fix bug: correct the blend function in DX12 #3447
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formula 1. Result_alpha = Src_alpha + Dst_alpha * (1 - Src_alpha) 2. Result_rgb = Result_color * Result_alpha = Src_color * Src_alpha + Dst_color * Dst_alpha * (1 - Src_alpha) assume 1. Dst_rgb = Dst_color * Dst_alpha 2. texture (like fonts) is (Src_color, Src_alpha), not premultiplied alpha, so Src_rgb = Src_color then 1. Result_alpha = Src_alpha + Dst_alpha * (1 - Src_alpha) 2. Result_rgb = Result_color * Result_alpha = Src_rgb * Src_alpha + Dst_rgb * (1 - Src_alpha) so 1. SrcBlendAlpha = D3D12_BLEND_ONE 2. DestBlendAlpha = D3D12_BLEND_INV_SRC_ALPHA
thedmd
reviewed
Oct 20, 2020
| @@ -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
erosnick
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Feb 28, 2023
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formula
assume
then
so
Why nobody found this bug? Because Dst_alpha is useless In the general case (only use Src_alpha, see
SrcBlendandDestBlend)I found this bug when I use two context, one for game, one for editor, and the error alpha in game context exposed in editor context.
Here I show a demo (there are two contexts, one in game window)
error result (text disappeared)
correct result
if fonts texture is premultiplied alpha, the blend mode should set to
SrcBlend = D3D12_BLEND_ONE