Rendering considerations.
diff --git a/ChangeLog b/ChangeLog
index 8797510..1f424d3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -195,6 +195,11 @@
the chord. The cubic distances, however, decrease less predictably
but are easy enough to calculate on each step.
+ The new algorithm produces slightly larger number of splits, which is
+ compensated by its simplicity. The overall rendering performance is
+ improved by 1-2%. The larger number of splits does not necessarily
+ result in higher quality, which stays comparable.
+
* src/smooth/ftgrays.c (gray_render_cubic): Replace the split
condition.
@@ -209,6 +214,10 @@
Optimize Bézier bisections.
+ This change makes bisections faster by 20-30%. When inlined into
+ `gray_render_cubic', this makes the function faster by 10% and is
+ noticeable in the overall rendering performance.
+
* src/raster/ftraster.c (Split_Conic, Split_Cubic): Use shifts and
refactor.
* src/smooth/ftgrays.c (gray_split_conic, gray_split_cubic): Ditto.
diff --git a/src/smooth/ftgrays.c b/src/smooth/ftgrays.c
index 8f87f84..4efc0d0 100644
--- a/src/smooth/ftgrays.c
+++ b/src/smooth/ftgrays.c
@@ -45,7 +45,7 @@
* This is a new anti-aliasing scan-converter for FreeType 2. The
* algorithm used here is _very_ different from the one in the standard
* `ftraster' module. Actually, `ftgrays' computes the _exact_
- * coverage of the outline on each pixel cell.
+ * coverage of the outline on each pixel cell by straight segments.
*
* It is based on ideas that I initially found in Raph Levien's
* excellent LibArt graphics library (see https://www.levien.com/libart
@@ -58,6 +58,14 @@
* different way, and I don't use sorted vector paths. Also, it doesn't
* use floating point values.
*
+ * Bézier segments are flattened by splitting them until their deviation
+ * from straight line becomes much smaller than a pixel. Therefore, the
+ * pixel coverage by a Bézier curve is calculated approximately. To
+ * estimate the deviation, we use the distance from the control point
+ * to the conic chord centre or the cubic chord trisection. These
+ * distances vanish fast after each split. In the conic case, they vanish
+ * predictably and the number of necessary splits can be calculated.
+ *
* This renderer has the following advantages:
*
* - It doesn't need an intermediate bitmap. Instead, one can supply a
@@ -67,7 +75,7 @@
* callback.
*
* - A perfect anti-aliaser, i.e., it computes the _exact_ coverage on
- * each pixel cell.
+ * each pixel cell by straight segments.
*
* - It performs a single pass on the outline (the `standard' FT2
* renderer makes two passes).
@@ -75,7 +83,7 @@
* - It can easily be modified to render to _any_ number of gray levels
* cheaply.
*
- * - For small (< 20) pixel sizes, it is faster than the standard
+ * - For small (< 80) pixel sizes, it is faster than the standard
* renderer.
*
*/
