From 806f35f589f4d2a45fd10cb48598d8d7001f8574 Mon Sep 17 00:00:00 2001
From: Brandon DeRosier <bdero@google.com>
Date: Sat, 3 Dec 2022 00:54:01 -0800
Subject: [PATCH] [Impeller] Make perspective transform resolve to left handed
 clip space

---
 impeller/geometry/geometry_unittests.cc | 20 ++++++++++----------
 impeller/geometry/matrix.h              |  2 +-
 impeller/renderer/renderer_unittests.cc |  3 ++-
 3 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/impeller/geometry/geometry_unittests.cc b/impeller/geometry/geometry_unittests.cc
index 9bafff468b1d8..95a0287eef600 100644
--- a/impeller/geometry/geometry_unittests.cc
+++ b/impeller/geometry/geometry_unittests.cc
@@ -240,7 +240,7 @@ TEST(GeometryTest, MatrixVectorMultiplication) {
     auto vector = Vector3(3, 3, -3);
 
     Vector3 result = matrix * vector;
-    auto expected = Vector3(1, 1, 0.673401);
+    auto expected = Vector3(-1, -1, 1.3468);
     ASSERT_VECTOR3_NEAR(result, expected);
   }
 
@@ -250,7 +250,7 @@ TEST(GeometryTest, MatrixVectorMultiplication) {
     auto point = Point(3, 3);
 
     Point result = matrix * point;
-    auto expected = Point(1, 1);
+    auto expected = Point(-1, -1);
     ASSERT_POINT_NEAR(result, expected);
   }
 
@@ -343,10 +343,10 @@ TEST(GeometryTest, MatrixMakePerspective) {
   {
     auto m = Matrix::MakePerspective(Degrees(60), Size(100, 200), 1, 10);
     auto expect = Matrix{
-        3.4641, 0,       0,        0,   //
-        0,      1.73205, 0,        0,   //
-        0,      0,       -1.11111, -1,  //
-        0,      0,       -1.11111, 0,   //
+        3.4641, 0,       0,        0,  //
+        0,      1.73205, 0,        0,  //
+        0,      0,       1.11111,  1,  //
+        0,      0,       -1.11111, 0,  //
     };
     ASSERT_MATRIX_NEAR(m, expect);
   }
@@ -354,10 +354,10 @@ TEST(GeometryTest, MatrixMakePerspective) {
   {
     auto m = Matrix::MakePerspective(Radians(1), 2, 10, 20);
     auto expect = Matrix{
-        0.915244, 0,       0,   0,   //
-        0,        1.83049, 0,   0,   //
-        0,        0,       -2,  -1,  //
-        0,        0,       -20, 0,   //
+        0.915244, 0,       0,   0,  //
+        0,        1.83049, 0,   0,  //
+        0,        0,       2,   1,  //
+        0,        0,       -20, 0,  //
     };
     ASSERT_MATRIX_NEAR(m, expect);
   }
diff --git a/impeller/geometry/matrix.h b/impeller/geometry/matrix.h
index 2d50f3045007b..9829317deb939 100644
--- a/impeller/geometry/matrix.h
+++ b/impeller/geometry/matrix.h
@@ -421,7 +421,7 @@ struct Matrix {
     return {
       1.0f / width, 0.0f,           0.0f,                                 0.0f,
       0.0f,         1.0f / height,  0.0f,                                 0.0f,
-      0.0f,         0.0f,           z_far / (z_near - z_far),            -1.0f,
+      0.0f,         0.0f,           z_far / (z_far - z_near),             1.0f,
       0.0f,         0.0f,          -(z_far * z_near) / (z_far - z_near),  0.0f,
     };
     // clang-format on
diff --git a/impeller/renderer/renderer_unittests.cc b/impeller/renderer/renderer_unittests.cc
index cfd3637f9a8c2..e5678dd2c26ae 100644
--- a/impeller/renderer/renderer_unittests.cc
+++ b/impeller/renderer/renderer_unittests.cc
@@ -117,6 +117,7 @@ TEST_P(RendererTest, CanRenderPerspectiveCube) {
   auto desc = PipelineBuilder<VS, FS>::MakeDefaultPipelineDescriptor(*context);
   ASSERT_TRUE(desc.has_value());
   desc->SetCullMode(CullMode::kBackFace);
+  desc->SetWindingOrder(WindingOrder::kCounterClockwise);
   desc->SetSampleCount(SampleCount::kCount4);
   auto pipeline =
       context->GetPipelineLibrary()->GetPipeline(std::move(desc)).Get();
@@ -184,7 +185,7 @@ TEST_P(RendererTest, CanRenderPerspectiveCube) {
 
     uniforms.mvp =
         Matrix::MakePerspective(fov_y, pass.GetRenderTargetSize(), 0, 10) *
-        Matrix::MakeTranslation({0, 0, -distance}) *
+        Matrix::MakeTranslation({0, 0, distance}) *
         Matrix::MakeRotationX(Radians(euler_angles.x)) *
         Matrix::MakeRotationY(Radians(euler_angles.y)) *
         Matrix::MakeRotationZ(Radians(euler_angles.z));