Simplify and extend trans-stacking-context subpixel accumulation

Simplify the situations in which subpixels are accumulated, so that it
is only done for translation or identity transformation matrices. Also,
apply accumulated subpixels to more operations in PaintContext. This
fixes several pre-existing reftests and hopefully will eliminate
off-by-one errors in flaky reftests.

Fixes #10881.

5 files changed, 40 insertions, 35 deletions

diff --git a/components/gfx/display_list/mod.rs b/components/gfx/display_list/mod.rs
index 0073b1b0a69..c7143457db9 100644
--- a/components/gfx/display_list/mod.rs
+++ b/components/gfx/display_list/mod.rs
@@ -18,7 +18,7 @@ use app_units::Au;
 use azure::azure::AzFloat;
 use azure::azure_hl::Color;
 use euclid::approxeq::ApproxEq;
-use euclid::num::Zero;
+use euclid::num::{One, Zero};
 use euclid::rect::TypedRect;
 use euclid::side_offsets::SideOffsets2D;
 use euclid::{Matrix2D, Matrix4D, Point2D, Rect, Size2D};
@@ -457,20 +457,18 @@ impl DisplayList {
                 let transform = transform.translate(pixel_snapped_origin.x as AzFloat,
                                                     pixel_snapped_origin.y as AzFloat,
                                                     0.0).mul(&stacking_context.transform);
-                let inverse_transform = transform.invert();
-
-                // Here we are trying to accumulate any subpixel distances across transformed
-                // stacking contexts. This allows us transform stacking context with a
-                // pixel-snapped transform, but continue to propagate any subpixels from stacking
-                // context origins to children.
-                let subpixel_offset = Point2D::new(origin.x.to_f32_px() - pixel_snapped_origin.x,
-                                                   origin.y.to_f32_px() - pixel_snapped_origin.y);
-                let subpixel_offset = inverse_transform.transform_point(&subpixel_offset) -
-                                      inverse_transform.transform_point(&Point2D::zero());;
-                let subpixel_offset = Point2D::new(Au::from_f32_px(subpixel_offset.x),
-                                                   Au::from_f32_px(subpixel_offset.y));
-
-                (transform, subpixel_offset)
+
+                if transform.is_identity_or_simple_translation() {
+                    let pixel_snapped_origin = Point2D::new(Au::from_f32_px(pixel_snapped_origin.x),
+                                                            Au::from_f32_px(pixel_snapped_origin.y));
+                    (transform, origin - pixel_snapped_origin)
+                } else {
+                    // In the case of a more complicated transformation, don't attempt to
+                    // preserve subpixel offsets. This causes problems with reference tests
+                    // that do scaling and rotation and it's unclear if we even want to be doing
+                    // this.
+                    (transform, Point2D::zero())
+                }
             }
         };
 
@@ -1462,3 +1460,18 @@ impl WebRenderImageInfo {
 /// The type of the scroll offset list. This is only populated if WebRender is in use.
 pub type ScrollOffsetMap = HashMap<StackingContextId, Point2D<f32>>;
 
+
+pub trait SimpleMatrixDetection {
+    fn is_identity_or_simple_translation(&self) -> bool;
+}
+
+impl SimpleMatrixDetection for Matrix4D<f32> {
+    #[inline]
+    fn is_identity_or_simple_translation(&self) -> bool {
+        let (_0, _1) = (Zero::zero(), One::one());
+        self.m11 == _1 && self.m12 == _0 && self.m13 == _0 && self.m14 == _0 &&
+        self.m21 == _0 && self.m22 == _1 && self.m23 == _0 && self.m24 == _0 &&
+        self.m31 == _0 && self.m32 == _0 && self.m33 == _1 && self.m34 == _0 &&
+        self.m44 == _1
+    }
+}
diff --git a/components/gfx/paint_context.rs b/components/gfx/paint_context.rs
index 15c4b169ba4..acfa5186fe1 100644
--- a/components/gfx/paint_context.rs
+++ b/components/gfx/paint_context.rs
@@ -122,6 +122,10 @@ struct CornerOrigin {
 }
 
 impl<'a> PaintContext<'a> {
+    pub fn to_nearest_azure_rect(&self, rect: &Rect<Au>) -> Rect<AzFloat> {
+        rect.translate(&self.subpixel_offset).to_nearest_azure_rect(self.screen_pixels_per_px())
+    }
+
     pub fn screen_pixels_per_px(&self) -> ScaleFactor<PagePx, ScreenPx, f32> {
         self.screen_rect.as_f32().size.width / self.page_rect.size.width
     }
@@ -132,7 +136,7 @@ impl<'a> PaintContext<'a> {
 
     pub fn draw_solid_color(&self, bounds: &Rect<Au>, color: Color) {
         self.draw_target.make_current();
-        self.draw_target.fill_rect(&bounds.to_nearest_azure_rect(self.screen_pixels_per_px()),
+        self.draw_target.fill_rect(&self.to_nearest_azure_rect(&bounds),
                                    PatternRef::Color(&ColorPattern::new(color)),
                                    None);
     }
@@ -160,7 +164,7 @@ impl<'a> PaintContext<'a> {
     }
 
     pub fn draw_push_clip(&self, bounds: &Rect<Au>) {
-        let rect = bounds.to_nearest_azure_rect(self.screen_pixels_per_px());
+        let rect = self.to_nearest_azure_rect(bounds);
         let path_builder = self.draw_target.create_path_builder();
 
         let left_top = Point2D::new(rect.origin.x, rect.origin.y);
@@ -211,7 +215,7 @@ impl<'a> PaintContext<'a> {
         let source_rect = Rect::new(Point2D::new(0.0, 0.0),
                                     Size2D::new(image_info.width as AzFloat,
                                                 image_info.height as AzFloat));
-        let dest_rect = bounds.to_nearest_azure_rect(scale);
+        let dest_rect = self.to_nearest_azure_rect(bounds);
 
         // TODO(pcwalton): According to CSS-IMAGES-3 § 5.3, nearest-neighbor interpolation is a
         // conforming implementation of `crisp-edges`, but it is not the best we could do.
@@ -1127,7 +1131,7 @@ impl<'a> PaintContext<'a> {
                                   radius: &BorderRadii<AzFloat>,
                                   color: Color,
                                   dash_size: DashSize) {
-        let rect = bounds.to_nearest_azure_rect(self.screen_pixels_per_px());
+        let rect = self.to_nearest_azure_rect(bounds);
         let draw_opts = DrawOptions::new(1.0, CompositionOp::Over, AntialiasMode::None);
         let border_width = match direction {
             Direction::Top => border.top,
@@ -1195,7 +1199,7 @@ impl<'a> PaintContext<'a> {
                                  border: &SideOffsets2D<f32>,
                                  radius: &BorderRadii<AzFloat>,
                                  color: Color) {
-        let rect = bounds.to_nearest_azure_rect(self.screen_pixels_per_px());
+        let rect = self.to_nearest_azure_rect(bounds);
         self.draw_border_path(&rect, direction, border, radius, color);
     }
 
@@ -1203,7 +1207,7 @@ impl<'a> PaintContext<'a> {
                              bounds: &Rect<Au>,
                              border: &SideOffsets2D<f32>,
                              shrink_factor: f32) -> Rect<f32> {
-        let rect            = bounds.to_nearest_azure_rect(self.screen_pixels_per_px());
+        let rect            = self.to_nearest_azure_rect(bounds);
         let scaled_border   = SideOffsets2D::new(shrink_factor * border.top,
                                                  shrink_factor * border.right,
                                                  shrink_factor * border.bottom,
@@ -1406,7 +1410,7 @@ impl<'a> PaintContext<'a> {
                                                  &end_point.to_nearest_azure_point(scale),
                                                  stops,
                                                  &Matrix2D::identity());
-        self.draw_target.fill_rect(&bounds.to_nearest_azure_rect(scale),
+        self.draw_target.fill_rect(&self.to_nearest_azure_rect(&bounds),
                                    PatternRef::LinearGradient(&pattern),
                                    None);
     }
@@ -1632,7 +1636,7 @@ impl<'a> PaintContext<'a> {
         self.draw_push_clip(&clip_region.main);
         for complex_region in &clip_region.complex {
             // FIXME(pcwalton): Actually draw a rounded rect.
-            self.push_rounded_rect_clip(&complex_region.rect.to_nearest_azure_rect(scale),
+            self.push_rounded_rect_clip(&self.to_nearest_azure_rect(&complex_region.rect),
                                         &complex_region.radii.to_radii_pixels(scale))
         }
         self.transient_clip = Some(clip_region)
diff --git a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini b/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini
deleted file mode 100644
index 4dc53b8f843..00000000000
--- a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[transform-input-017.htm]
-  type: reftest
-  expected: FAIL
-  bug: https://github.com/servo/servo/issues/10881
diff --git a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini b/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini
deleted file mode 100644
index 69c5b778d30..00000000000
--- a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[transform-input-018.htm]
-  type: reftest
-  expected: FAIL
-  bug: https://github.com/servo/servo/issues/10881
diff --git a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-019.htm.ini b/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-019.htm.ini
deleted file mode 100644
index 8e2a7b49f90..00000000000
--- a/tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-019.htm.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[transform-input-019.htm]
-  type: reftest
-  expected:
-    if os == "linux": FAIL