From 8adab259053eb0a9ada89815f3680736f4a1f17b Mon Sep 17 00:00:00 2001
From: Teodor Tanasoaia <28601907+teoxoy@users.noreply.github.com>
Date: Mon, 18 Sep 2023 16:43:11 +0200
Subject: [PATCH] [d3d12] Document `map_blend_factor` (#4151)

---
 wgpu-hal/src/dx12/conv.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/wgpu-hal/src/dx12/conv.rs b/wgpu-hal/src/dx12/conv.rs
index 8b44ae9c4b..908944567a 100644
--- a/wgpu-hal/src/dx12/conv.rs
+++ b/wgpu-hal/src/dx12/conv.rs
@@ -222,6 +222,10 @@ pub fn map_polygon_mode(mode: wgt::PolygonMode) -> d3d12_ty::D3D12_FILL_MODE {
     }
 }
 
+/// D3D12 doesn't support passing factors ending in `_COLOR` for alpha blending
+/// (see https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ns-d3d12-d3d12_render_target_blend_desc).
+/// Therefore this function takes an additional `is_alpha` argument
+/// which if set will return an equivalent `_ALPHA` factor.
 fn map_blend_factor(factor: wgt::BlendFactor, is_alpha: bool) -> d3d12_ty::D3D12_BLEND {
     use wgt::BlendFactor as Bf;
     match factor {