Bug 1918529 - fix some subpixel misalignment issues with gfx.webrender.svg-filter-effects r=gfx-reviewers,gw

Differential Revision: https://phabricator.services.mozilla.com/D223453

Author: ladyhavoc

webrender/src/picture.rs

@@ -4183,6 +4183,8 @@ struct SurfaceAllocInfo {
     // Only used for SVGFEGraph currently, this is the source pixels needed to
     // render the pixels in clipped.
     source: DeviceRect,
+    // Only used for SVGFEGraph, this is the same as clipped before rounding.
+    clipped_notsnapped: DeviceRect,
     clipped_local: PictureRect,
     uv_rect_kind: UvRectKind,
 }
@@ -6378,6 +6380,14 @@ impl PicturePrimitive {
                             )
                         );
 
+                        // Determine the local space to device pixel scaling in the most robust
+                        // way, this accounts for local to device transform and
+                        // device_pixel_scale (if the task is shrunk in get_surface_rects).
+                        let subregion_to_device_scale_x = surface_rects.clipped_notsnapped.width() / surface_rects.clipped_local.width();
+                        let subregion_to_device_scale_y = surface_rects.clipped_notsnapped.height() / surface_rects.clipped_local.height();
+                        let subregion_to_device_offset_x = surface_rects.clipped_notsnapped.min.x - (surface_rects.clipped_local.min.x * subregion_to_device_scale_x).floor();
+                        let subregion_to_device_offset_y = surface_rects.clipped_notsnapped.min.y - (surface_rects.clipped_local.min.y * subregion_to_device_scale_y).floor();
+
                         // Produce the target pixels, this is the result of the
                         // composite op
                         let filter_task_id = request_render_task(
@@ -6396,8 +6406,10 @@ impl PicturePrimitive {
                                     source_subregion.cast_unit(),
                                     target_subregion.cast_unit(),
                                     prim_subregion.cast_unit(),
-                                    surface_rects.clipped.cast_unit(),
-                                    surface_rects.clipped_local.cast_unit(),
+                                    subregion_to_device_scale_x,
+                                    subregion_to_device_scale_y,
+                                    subregion_to_device_offset_x,
+                                    subregion_to_device_offset_y,
                                 )
                             }
                         );
@@ -8151,6 +8163,7 @@ fn get_surface_rects(
         clipped: clipped_snapped,
         unclipped,
         source,
+        clipped_notsnapped: clipped,
         clipped_local,
         uv_rect_kind,
     })

webrender/src/render_task.rs

@@ -1649,8 +1649,10 @@ impl RenderTask {
         source_subregion: LayoutRect,
         target_subregion: LayoutRect,
         prim_subregion: LayoutRect,
-        surface_rects_clipped: LayoutRect,
-        surface_rects_clipped_local: LayoutRect,
+        subregion_to_device_scale_x: f32,
+        subregion_to_device_scale_y: f32,
+        subregion_to_device_offset_x: f32,
+        subregion_to_device_offset_y: f32,
     ) -> RenderTaskId {
         const BUFFER_LIMIT: usize = SVGFE_GRAPH_MAX;
         let mut task_by_buffer_id: [RenderTaskId; BUFFER_LIMIT] = [RenderTaskId::INVALID; BUFFER_LIMIT];
@@ -1699,20 +1701,6 @@ impl RenderTask {
             }
         }
 
-        // Determine the local space to device pixel scaling in the most robust
-        // way, this accounts for local to device transform and
-        // device_pixel_scale (if the task is shrunk in get_surface_rects).
-        //
-        // This has some precision issues because surface_rects_clipped was
-        // rounded already, so it's not exactly the same transform that
-        // get_surface_rects performed, but it is very close, since it is not
-        // quite the same we have to round the offset a certain way to avoid
-        // introducing subpixel offsets caused by the slight deviation.
-        let subregion_to_device_scale_x = surface_rects_clipped.width() / surface_rects_clipped_local.width();
-        let subregion_to_device_scale_y = surface_rects_clipped.height() / surface_rects_clipped_local.height();
-        let subregion_to_device_offset_x = surface_rects_clipped.min.x - (surface_rects_clipped_local.min.x * subregion_to_device_scale_x).floor();
-        let subregion_to_device_offset_y = surface_rects_clipped.min.y - (surface_rects_clipped_local.min.y * subregion_to_device_scale_y).floor();
-
         // Iterate the filter nodes and create tasks
         let mut made_dependency_on_source = false;
         for (filter_index, (filter_node, op)) in filter_nodes.iter().enumerate() {