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.

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
+                if transform.is_identity_or_simple_translation() {
-                // stacking contexts. This allows us transform stacking context with a
+                    let pixel_snapped_origin = Point2D::new(Au::from_f32_px(pixel_snapped_origin.x),
-                // pixel-snapped transform, but continue to propagate any subpixels from stacking
+                                                            Au::from_f32_px(pixel_snapped_origin.y));
-                // context origins to children.
+                    (transform, origin - pixel_snapped_origin)
-                let subpixel_offset = Point2D::new(origin.x.to_f32_px() - pixel_snapped_origin.x,
+                } else {
-                                                   origin.y.to_f32_px() - pixel_snapped_origin.y);
+                    // In the case of a more complicated transformation, don't attempt to
-                let subpixel_offset = inverse_transform.transform_point(&subpixel_offset) -
+                    // preserve subpixel offsets. This causes problems with reference tests
-                                      inverse_transform.transform_point(&Point2D::zero());;
+                    // that do scaling and rotation and it's unclear if we even want to be doing
-                let subpixel_offset = Point2D::new(Au::from_f32_px(subpixel_offset.x),
+                    // this.
-                                                   Au::from_f32_px(subpixel_offset.y));
+                    (transform, Point2D::zero())
-
+                }
-                (transform, subpixel_offset)
             }
         };
 
@@ -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
+    }
+}

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)

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini

@@ -1,4 +0,0 @@
-[transform-input-017.htm]
-  type: reftest
-  expected: FAIL
-  bug: https://github.com/servo/servo/issues/10881

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini

@@ -1,4 +0,0 @@
-[transform-input-018.htm]
-  type: reftest
-  expected: FAIL
-  bug: https://github.com/servo/servo/issues/10881

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-019.htm.ini

@@ -1,4 +0,0 @@
-[transform-input-019.htm]
-  type: reftest
-  expected:
-    if os == "linux": FAIL