[freetype2] master 101e62a: ftimage.h: Improve description of `FT_Span`.

[Werner LEMBERG]

branch: master
commit 101e62a286bd2b5a90d7d0fc07ee35cd32775212
Author: Werner Lemberg <wl@gnu.org>
Commit: Werner Lemberg <wl@gnu.org>

    ftimage.h: Improve description of `FT_Span`.
    
    From Piotr Grochowski <piotrunio-2004@wp.pl>.
---
 include/freetype/ftimage.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/include/freetype/ftimage.h b/include/freetype/ftimage.h
index 7491162..4b5e3b4 100644
--- a/include/freetype/ftimage.h
+++ b/include/freetype/ftimage.h
@@ -853,7 +853,19 @@ FT_BEGIN_HEADER
    *   @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_
   {