When triangulating quads, send edges in consistent order.

Due to the underlying implementation, this probably doesn't
make a difference. But it's more aesthetically pleasing.

Most importantly, add a comment so we know what the tradeoffs are
here.

1 files changed, 20 insertions, 1 deletions

diff --git a/fitz/res_shade.c b/fitz/res_shade.c
index ae024073..fd5e0a85 100644
--- a/fitz/res_shade.c
+++ b/fitz/res_shade.c
@@ -11,8 +11,27 @@ paint_tri(fz_mesh_processor *painter, fz_vertex *v0, fz_vertex *v1, fz_vertex *v
 static void
 paint_quad(fz_mesh_processor *painter, fz_vertex *v0, fz_vertex *v1, fz_vertex *v2, fz_vertex *v3)
 {
+	/* For a quad with corners (in clockwise or anticlockwise order) are
+	 * v0, v1, v2, v3. We can choose to split in in various different ways.
+	 * Arbitrarily we can pick v0, v1, v3 for the first triangle. We then
+	 * have to choose between v1, v2, v3 or v3, v2, v1 (or their equivalent
+	 * rotations) for the second triangle.
+	 *
+	 * v1, v2, v3 has the property that both triangles share the same
+	 * winding (useful if we were ever doing simple back face culling).
+	 *
+	 * v3, v2, v1 has the property that all the 'shared' edges (both
+	 * within this quad, and with adjacent quads) are walked in the same
+	 * direction every time. This can be useful in that depending on the
+	 * implementation/rounding etc walking from A -> B can hit different
+	 * pixels than walking from B->A.
+	 *
+	 * In the event neither of these things matter at the moment, as all
+	 * the process functions where it matters order the edges from top to
+	 * bottom before walking them.
+	 */
 	painter->process(painter->process_arg, v0, v1, v3);
-	painter->process(painter->process_arg, v2, v3, v1);
+	painter->process(painter->process_arg, v3, v2, v1);
 }
 
 static void