From 7ff7eb2f227c8131b8659996f1074ad707ebef4d Mon Sep 17 00:00:00 2001
From: Dzmitry Malyshau <kvarkus@gmail.com>
Date: Thu, 26 Mar 2020 12:33:50 -0400
Subject: [PATCH] [rs] Change NDC Y axis

---
 wgpu/examples/framework.rs        | 2 +-
 wgpu/examples/shadow/forward.frag | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/wgpu/examples/framework.rs b/wgpu/examples/framework.rs
index bbadbc2610..6a33a0222b 100644
--- a/wgpu/examples/framework.rs
+++ b/wgpu/examples/framework.rs
@@ -4,7 +4,7 @@ use winit::event::WindowEvent;
 #[allow(unused)]
 pub const OPENGL_TO_WGPU_MATRIX: cgmath::Matrix4<f32> = cgmath::Matrix4::new(
     1.0, 0.0, 0.0, 0.0,
-    0.0, -1.0, 0.0, 0.0,
+    0.0, 1.0, 0.0, 0.0,
     0.0, 0.0, 0.5, 0.0,
     0.0, 0.0, 0.5, 1.0,
 );
diff --git a/wgpu/examples/shadow/forward.frag b/wgpu/examples/shadow/forward.frag
index fdd587ae6d..8a3e2579dc 100644
--- a/wgpu/examples/shadow/forward.frag
+++ b/wgpu/examples/shadow/forward.frag
@@ -32,9 +32,11 @@ float fetch_shadow(int light_id, vec4 homogeneous_coords) {
     if (homogeneous_coords.w <= 0.0) {
         return 1.0;
     }
+    // compensate for the Y-flip difference between the NDC and texture coordinates
+    const vec2 flip_correction = vec2(0.5, -0.5);
     // compute texture coordinates for shadow lookup
     vec4 light_local = vec4(
-        (homogeneous_coords.xy/homogeneous_coords.w + 1.0) / 2.0,
+        homogeneous_coords.xy * flip_correction/homogeneous_coords.w + 0.5,
         light_id,
         homogeneous_coords.z / homogeneous_coords.w
     );