From b52bc313d1d1f908652253b4fdf43ad2ced0e28f Mon Sep 17 00:00:00 2001
From: Alexey Panteleev <alpanteleev@nvidia.com>
Date: Tue, 3 Dec 2024 14:58:53 -0800
Subject: [PATCH] Fixed dual-source blending in the forward shading pass on
 Vulkan.

---
 include/donut/shaders/binding_helpers.hlsli | 8 ++++++++
 shaders/passes/forward_ps.hlsl              | 4 ++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/donut/shaders/binding_helpers.hlsli b/include/donut/shaders/binding_helpers.hlsli
index 16c800b..7d36a67 100644
--- a/include/donut/shaders/binding_helpers.hlsli
+++ b/include/donut/shaders/binding_helpers.hlsli
@@ -29,11 +29,19 @@
 #define VK_BINDING(reg,dset) [[vk::binding(reg,dset)]]
 #define VK_DESCRIPTOR_SET(dset) ,space##dset
 
+// To use dual-source blending on Vulkan, a pixel shader must specify the same Location but different Index
+// decorations for two outputs. In HLSL, that can only be achieved with explicit attributes.
+// Use on the declarations of pixel shader outputs.
+#define VK_LOCATION(loc) [[vk::location(loc)]]
+#define VK_LOCATION_INDEX(loc,idx) [[vk::location(loc)]] [[vk::index(idx)]]
+
 #else
 
 #define VK_PUSH_CONSTANT
 #define VK_BINDING(reg,dset) 
 #define VK_DESCRIPTOR_SET(dset)
+#define VK_LOCATION(loc)
+#define VK_LOCATION_INDEX(loc,idx)
 
 #endif
 
diff --git a/shaders/passes/forward_ps.hlsl b/shaders/passes/forward_ps.hlsl
index b99f403..972e239 100644
--- a/shaders/passes/forward_ps.hlsl
+++ b/shaders/passes/forward_ps.hlsl
@@ -71,9 +71,9 @@ void main(
     in float4 i_position : SV_Position,
     in SceneVertex i_vtx,
     in bool i_isFrontFace : SV_IsFrontFace,
-    out float4 o_color : SV_Target0
+    VK_LOCATION_INDEX(0, 0) out float4 o_color : SV_Target0
 #if TRANSMISSIVE_MATERIAL
-    , out float4 o_backgroundBlendFactor : SV_Target1
+    , VK_LOCATION_INDEX(0, 1) out float4 o_backgroundBlendFactor : SV_Target1
 #endif
 )
 {