ref: 101e62a286bd2b5a90d7d0fc07ee35cd32775212
parent: 804e625def2cfb64ef2f4c8877cd3fa11e86e208
author: Werner Lemberg <wl@gnu.org>
date: Fri Oct 30 04:04:01 EDT 2020
ftimage.h: Improve description of `FT_Span`. From Piotr Grochowski <piotrunio-2004@wp.pl>.
--- a/include/freetype/ftimage.h
+++ b/include/freetype/ftimage.h
@@ -853,7 +853,19 @@
* @FT_SpanFunc that takes the y~coordinate of the span as a parameter.
*
* The coverage value is always between 0 and 255. If you want less gray
- * values, the callback function has to reduce them.
+ * values, the callback function has to reduce them by scaling the
+ * outline four times and using bilevel monochrome renderer to then
+ * average 16 pixels in each 4×4 box.
+ *
+ * The only correct way to blend colors is to convert the colors from
+ * sRGB to linear colorspace, then perform the weighted average, then
+ * convert to sRGB, but this is out of scope of FreeType. Using an
+ * approximation such as gamma 2.2 or 2.3 or interpolation from a lookup
+ * table of more than three entries or sampling from a lookup table of
+ * more than fifteen entries is acceptable, but any implementation
+ * significantly deviating from that (for example a gamma of 1.8 or less,
+ * or a gamma of 2.7 or greater, or not implementing gamma properly at
+ * all) will most likely yield bad results.
*/
typedef struct FT_Span_
{