithm if the opacity is used instead of alpha,
+and alpha is set to 1, on the foregrund.
+
+
+However, since libskrift is a text drawing library,
+which subpixel rendering support, the opacity is
+multiply with the subpixel's value in the glyph, and
+they highest subpixel's value for the alpha channel
+(since it only support surfaces with per-pixel alpha
+and not per-subpixel alpha).