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| -rw-r--r-- | README | 3 | ||||
| -rw-r--r-- | algorithms/dual-alpha_blending | 47 |
2 files changed, 49 insertions, 1 deletions
@@ -18,7 +18,8 @@ Currently implemented: Drawing coloured text with 3 primaries, alpha and opacity, (100% opacity but 0% alpha will cut out the text so it is - transparent instead of opaque where the text is.) + transparent, rather than at least as opaque as the background, + where the text is.) Drawing on RGB, XRGB, ARGB, and RGBA with 8-bit, 16-bit, 32-bit, and 64-bit subpixels, RGB, ARGB, and RGBA with diff --git a/algorithms/dual-alpha_blending b/algorithms/dual-alpha_blending new file mode 100644 index 0000000..ef16f3e --- /dev/null +++ b/algorithms/dual-alpha_blending @@ -0,0 +1,47 @@ +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). |