Bug 1523495 - Adjust shadow blur target sizes to avoid down-scaling artifacts. r=gw

Differential Revision: https://phabricator.services.mozilla.com/D18856

gfx/wr/webrender/src/picture.rs

@@ -2960,7 +2960,7 @@ impl PicturePrimitive {
             PictureCompositeMode::Filter(FilterOp::DropShadow(offset, blur_radius, color)) => {
                 let blur_std_deviation = blur_radius * frame_context.device_pixel_scale.0;
                 let blur_range = (blur_std_deviation * BLUR_SAMPLE_SCALE).ceil() as i32;
-
+                let rounded_std_dev = blur_std_deviation.round();
                 // The clipped field is the part of the picture that is visible
                 // on screen. The unclipped field is the screen-space rect of
                 // the complete picture, if no screen / clip-chain was applied
@@ -2969,10 +2969,13 @@ impl PicturePrimitive {
                 // blur results, inflate that clipped area by the blur range, and
                 // then intersect with the total screen rect, to minimize the
                 // allocation size.
-                let device_rect = clipped
-                    .inflate(blur_range, blur_range)
-                    .intersection(&unclipped.to_i32())
-                    .unwrap();
+                let mut device_rect = clipped.inflate(blur_range, blur_range)
+                        .intersection(&unclipped.to_i32())
+                        .unwrap();
+                device_rect.size = RenderTask::adjusted_blur_source_size(
+                    device_rect.size,
+                    rounded_std_dev,
+                );
 
                 let uv_rect_kind = calculate_uv_rect_kind(
                     &pic_rect,
@@ -2996,7 +2999,7 @@ impl PicturePrimitive {
                 let picture_task_id = frame_state.render_tasks.add(picture_task);
 
                 let blur_render_task = RenderTask::new_blur(
-                    blur_std_deviation.round(),
+                    rounded_std_dev,
                     picture_task_id,
                     frame_state.render_tasks,
                     RenderTargetKind::Color,

gfx/wr/webrender/src/render_task.rs

@@ -626,6 +626,25 @@ impl RenderTask {
         )
     }
 
+    // In order to do the blur down-scaling passes without introducing errors, we need the
+    // source of each down-scale pass to be a multuple of two. If need be, this inflates
+    // the source size so that each down-scale pass will sample correctly.
+    pub fn adjusted_blur_source_size(original_size: DeviceIntSize, mut std_dev: f32) -> DeviceIntSize {
+        let mut adjusted_size = original_size;
+        let mut scale_factor = 1.0;
+        while std_dev > MAX_BLUR_STD_DEVIATION {
+            if adjusted_size.width < MIN_DOWNSCALING_RT_SIZE ||
+               adjusted_size.height < MIN_DOWNSCALING_RT_SIZE {
+                break;
+            }
+            std_dev *= 0.5;
+            scale_factor *= 2.0;
+            adjusted_size = (original_size.to_f32() / scale_factor).ceil().to_i32();
+        }
+
+        adjusted_size * scale_factor as i32
+    }
+
     // Construct a render task to apply a blur to a primitive.
     // The render task chain that is constructed looks like:
     //