Composite multiple layers in Windows software rendering (#51759)

Blend pixels per-alpha when presenting multiple layers from the software
compositor.

Part of flutter/flutter#143375

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---------

Co-authored-by: Loïc Sharma <[email protected]>

shell/platform/windows/compositor_software.cc

@@ -9,6 +9,78 @@
 
 namespace flutter {
 
+constexpr int kOpaqueBlack = 0xff000000;
+
+namespace {
+
+/// Calculate the minimum and maximum x and y coordinates to enclose all layers.
+FlutterRect CalculateBounds(const FlutterLayer** layers, size_t layers_count) {
+  double x_min = HUGE_VAL;
+  double x_max = -HUGE_VAL;
+  double y_min = HUGE_VAL;
+  double y_max = -HUGE_VAL;
+  for (const FlutterLayer** layer = layers; layer < layers + layers_count;
+       layer++) {
+    const FlutterPoint& offset = (*layer)->offset;
+    const FlutterSize& size = (*layer)->size;
+    x_min = std::min(x_min, offset.x);
+    y_min = std::min(y_min, offset.y);
+    x_max = std::max(x_max, offset.x + size.width);
+    y_max = std::max(y_max, offset.y + size.height);
+  }
+  return FlutterRect{x_min, y_min, x_max, y_max};
+}
+
+/// Blend layer in-place onto allocation, which already holds the previous
+/// results of composition.
+void BlendLayer(std::vector<uint32_t>& allocation,
+                const FlutterLayer& layer,
+                int x_min,
+                int y_min,
+                int width,
+                int height) {
+  FML_DCHECK(layer.type == kFlutterLayerContentTypeBackingStore);
+  auto& backing_store = *layer.backing_store;
+  FML_DCHECK(backing_store.type == kFlutterBackingStoreTypeSoftware);
+  auto src_data =
+      static_cast<const uint32_t*>(backing_store.software.allocation);
+  const FlutterPoint& offset = layer.offset;
+  const FlutterSize& size = layer.size;
+
+  // Bounds for iteration to prevent out-of-bounds destination coordinates.
+  int y_src_min = std::max(0., y_min - offset.y);
+  int y_src_max = std::min(size.height, height + y_min - offset.y);
+  int x_src_min = std::max(0., x_min - offset.x);
+  int x_src_max = std::min(size.width, width + x_min - offset.x);
+  for (int y_src = y_src_min; y_src < y_src_max; y_src++) {
+    int y_dst = y_src + offset.y - y_min;
+    for (int x_src = x_src_min; x_src < x_src_max; x_src++) {
+      int x_dst = x_src + offset.x + x_min;
+      size_t i_src = y_src * size.width + x_src;
+      size_t i_dst = y_dst * width + x_dst;
+      uint32_t src = src_data[i_src];
+      uint32_t dst = allocation[i_dst];
+
+      int r_src = (src >> 0) & 0xff;
+      int g_src = (src >> 8) & 0xff;
+      int b_src = (src >> 16) & 0xff;
+      int a_src = (src >> 24) & 0xff;
+
+      int r_dst = (dst >> 0) & 0xff;
+      int g_dst = (dst >> 8) & 0xff;
+      int b_dst = (dst >> 16) & 0xff;
+
+      int r = (r_dst * 255 + (r_src - r_dst) * a_src) / 255;
+      int g = (g_dst * 255 + (g_src - g_dst) * a_src) / 255;
+      int b = (b_dst * 255 + (b_src - b_dst) * a_src) / 255;
+
+      allocation[i_dst] = (r << 0) | (g << 8) | (b << 16) | (0xff << 24);
+    }
+  }
+}
+
+}  // namespace
+
 CompositorSoftware::CompositorSoftware() {}
 
 bool CompositorSoftware::CreateBackingStore(
@@ -47,18 +119,46 @@ bool CompositorSoftware::Present(FlutterWindowsView* view,
     return view->ClearSoftwareBitmap();
   }
 
-  // TODO: Support compositing layers and platform views.
-  // See: https://github.com/flutter/flutter/issues/31713
-  FML_DCHECK(layers_count == 1);
-  FML_DCHECK(layers[0]->offset.x == 0 && layers[0]->offset.y == 0);
-  FML_DCHECK(layers[0]->type == kFlutterLayerContentTypeBackingStore);
-  FML_DCHECK(layers[0]->backing_store->type ==
-             kFlutterBackingStoreTypeSoftware);
+  // Bypass composition logic if there is only one layer.
+  if (layers_count == 1) {
+    const FlutterLayer* layer = layers[0];
+    FML_DCHECK(layer != nullptr);
+    if (layer->type == kFlutterLayerContentTypeBackingStore &&
+        layer->offset.x == 0 && layer->offset.y == 0) {
+      auto& backing_store = *layer->backing_store;
+      FML_DCHECK(backing_store.type == kFlutterBackingStoreTypeSoftware);
+      auto& software = backing_store.software;
+      return view->PresentSoftwareBitmap(software.allocation,
+                                         software.row_bytes, software.height);
+    }
+  }
+
+  // Composite many layers.
+  FlutterRect bounds = CalculateBounds(layers, layers_count);
+  // Truncate from double to integer to represent whole pixels.
+  int x_min = static_cast<int>(bounds.left);
+  int x_max = static_cast<int>(bounds.right);
+  int y_min = static_cast<int>(bounds.top);
+  int y_max = static_cast<int>(bounds.bottom);
 
-  const auto& backing_store = layers[0]->backing_store->software;
+  int width = x_max - x_min;
+  int height = y_max - y_min;
+  std::vector<uint32_t> allocation(width * height, kOpaqueBlack);
+
+  for (const FlutterLayer** layer = layers; layer < layers + layers_count;
+       layer++) {
+    // TODO(schectman): handle platform view type layers.
+    // https://github.com/flutter/flutter/issues/143375
+    if ((*layer)->type == kFlutterLayerContentTypeBackingStore) {
+      BlendLayer(allocation, **layer, x_min, y_min, width, height);
+    } else {
+      FML_UNREACHABLE();
+      return false;
+    }
+  }
 
-  return view->PresentSoftwareBitmap(
-      backing_store.allocation, backing_store.row_bytes, backing_store.height);
+  return view->PresentSoftwareBitmap(static_cast<void*>(allocation.data()),
+                                     width * sizeof(uint32_t), height);
 }
 
 }  // namespace flutter

shell/platform/windows/compositor_software_unittests.cc

@@ -107,5 +107,114 @@ TEST_F(CompositorSoftwareTest, PresentEmpty) {
   EXPECT_TRUE(compositor.Present(view(), nullptr, 0));
 }
 
+// Test compositing an upper layer on a base layer, each 2x2 pixels.
+// Base layer has opaque pixel values:
+// BLACK  RED
+// GREEN  WHITE
+// Overlay layer has pixel values:
+// RED: 127   WHITE: 0
+// BLUE: 127  BLACK: 255
+TEST_F(CompositorSoftwareTest, PresentMultiLayers) {
+  UseEngineWithView();
+
+  CompositorSoftware compositor;
+
+  FlutterBackingStoreConfig config = {sizeof(config), {2, 2}};
+  FlutterBackingStore backing_store0 = {sizeof(FlutterBackingStore), nullptr};
+  FlutterBackingStore backing_store1 = {sizeof(FlutterBackingStore), nullptr};
+
+  ASSERT_TRUE(compositor.CreateBackingStore(config, &backing_store0));
+  ASSERT_TRUE(compositor.CreateBackingStore(config, &backing_store1));
+
+  uint32_t pixels0[4] = {0xff000000, 0xff0000ff, 0xff00ff00, 0xffffffff};
+  uint32_t pixels1[4] = {0x7f0000ff, 0x00ffffff, 0x7fff0000, 0xff000000};
+
+  std::memcpy(const_cast<void*>(backing_store0.software.allocation), pixels0,
+              sizeof(uint32_t) * 4);
+  std::memcpy(const_cast<void*>(backing_store1.software.allocation), pixels1,
+              sizeof(uint32_t) * 4);
+
+  FlutterLayer layer0 = {};
+  layer0.type = kFlutterLayerContentTypeBackingStore;
+  layer0.backing_store = &backing_store0;
+  layer0.offset = {0, 0};
+  layer0.size = {2, 2};
+
+  FlutterLayer layer1 = layer0;
+  layer1.backing_store = &backing_store1;
+  const FlutterLayer* layer_ptr[2] = {&layer0, &layer1};
+
+  EXPECT_CALL(*view(), PresentSoftwareBitmap)
+      .WillOnce([&](const void* allocation, size_t row_bytes, size_t height) {
+        auto pixel_data = static_cast<const uint32_t*>(allocation);
+        EXPECT_EQ(row_bytes, 2 * sizeof(uint32_t));
+        EXPECT_EQ(height, 2);
+        EXPECT_EQ(pixel_data[0], 0xff00007f);
+        EXPECT_EQ(pixel_data[1], 0xff0000ff);
+        EXPECT_EQ(pixel_data[2], 0xff7f8000);
+        EXPECT_EQ(pixel_data[3], 0xff000000);
+        return true;
+      });
+  EXPECT_TRUE(compositor.Present(view(), layer_ptr, 2));
+
+  ASSERT_TRUE(compositor.CollectBackingStore(&backing_store0));
+  ASSERT_TRUE(compositor.CollectBackingStore(&backing_store1));
+}
+
+// Test compositing layers with offsets.
+// 0th layer is a single red pixel in the top-left.
+// 1st layer is a row of two blue pixels on the second row.
+TEST_F(CompositorSoftwareTest, PresentOffsetLayers) {
+  UseEngineWithView();
+
+  CompositorSoftware compositor;
+
+  FlutterBackingStoreConfig config0 = {sizeof(FlutterBackingStoreConfig),
+                                       {1, 1}};
+  FlutterBackingStore backing_store0 = {sizeof(FlutterBackingStore), nullptr};
+  FlutterBackingStoreConfig config1 = {sizeof(FlutterBackingStoreConfig),
+                                       {2, 1}};
+  FlutterBackingStore backing_store1 = {sizeof(FlutterBackingStore), nullptr};
+
+  ASSERT_TRUE(compositor.CreateBackingStore(config0, &backing_store0));
+  ASSERT_TRUE(compositor.CreateBackingStore(config1, &backing_store1));
+
+  uint32_t pixels0 = 0xff0000ff;
+  uint32_t pixels1[2] = {0xffff0000, 0xffff0000};
+
+  std::memcpy(const_cast<void*>(backing_store0.software.allocation), &pixels0,
+              sizeof(uint32_t) * 1);
+  std::memcpy(const_cast<void*>(backing_store1.software.allocation), pixels1,
+              sizeof(uint32_t) * 2);
+
+  FlutterLayer layer0 = {};
+  layer0.type = kFlutterLayerContentTypeBackingStore;
+  layer0.backing_store = &backing_store0;
+  layer0.offset = {0, 0};
+  layer0.size = {1, 1};
+
+  FlutterLayer layer1 = layer0;
+  layer1.backing_store = &backing_store1;
+  layer1.offset = {0, 1};
+  layer1.size = {2, 1};
+  const FlutterLayer* layer_ptr[2] = {&layer0, &layer1};
+
+  EXPECT_CALL(*view(), PresentSoftwareBitmap)
+      .WillOnce([&](const void* allocation, size_t row_bytes, size_t height) {
+        auto pixel_data = static_cast<const uint32_t*>(allocation);
+        EXPECT_EQ(row_bytes, 2 * sizeof(uint32_t));
+        EXPECT_EQ(height, 2);
+        EXPECT_EQ(pixel_data[0], 0xff0000ff);
+        EXPECT_EQ(pixel_data[1], 0xff000000);
+        EXPECT_EQ(pixel_data[2], 0xffff0000);
+        EXPECT_EQ(pixel_data[3], 0xffff0000);
+        return true;
+      });
+  EXPECT_TRUE(compositor.Present(view(), layer_ptr, 2));
+
+  ASSERT_TRUE(compositor.CollectBackingStore(&backing_store0));
+  ASSERT_TRUE(compositor.CollectBackingStore(&backing_store1));
+}
+
 }  // namespace testing
 }  // namespace flutter