Fix alpha blending for wgpu msaa #1367
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tarkah
commented
Jun 16, 2022
| blend: Some(wgpu::BlendState { | ||
| color: wgpu::BlendComponent { | ||
| src_factor: wgpu::BlendFactor::SrcAlpha, | ||
| dst_factor: wgpu::BlendFactor::OneMinusSrcAlpha, | ||
| operation: wgpu::BlendOperation::Add, | ||
| }, | ||
| alpha: wgpu::BlendComponent { | ||
| src_factor: wgpu::BlendFactor::One, | ||
| dst_factor: wgpu::BlendFactor::OneMinusSrcAlpha, | ||
| operation: wgpu::BlendOperation::Add, | ||
| }, | ||
| }), | ||
| blend: Some(wgpu::BlendState::ALPHA_BLENDING), |
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ALPHA_BLENDING is equal to this
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Awesome. I see they added new useful variants 👌
hecrj
approved these changes
Jun 17, 2022
| blend: Some(wgpu::BlendState { | ||
| color: wgpu::BlendComponent { | ||
| src_factor: wgpu::BlendFactor::SrcAlpha, | ||
| dst_factor: wgpu::BlendFactor::OneMinusSrcAlpha, | ||
| operation: wgpu::BlendOperation::Add, | ||
| }, | ||
| alpha: wgpu::BlendComponent { | ||
| src_factor: wgpu::BlendFactor::One, | ||
| dst_factor: wgpu::BlendFactor::OneMinusSrcAlpha, | ||
| operation: wgpu::BlendOperation::Add, | ||
| }, | ||
| }), | ||
| blend: Some(wgpu::BlendState::ALPHA_BLENDING), |
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Awesome. I see they added new useful variants 👌
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Alpha is not blended correctly when using MSAA on wgpu. When the fragments are stored to the multisampled color attachment, the color components are multiplied by alpha. When they are sampled and then stored to the non-multisampled resolve attachment, they are getting multiplied by alpha again. Using
PREMULTIPLIED_ALPHA_BLENDINGfor the resolve attachment fixes this since it uses asrc_factorofOnefor color components.This is evident when adding a background color to the
solar_systemexample and removing theframe.fillcall to cover the canvas in a background color. The orbit, which has a small alpha value to begin with, becomes nearly fully transparent due to the double alpha blending. This fixes it so the orbit looks identical in alpha to the example without anti-aliasing enabled.