Bug 1946361 - Fallback to a transparent image for missing snapshots. r=gw

A followup patch will add debug prefs to panic or use pink instead of transparent
for the fallback image.

Differential Revision: https://phabricator.services.mozilla.com/D237007

gfx/wr/webrender/src/image_source.rs

@@ -93,5 +93,5 @@ pub fn resolve_cached_render_task(
     let rt_cache_entry = resource_cache
         .get_cached_render_task(&handle);
 
-    resource_cache.get_texture_cache_item(&rt_cache_entry.handle)
+    resource_cache.get_texture_cache_item(&rt_cache_entry.handle).unwrap()
 }

gfx/wr/webrender/src/resource_cache.rs

@@ -501,6 +501,21 @@ pub struct ResourceCache {
 
     /// A pool of render targets for use by the render task graph
     render_target_pool: Vec<RenderTarget>,
+
+    /// An empty (1x1 transparent) image used when a stacking context snapshot
+    /// is missing.
+    ///
+    /// For now it acts as a catch-all solution for cases where WebRender fails
+    /// to produce a texture cache item for a snapshotted tacking context.
+    /// These cases include:
+    /// - Empty stacking contexts.
+    /// - Stacking contexts that are more aggressively culled out than they
+    ///   should, for example when they are in a perspective transform that
+    ///   cannot be projected to screen space.
+    /// - Likely other cases we have not found yet.
+    /// Over time it would be better to handle each of these cases explicitly
+    /// and make it a hard error to fail to snapshot a stacking context.
+    fallback_handle: TextureCacheHandle,
 }
 
 impl ResourceCache {
@@ -538,6 +553,7 @@ impl ResourceCache {
             image_templates_memory: 0,
             font_templates_memory: 0,
             render_target_pool: Vec::new(),
+            fallback_handle: TextureCacheHandle::invalid(),
         }
     }
 
@@ -661,7 +677,7 @@ impl ResourceCache {
             self.texture_cache.shared_color_expected_format(),
             flags,
         );
- 
+
         // Allocate space in the texture cache, but don't supply
         // and CPU-side data to be uploaded.
         let user_data = [0.0; 4];
@@ -1342,7 +1358,19 @@ impl ResourceCache {
     pub fn get_cached_image(&self, request: ImageRequest) -> Result<CacheItem, ()> {
         debug_assert_eq!(self.state, State::QueryResources);
         let image_info = self.get_image_info(request)?;
-        Ok(self.get_texture_cache_item(&image_info.texture_cache_handle))
+
+        if let Ok(item) = self.get_texture_cache_item(&image_info.texture_cache_handle) {
+            // Common path.
+            return Ok(item);
+        }
+
+        if self.resources.image_templates
+            .get(request.key)
+            .map_or(false, |img| img.data.is_snapshot()) {
+            return self.get_texture_cache_item(&self.fallback_handle);
+        }
+
+        panic!("Requested image missing from the texture cache");
     }
 
     pub fn get_cached_render_task(
@@ -1364,8 +1392,12 @@ impl ResourceCache {
     }
 
     #[inline]
-    pub fn get_texture_cache_item(&self, handle: &TextureCacheHandle) -> CacheItem {
-        self.texture_cache.get(handle)
+    pub fn get_texture_cache_item(&self, handle: &TextureCacheHandle) -> Result<CacheItem, ()> {
+        if let Some(item) = self.texture_cache.try_get(handle) {
+            return Ok(item);
+        }
+
+        Err(())
     }
 
     pub fn get_image_properties(&self, image_key: ImageKey) -> Option<ImageProperties> {
@@ -1480,6 +1512,29 @@ impl ResourceCache {
 
     fn update_texture_cache(&mut self, gpu_cache: &mut GpuCache) {
         profile_scope!("update_texture_cache");
+
+        if self.fallback_handle == TextureCacheHandle::invalid() {
+            self.texture_cache.update(
+                &mut self.fallback_handle,
+                ImageDescriptor {
+                    size: size2(1, 1),
+                    stride: None,
+                    format: ImageFormat::BGRA8,
+                    flags: ImageDescriptorFlags::empty(),
+                    offset: 0,
+                },
+                TextureFilter::Linear,
+                Some(CachedImageData::Raw(Arc::new(vec![0, 0, 0, 0]))),
+                [0.0; 4],
+                DirtyRect::All,
+                gpu_cache,
+                None,
+                UvRectKind::Rect,
+                Eviction::Manual,
+                TargetShader::Default,
+            );
+        }
+
         for request in self.pending_image_requests.drain() {
             let image_template = self.resources.image_templates.get_mut(request.key).unwrap();
             debug_assert!(image_template.data.uses_texture_cache());

gfx/wr/webrender/src/texture_cache.rs

@@ -774,7 +774,7 @@ impl TextureCache {
         // Number of moved pixels after which we stop attempting to move more items for this frame.
         // The constant is up for adjustment, the main goal is to avoid causing frame spikes on
         // low end GPUs.
-        let area_threshold = 512*512; 
+        let area_threshold = 512*512;
 
         let mut changes = Vec::new();
         allocator_lists[idx].try_compaction(area_threshold, &mut changes);
@@ -1016,6 +1016,36 @@ impl TextureCache {
         }
     }
 
+    pub fn try_get(&self, handle: &TextureCacheHandle) -> Option<CacheItem> {
+        let (texture_id, uv_rect, swizzle, uv_rect_handle, user_data) = self.try_get_cache_location(handle)?;
+        Some(CacheItem {
+            uv_rect_handle,
+            texture_id: TextureSource::TextureCache(
+                texture_id,
+                swizzle,
+            ),
+            uv_rect,
+            user_data,
+        })
+    }
+
+    pub fn try_get_cache_location(
+        &self,
+        handle: &TextureCacheHandle,
+    ) -> Option<(CacheTextureId, DeviceIntRect, Swizzle, GpuCacheHandle, [f32; 4])> {
+        let entry = self.get_entry_opt(handle)?;
+
+        debug_assert_eq!(entry.last_access, self.now);
+        let origin = entry.details.describe();
+        Some((
+            entry.texture_id,
+            DeviceIntRect::from_origin_and_size(origin, entry.size),
+            entry.swizzle,
+            entry.uv_rect_handle,
+            entry.user_data,
+        ))
+    }
+
     /// A more detailed version of get(). This allows access to the actual
     /// device rect of the cache allocation.
     ///

gfx/wr/wrench/reftests/image/empty.yaml

@@ -0,0 +1,7 @@
+---
+root:
+  items:
+    -
+      bounds: [200, 200, 1000, 1000]
+      type: "stacking-context"
+      items:

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

@@ -26,3 +26,9 @@ fuzzy(1,160000) == image-filter-stretch-tile.yaml green-alpha-ref.yaml
 == snapshot-filters-02.yaml snapshot-filters-02-ref.yaml
 fuzzy(3,3000) == snapshot-shadow.yaml snapshot-shadow-ref.yaml
 == snapshot-multiframe.yaml snapshot-multiframe-ref.yaml
+== snapshot-empty.yaml empty.yaml
+# TODO: At the moment snapshot-perspective-01.yaml renders incorrectly, so the
+# reftest acts more as a crash test. When bug 1941577 is fixed the reftest
+# reference should be updated to reflect that something needs to be rendered
+# instead of leaving the snapshot empty.
+== snapshot-perspective-01.yaml empty.yaml

gfx/wr/wrench/reftests/image/snapshot-empty.yaml

@@ -0,0 +1,17 @@
+---
+root:
+  items:
+    -
+      bounds: [200, 200, 1000, 1000]
+      type: "stacking-context"
+      perspective: 256
+      items:
+        -
+          bounds: [0, 0, 100, 100]
+          type: "stacking-context"
+          snapshot:
+            name: "snap0"
+            area: [0, 0, 100, 100]
+          items:
+    - image: snapshot(snap0)
+      bounds: [10, 10, 200, 200]

gfx/wr/wrench/reftests/image/snapshot-perspective-01.yaml

@@ -0,0 +1,33 @@
+---
+root:
+  items:
+    -
+      bounds: [200, 200, 1000, 1000]
+      type: "stacking-context"
+      perspective: 256
+      items:
+        -
+          bounds: [128, 128, 256, 256]
+          type: "stacking-context"
+          transform: rotate-x(-60) rotate-y(-120)
+          snapshot:
+            name: "snap0"
+            area: [0, 0, 100, 200]
+          items:
+            - type: clip
+              id: 101
+              complex:
+                - rect: [0, 0, 100, 200]
+                  radius: [32, 32]
+            - type: clip-chain
+              id: 201
+              clips: [101]
+            -
+              bounds: [0, 0, 100, 200]
+              type: rect
+              color: blue
+              clip-chain: 201
+    - image: snapshot(snap0)
+      bounds: [300, 0, 100, 200]
+
+