[freetype2] 1100-sdf-improvements 5f9bc4b85 1/3: * include/freetype/freetype.h (FT_RENDER_MODE_SDF): Improve documentation.

[Werner Lemberg]

branch: 1100-sdf-improvements
commit 5f9bc4b8594c7051ce700eb52fb593da924314cb
Author: Anuj Verma <anuj@womp.xyz>
Commit: Anuj Verma <anuj@womp.xyz>

    * include/freetype/freetype.h (FT_RENDER_MODE_SDF): Improve documentation.
    
    * include/freetype/freetype.h (FT_RENDER_MODE_SDF): Added more
      documentation for the render mode regarding the stability and point
      where artifacts are expected to occur.  Also added more information
      for both `sdf`, `bsdf` and when they are used.
---
 include/freetype/freetype.h | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/include/freetype/freetype.h b/include/freetype/freetype.h
index 0489c8448..40a4dc215 100644
--- a/include/freetype/freetype.h
+++ b/include/freetype/freetype.h
@@ -3409,6 +3409,45 @@ FT_BEGIN_HEADER
    *   }
    *
    *   ```
+   *
+   *   FreeType has two rasterizers for generating SDF, namely:
+   *
+   *   1. `sdf`  - For generating SDF directly from glyph's outline.
+   *
+   *   2. `bsdf` - For generating SDF from rasterized bitmaps.
+   *
+   *   Depending on the glyph type (i.e. outline or bitmap), one of the two
+   *   rasterizer is chosen at runtime and used for generating SDF.  To
+   *   force use `bsdf` you can simply render the glyph using any of the
+   *   FreeType's other rendering modes (e.g. `FT_RENDER_MODE_NORMAL`) and
+   *   then re-render using `FT_RENDER_MODE_SDF`.
+   *
+   *   Now, a few notes on the stability and point of failures for the SDF
+   *   renderers (specifically `sdf`):
+   *
+   *   1. The `sdf` rasterizer is sensitive to really small features (e.g.
+   *      sharp turns which are less than 1 pixel) and imperfections in the
+   *      glyph's outline.  Having these in the outline can cause artifats
+   *      in the final output.
+   *
+   *   2. The `sdf` rasterizer has limited support for handling intersecting
+   *      contours and *cannot* handle self-intersecting contours whatsoever.
+   *      Self intersection happens when a single connected contour intersect
+   *      itself at some point and having these in your font will definately
+   *      pose a problem to the rasterizer and cause artifacts.
+   *
+   *   3. Generating SDF for really small glyphs may result in undesirable
+   *      output due to limited availability of pixel grid to store distance
+   *      information.
+   *
+   *   4. Since the output buffer is normalized, precision at smaller spread
+   *      will be greater than precision at larger spread values, because the
+   *      output range of [0 .. 255] will be mapped to a smaller sdf range.
+   *      A spread of 2 should be sufficient in most cases.
+   *
+   *   Point (1) and (2) can be avoided by using `bsdf` and overall it is
+   *   more stable than the `sdf` rasterizer.
+   *
    */
   typedef enum  FT_Render_Mode_
   {