Bug 1559861: WR - Scale picture tasks based on their surface to parent transform scale factors r=gw,Bert

This patch adjusts the device pixel scale for raster root surfaces so that the surface's content is not
pixelated/blurred when it is composited into its parent surface where a transform scales up the surface.

This only fixes the referenced bug when WebRender is used.

Differential Revision: https://phabricator.services.mozilla.com/D75236

gfx/wr/webrender/src/picture.rs

@@ -5941,16 +5941,26 @@ impl PicturePrimitive {
                 false
             };
 
+            let surface_to_parent_transform = frame_context.spatial_tree
+                .get_relative_transform(surface_spatial_node_index, parent_raster_node_index);
+
             // Check if there is perspective or if an SVG filter is applied, and thus whether a new
             // rasterization root should be established.
-            let establishes_raster_root = has_svg_filter || frame_context.spatial_tree
-                .get_relative_transform(surface_spatial_node_index, parent_raster_node_index)
-                .is_perspective();
+            let establishes_raster_root = has_svg_filter || surface_to_parent_transform.is_perspective();
 
-            let raster_spatial_node_index = if establishes_raster_root {
-                surface_spatial_node_index
+            let (raster_spatial_node_index, device_pixel_scale) = if establishes_raster_root {
+                // If a raster root is established, this surface should be scaled based on the scale factors of the surface raster to parent raster transform.
+                // This scaling helps ensure that the content in this surface does not become blurry or pixelated when composited in the parent surface.
+                let scale_factors = surface_to_parent_transform.scale_factors();
+
+                // Pick the largest scale factor of the transform for the scaling factor.
+                // Currently, we ensure that the scaling factor is >= 1.0 as a smaller scale factor can result in blurry output.
+                let scaling_factor = scale_factors.0.max(scale_factors.1).max(1.0);
+
+                let device_pixel_scale = frame_context.global_device_pixel_scale * Scale::new(scaling_factor);
+                (surface_spatial_node_index, device_pixel_scale)
             } else {
-                parent_raster_node_index
+                (parent_raster_node_index, frame_context.global_device_pixel_scale)
             };
 
             let scale_factors = frame_context
@@ -6002,7 +6012,7 @@ impl PicturePrimitive {
                 inflation_factor,
                 frame_context.global_screen_world_rect,
                 &frame_context.spatial_tree,
-                frame_context.global_device_pixel_scale,
+                device_pixel_scale,
                 scale_factors,
             );
 

gfx/wr/wrench/reftests/transforms/raster-root-scaling-ref.yaml

@@ -0,0 +1,10 @@
+
+# Tests that surfaces created by raster roots are scaled based on the surface to parent transform.
+---
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 0, 0]
+      items:
+        - image: checkerboard(2, 16, 16)
+          bounds: [0, 0, 260, 260]

gfx/wr/wrench/reftests/transforms/raster-root-scaling.yaml

@@ -0,0 +1,16 @@
+# Tests that surfaces created by raster roots are scaled based on the surface to parent transform.
+---
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 0, 0]
+      # Force WebRender to form a raster root
+      transform: perspective(1000)
+      items:
+      - type: stacking-context
+        bounds: [0, 0, 0, 0]
+        transform-style: preserve-3d
+        transform: scale(10,10)
+        items:
+          - image: checkerboard(2, 16, 16)
+            bounds: [0, 0, 26, 26]

gfx/wr/wrench/reftests/transforms/reftest.list

@@ -48,3 +48,4 @@ skip_on(android) fuzzy-range(<=3,*3077,<=10,*133,<=93,*490) == raster_root_A_819
 skip_on(android) fuzzy(60,917) == raster_root_B_8192.yaml raster_root_B_ref.yaml
 # Make sure we don't panic
 != raster-root-large-mask.yaml blank.yaml
+skip_on(android) == raster-root-scaling.yaml raster-root-scaling-ref.yaml