From c9cbb5b071f6a3a76d3f25e74ece52ce580c26c9 Mon Sep 17 00:00:00 2001
From: Peter Reid <peter.d.reid@gmail.com>
Date: Sat, 17 Jan 2015 23:10:05 -0500
Subject: [PATCH] Handle overlapping border-radius corners

Conforming to section 5.5 (Rounded Corners/Overlapping Curves) of "CSS
Background and Borders Module Level 3", border radii on elements whose
border curves would have overlapped are uniformly scaled down to the
point that they no longer do.

http://dev.w3.org/csswg/css-backgrounds/#corner-overlap
---
 components/layout/display_list_builder.rs    | 36 ++++++++++++++++++--
 tests/ref/basic.list                         |  1 +
 tests/ref/border_radius_overlapping_a.html   |  6 ++++
 tests/ref/border_radius_overlapping_ref.html |  6 ++++
 4 files changed, 46 insertions(+), 3 deletions(-)
 create mode 100644 tests/ref/border_radius_overlapping_a.html
 create mode 100644 tests/ref/border_radius_overlapping_ref.html

diff --git a/components/layout/display_list_builder.rs b/components/layout/display_list_builder.rs
index 44d9a286bf2..1c81e65459b 100644
--- a/components/layout/display_list_builder.rs
+++ b/components/layout/display_list_builder.rs
@@ -40,7 +40,7 @@ use servo_msg::constellation_msg::Msg as ConstellationMsg;
 use servo_msg::constellation_msg::ConstellationChan;
 use servo_net::image::holder::ImageHolder;
 use servo_util::cursor::Cursor;
-use servo_util::geometry::{self, Au, to_px};
+use servo_util::geometry::{self, Au, to_px, to_frac_px};
 use servo_util::logical_geometry::{LogicalPoint, LogicalRect, LogicalSize};
 use servo_util::opts;
 use std::default::Default;
@@ -218,12 +218,42 @@ pub trait FragmentDisplayListBuilding {
                                                   clip: &ClippingRegion);
 }
 
+fn handle_overlapping_radii(size: &Size2D<Au>, radii: &BorderRadii<Au>) -> BorderRadii<Au> {
+    // No two corners' border radii may add up to more than the length of the edge
+    // between them. To prevent that, all radii are scaled down uniformly.
+    fn scale_factor(radius_a: Au, radius_b: Au, edge_length: Au) -> f64 {
+        let required = radius_a + radius_b;
+
+        if required <= edge_length {
+            1.0
+        } else {
+            to_frac_px(edge_length) / to_frac_px(required)
+        }
+    }
+
+    let top_factor = scale_factor(radii.top_left, radii.top_right, size.width);
+    let bottom_factor = scale_factor(radii.bottom_left, radii.bottom_right, size.width);
+    let left_factor = scale_factor(radii.top_left, radii.bottom_left, size.height);
+    let right_factor = scale_factor(radii.top_right, radii.bottom_right, size.height);
+    let min_factor = top_factor.min(bottom_factor).min(left_factor).min(right_factor);
+    if min_factor < 1.0 {
+        BorderRadii {
+            top_left:     radii.top_left    .scale_by(min_factor),
+            top_right:    radii.top_right   .scale_by(min_factor),
+            bottom_left:  radii.bottom_left .scale_by(min_factor),
+            bottom_right: radii.bottom_right.scale_by(min_factor),
+        }
+    } else {
+        *radii
+    }
+}
+
 fn build_border_radius(abs_bounds: &Rect<Au>, border_style: &Border) -> BorderRadii<Au> {
     // TODO(cgaebel): Support border radii even in the case of multiple border widths.
     // This is an extension of supporting elliptical radii. For now, all percentage
     // radii will be relative to the width.
 
-    BorderRadii {
+    handle_overlapping_radii(&abs_bounds.size, &BorderRadii {
         top_left:     model::specified(border_style.border_top_left_radius,
                                        abs_bounds.size.width),
         top_right:    model::specified(border_style.border_top_right_radius,
@@ -232,7 +262,7 @@ fn build_border_radius(abs_bounds: &Rect<Au>, border_style: &Border) -> BorderRa
                                        abs_bounds.size.width),
         bottom_left:  model::specified(border_style.border_bottom_left_radius,
                                        abs_bounds.size.width),
-    }
+    })
 }
 
 impl FragmentDisplayListBuilding for Fragment {
diff --git a/tests/ref/basic.list b/tests/ref/basic.list
index e53bcc6da93..3587b59e181 100644
--- a/tests/ref/basic.list
+++ b/tests/ref/basic.list
@@ -223,6 +223,7 @@ fragment=top != ../html/acid2.html acid2_ref.html
 != inset_blackborder.html blackborder_ref.html
 != outset_blackborder.html blackborder_ref.html
 == border_radius_clip_a.html border_radius_clip_ref.html
+== border_radius_overlapping_a.html border_radius_overlapping_ref.html
 == stacking_context_overflow_a.html stacking_context_overflow_ref.html
 == stacking_context_overflow_relative_outline_a.html stacking_context_overflow_relative_outline_ref.html
 == word_break_a.html word_break_ref.html
diff --git a/tests/ref/border_radius_overlapping_a.html b/tests/ref/border_radius_overlapping_a.html
new file mode 100644
index 00000000000..58baa1720ca
--- /dev/null
+++ b/tests/ref/border_radius_overlapping_a.html
@@ -0,0 +1,6 @@
+<!DOCTYPE html>
+<html>
+    <body>
+        <div style='box-sizing:border-box; width:6em; height:2em; border-radius:.5em 2em .5em 2em; background-color:red;'></div>
+    </body>
+</html>
diff --git a/tests/ref/border_radius_overlapping_ref.html b/tests/ref/border_radius_overlapping_ref.html
new file mode 100644
index 00000000000..287a3d52085
--- /dev/null
+++ b/tests/ref/border_radius_overlapping_ref.html
@@ -0,0 +1,6 @@
+<!DOCTYPE html>
+<html>
+    <body>
+        <div style='box-sizing:border-box; width:6em; height:2em; border-radius:.4em 1.6em .4em 1.6em; background-color:red;'></div>
+    </body>
+</html>