ref: 5499d7bf8155289dd8907d0fd11185089f0f642a
parent: 335224beee2e72caab4ae56b76d6eb72001c3753
author: Anuj Verma <[email protected]>
date: Fri Mar 4 11:42:02 EST 2022
* include/freetype/freetype.h: Improve SDF documentation.
--- a/include/freetype/freetype.h
+++ b/include/freetype/freetype.h
@@ -3409,6 +3409,44 @@
* }
*
* ```
+ *
+ * FreeType has two rasterizers for generating SDF, namely:
+ *
+ * 1. `sdf` for generating SDF directly from glyph's outline, and
+ *
+ * 2. `bsdf` for generating SDF from rasterized bitmaps.
+ *
+ * Depending on the glyph type (i.e., outline or bitmap), one of the two
+ * rasterizers is chosen at runtime and used for generating SDFs. To
+ * force the use of `bsdf` you should render the glyph with any of the
+ * FreeType's other rendering modes (e.g., `FT_RENDER_MODE_NORMAL`) and
+ * then re-render with `FT_RENDER_MODE_SDF`.
+ *
+ * There are some issues with stability and possible failures of the SDF
+ * renderers (specifically `sdf`).
+ *
+ * 1. The `sdf` rasterizer is sensitive to really small features (e.g.,
+ * sharp turns that are less than 1~pixel) and imperfections in the
+ * glyph's outline, causing artifacts 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; having these in your font definitely pose a
+ * problem to the rasterizer and cause artifacts, too.
+ *
+ * 3. Generating SDF for really small glyphs may result in undesirable
+ * output; the pixel grid (which stores distance information) becomes
+ * too coarse.
+ *
+ * 4. Since the output buffer is normalized, precision at smaller spreads
+ * is greater than precision at larger spread values because the
+ * output range of [0..255] gets mapped to a smaller SDF range. A
+ * spread of~2 should be sufficient in most cases.
+ *
+ * Points (1) and (2) can be avoided by using the `bsdf` rasterizer,
+ * which is more stable than the `sdf` rasterizer in general.
+ *
*/
typedef enum FT_Render_Mode_
{