Add viewport and scissor support (flutter/engine#9)

engine/src/flutter/impeller/geometry/geometry_unittests.cc

@@ -335,5 +335,37 @@ TEST(GeometryTest, RectContainsPoint) {
   }
 }
 
+TEST(GeometryTest, RectContainsRect) {
+  {
+    Rect a(100, 100, 100, 100);
+    ASSERT_TRUE(a.Contains(a));
+  }
+  {
+    Rect a(100, 100, 100, 100);
+    Rect b(0, 0, 0, 0);
+    ASSERT_FALSE(a.Contains(b));
+  }
+  {
+    Rect a(100, 100, 100, 100);
+    Rect b(150, 150, 20, 20);
+    ASSERT_TRUE(a.Contains(b));
+  }
+  {
+    Rect a(100, 100, 100, 100);
+    Rect b(150, 150, 100, 100);
+    ASSERT_FALSE(a.Contains(b));
+  }
+  {
+    Rect a(100, 100, 100, 100);
+    Rect b(50, 50, 100, 100);
+    ASSERT_FALSE(a.Contains(b));
+  }
+  {
+    Rect a(100, 100, 100, 100);
+    Rect b(0, 0, 300, 300);
+    ASSERT_FALSE(a.Contains(b));
+  }
+}
+
 }  // namespace testing
 }  // namespace impeller

engine/src/flutter/impeller/geometry/rect.h

@@ -84,6 +84,10 @@ struct TRect {
            p.y < origin.y + size.height;
   }
 
+  constexpr bool Contains(const TRect& o) const {
+    return Union(o).size == size;
+  }
+
   constexpr bool IsZero() const { return size.IsZero(); }
 
   constexpr bool IsEmpty() const { return size.IsEmpty(); }

engine/src/flutter/impeller/renderer/backend/metal/render_pass_mtl.mm

@@ -428,6 +428,28 @@ bool RenderPassMTL::EncodeCommands(Allocator& allocator,
                                        : MTLWindingCounterClockwise];
     [encoder setCullMode:MTLCullModeNone];
     [encoder setStencilReferenceValue:command.stencil_reference];
+    if (command.viewport.has_value()) {
+      auto v = command.viewport.value();
+      MTLViewport viewport = {
+          .originX = v.rect.origin.x,
+          .originY = v.rect.origin.y,
+          .width = v.rect.size.width,
+          .height = v.rect.size.height,
+          .znear = v.znear,
+          .zfar = v.zfar,
+      };
+      [encoder setViewport:viewport];
+    }
+    if (command.scissor.has_value()) {
+      auto s = command.scissor.value();
+      MTLScissorRect scissor = {
+          .x = static_cast<NSUInteger>(s.origin.x),
+          .y = static_cast<NSUInteger>(s.origin.y),
+          .width = static_cast<NSUInteger>(s.size.width),
+          .height = static_cast<NSUInteger>(s.size.height),
+      };
+      [encoder setScissorRect:scissor];
+    }
     if (!bind_stage_resources(command.vertex_bindings, ShaderStage::kVertex)) {
       return false;
     }
@@ -472,6 +494,15 @@ bool RenderPassMTL::AddCommand(Command command) {
     return false;
   }
 
+  if (command.scissor.has_value()) {
+    auto target_rect = IRect({}, render_target_.GetRenderTargetSize());
+    if (!target_rect.Contains(command.scissor.value())) {
+      VALIDATION_LOG << "Cannot apply a scissor that lies outside the bounds "
+                        "of the render target.";
+      return false;
+    }
+  }
+
   commands_.emplace_back(std::move(command));
   return true;
 }

engine/src/flutter/impeller/renderer/command.h

@@ -6,10 +6,12 @@
 
 #include <map>
 #include <memory>
+#include <optional>
 #include <string>
 
 #include "flutter/fml/logging.h"
 #include "flutter/fml/macros.h"
+#include "impeller/geometry/rect.h"
 #include "impeller/renderer/buffer_view.h"
 #include "impeller/renderer/formats.h"
 #include "impeller/renderer/pipeline.h"
@@ -69,6 +71,19 @@ struct Command {
   PrimitiveType primitive_type = PrimitiveType::kTriangle;
   WindingOrder winding = WindingOrder::kClockwise;
   uint32_t stencil_reference = 0u;
+  //----------------------------------------------------------------------------
+  /// The viewport coordinates that the rasterizer linearly maps normalized
+  /// device coordinates to.
+  /// If unset, the viewport is the size of the render target with a zero
+  /// origin, znear=0, and zfar=1.
+  ///
+  std::optional<Viewport> viewport;
+  //----------------------------------------------------------------------------
+  /// The scissor rect to use for clipping writes to the render target. The
+  /// scissor rect must lie entirely within the render target.
+  /// If unset, no scissor is applied.
+  ///
+  std::optional<IRect> scissor;
 
   bool BindVertices(const VertexBuffer& buffer);
 

engine/src/flutter/impeller/renderer/formats.h

@@ -11,6 +11,8 @@
 
 #include "flutter/fml/hash_combine.h"
 #include "flutter/fml/macros.h"
+#include "impeller/geometry/rect.h"
+#include "impeller/geometry/scalar.h"
 #include "impeller/geometry/color.h"
 
 namespace impeller {
@@ -149,6 +151,12 @@ enum class PrimitiveType {
   // checks. Hence, they are not supported here.
 };
 
+struct Viewport {
+  Rect rect;
+  Scalar znear = 0.0f;
+  Scalar zfar = 1.0f;
+};
+
 enum class MinMagFilter {
   /// Select nearest to the sample point. Most widely supported.
   kNearest,