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* include/freetype/ftlcdfil.h: Add description of color filtering. 

Based on a contribution from Antti S. Lankila <alankila@bel.fi>.
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Werner Lemberg 2013-03-29 19:35:39 +01:00
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@ -1,3 +1,9 @@
2013-03-29 Werner Lemberg <wl@gnu.org>
* include/freetype/ftlcdfil.h: Add description of color filtering.
Based on a contribution from Antti S. Lankila <alankila@bel.fi>.
2013-03-23 Werner Lemberg <wl@gnu.org>
[autofit] Minor.

38
include/freetype/ftlcdfil.h
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@ -53,6 +53,44 @@ FT_BEGIN_HEADER
* *not* implemented in default builds of the library. You need to
* #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your `ftoption.h' file
* in order to activate it.
*
* FreeType generates alpha coverage maps, which are linear by nature.
* For instance, the value 0x80 in bitmap representation means that
* (within numerical precision) 0x80/0xff fraction of that pixel is
* covered by the glyph's outline. The blending function for placing
* text over a background is
*
* {
* dst = alpha * src + (1 - alpha) * dst ,
* }
*
* which is known as OVER. However, when calculating the output of the
* OVER operator, the source colors should first be transformed to a
* linear color space, then alpha blended in that space, and transformed
* back to the output color space.
*
* When linear light blending is used, the default FIR5 filtering
* weights (as given by FT_LCD_FILTER_DEFAULT) are no longer optimal, as
* they have been designed for black on white rendering while lacking
* gamma correction. To preserve color neutrality, weights for a FIR5
* filter should be chosen according to two free parameters `a' and `c',
* and the FIR weights should be
*
* {
* [a - c, a + c, 2 * a, a + c, a - c] .
* }
*
* This formula generates equal weights for all the color primaries
* across the filter kernel, which makes it colorless. One suggested
* set of weights is
*
* {
* [0x10, 0x50, 0x60, 0x50, 0x10] ,
* }
*
* where `a' has value 0x30 and `b' value 0x20. The weights in filter
* may have a sum larger than 0x100, which increases coloration slightly
* but also improves contrast.
*/