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fix bug: correct the blend function in DX12 #3447

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fix bug: correct the blend function in DX12 #3447

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5 changes: 3 additions & 2 deletions examples/imgui_impl_dx12.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@

// CHANGELOG
// (minor and older changes stripped away, please see git history for details)
// 2020-09-01: DirectX12: Corrected the blend function.
// 2019-10-18: DirectX12: *BREAKING CHANGE* Added extra ID3D12DescriptorHeap parameter to ImGui_ImplDX12_Init() function.
// 2019-05-29: DirectX12: Added support for large mesh (64K+ vertices), enable ImGuiBackendFlags_RendererHasVtxOffset flag.
// 2019-04-30: DirectX12: Added support for special ImDrawCallback_ResetRenderState callback to reset render state.
Expand Down Expand Up @@ -552,8 +553,8 @@ bool ImGui_ImplDX12_CreateDeviceObjects()
desc.RenderTarget[0].SrcBlend = D3D12_BLEND_SRC_ALPHA;
desc.RenderTarget[0].DestBlend = D3D12_BLEND_INV_SRC_ALPHA;
desc.RenderTarget[0].BlendOp = D3D12_BLEND_OP_ADD;
desc.RenderTarget[0].SrcBlendAlpha = D3D12_BLEND_INV_SRC_ALPHA;
desc.RenderTarget[0].DestBlendAlpha = D3D12_BLEND_ZERO;
desc.RenderTarget[0].SrcBlendAlpha = D3D12_BLEND_ONE;
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I intended to write that D3D12_BLEND_SRC_ALPHA would be consistent with other backends, but...

It appears backends has different idea how to blend alpha channel into final image:

Backend Alpha Channel Blend Equation
D3D9 src * SRC_ALPHA + dest * INV_SRC_ALPHA
D3D10 src * INV_SRC_ALPHA + dest * 0
D3D11 src * INV_SRC_ALPHA + dest * 0
D3D12 src * 1 + dest * INV_SRC_ALPHA
OpenGL src * SRC_ALPHA + dest * INV_SRC_ALPHA
Metal src * SRC_ALPHA + dest * INV_SRC_ALPHA
Vulkan src * INV_SRC_ALPHA + dest * 0
Allegro src * SRC_ALPHA + dest * INV_SRC_ALPHA

I guess it does not matter until ImGui is rendered into the texture.
What is desired equation?

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result alpha = 1 - result transparency
result transparency = src transparency * dest transparency
src transparency = 1 - src alpha
dest transparency = 1 - dst alpha

so
result alpha = src alpha * 1 + dest alpha * (1 - src alpha)

D3D12 is correct

desc.RenderTarget[0].DestBlendAlpha = D3D12_BLEND_INV_SRC_ALPHA;
desc.RenderTarget[0].BlendOpAlpha = D3D12_BLEND_OP_ADD;
desc.RenderTarget[0].RenderTargetWriteMask = D3D12_COLOR_WRITE_ENABLE_ALL;
}
Expand Down