From 24681fa94ef840dcc6c0793f42bbaa8d3be013c9 Mon Sep 17 00:00:00 2001
From: Arman Uguray <armansito@google.com>
Date: Wed, 8 Nov 2023 08:46:36 -0800
Subject: [PATCH] Minor documentation and cleanup to zero-length tangent
 detection

---
 crates/encoding/src/path.rs | 24 +++++++++++++++---------
 shader/flatten.wgsl         |  8 +++++++-
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/crates/encoding/src/path.rs b/crates/encoding/src/path.rs
index d35edc3e1..6118ff414 100644
--- a/crates/encoding/src/path.rs
+++ b/crates/encoding/src/path.rs
@@ -525,9 +525,11 @@ impl<'a> PathEncoder<'a> {
         if self.state == PathState::MoveTo {
             let x0 = self.first_point[0];
             let y0 = self.first_point[1];
-            // TODO: should this be an exact match?
-            if (x - x0).abs() < 1e-12 && (y - y0).abs() < 1e-12 {
+            // Ensure that we don't end up with a zero-length start tangent.
+            const EPS: f32 = 1e-12;
+            if (x - x0).abs() < EPS && (y - y0).abs() < EPS {
                 // Drop the segment if its length is zero
+                // TODO: do this for all not segments, not just start?
                 return;
             }
             self.first_start_tangent_end = [x, y];
@@ -552,13 +554,15 @@ impl<'a> PathEncoder<'a> {
         if self.state == PathState::MoveTo {
             let x0 = self.first_point[0];
             let y0 = self.first_point[1];
-            // TODO clean this up
-            let (x, y) = if (x1 - x0).abs() > 1e-12 || (y1 - y0).abs() > 1e-12 {
+            // Ensure that we don't end up with a zero-length start tangent.
+            const EPS: f32 = 1e-12;
+            let (x, y) = if (x1 - x0).abs() > EPS || (y1 - y0).abs() > EPS {
                 (x1, y1)
-            } else if (x2 - x0).abs() > 1e-12 || (y2 - y0).abs() > 1e-12 {
+            } else if (x2 - x0).abs() > EPS || (y2 - y0).abs() > EPS {
                 (x2, y2)
             } else {
                 // Drop the segment if its length is zero
+                // TODO: do this for all not segments, not just start?
                 return;
             };
             self.first_start_tangent_end = [x, y];
@@ -583,15 +587,17 @@ impl<'a> PathEncoder<'a> {
         if self.state == PathState::MoveTo {
             let x0 = self.first_point[0];
             let y0 = self.first_point[1];
-            // TODO clean this up
-            let (x, y) = if (x1 - x0).abs() > 1e-12 || (y1 - y0).abs() > 1e-12 {
+            // Ensure that we don't end up with a zero-length start tangent.
+            const EPS: f32 = 1e-12;
+            let (x, y) = if (x1 - x0).abs() > EPS || (y1 - y0).abs() > EPS {
                 (x1, y1)
-            } else if (x2 - x0).abs() > 1e-12 || (y2 - y0).abs() > 1e-12 {
+            } else if (x2 - x0).abs() > EPS || (y2 - y0).abs() > EPS {
                 (x2, y2)
-            } else if (x3 - x0).abs() > 1e-12 || (y3 - y0).abs() > 1e-12 {
+            } else if (x3 - x0).abs() > EPS || (y3 - y0).abs() > EPS {
                 (x3, y3)
             } else {
                 // Drop the segment if its length is zero
+                // TODO: do this for all not segments, not just start?
                 return;
             };
             self.first_start_tangent_end = [x, y];
diff --git a/shader/flatten.wgsl b/shader/flatten.wgsl
index 13f53f18f..43e1afa53 100644
--- a/shader/flatten.wgsl
+++ b/shader/flatten.wgsl
@@ -328,7 +328,13 @@ fn read_path_segment(tag: PathTagData, transform: Transform, is_stroke: bool) ->
     }
 
     if is_stroke_cap_marker && is_open {
-        // TODO: document
+        // The stroke cap marker for an open path is encoded as a quadto where the p1 and p2 store
+        // the start control point of the subpath and together with p2 forms the start tangent. p0
+        // is ignored.
+        //
+        // This is encoded this way because encoding this as a lineto would require adding a moveto,
+        // which would terminate the subpath too early (by setting the SUBPATH_END_BIT on the
+        // segment preceding the cap marker). This scheme is only used for strokes.
         p0 = transform_apply(transform, p1);
         p1 = transform_apply(transform, p2);
         seg_type = PATH_TAG_LINETO;