AttachmentBlend::Default returns a non-intuitive value

[tom-leys]

Version of vulkano

0.35.2

OS (incl. version)

Linux Pop OS

Selected physical device

NVIDIA GeForce RTX 5070

Driver (incl. version)

NVIDIA-SMI 570.172.08

Issue description

I would expect AttachmentBlend::Default (used in pipeline construction) to discard dest and overwrite src as-is. Instead it n^2 (squares) source (shader output) before writing it to texture.

If the shader returns 0.5 flat grey, the current blend will cause 0.25 to be stored into the output texture.

Instead of

fn default() -> Self {
        Self {
            src_color_blend_factor: BlendFactor::SrcColor,
            dst_color_blend_factor: BlendFactor::Zero,
            color_blend_op: BlendOp::Add,
            src_alpha_blend_factor: BlendFactor::SrcColor,
            dst_alpha_blend_factor: BlendFactor::Zero,
            alpha_blend_op: BlendOp::Add,
        }
    }

A sensible default would be

fn default() -> Self {
        Self {
                src_color_blend_factor: BlendFactor::One,
                dst_color_blend_factor: BlendFactor::Zero,
                color_blend_op: BlendOp::Add,
                src_alpha_blend_factor: BlendFactor::One,
                dst_alpha_blend_factor: BlendFactor::Zero,
                alpha_blend_op: BlendOp::Add,        
          }
    }

Minimal reproducible example

You could use the triangle example, set up the blend as default and hardcode the output of the fragment shader to always return 0.5. Then look in renderdoc at the output texture to read back the exact value.

[tom-leys]

On reflection it was my bad to set the blend at all, I guess I should have set it to None. Anyway it caused me confusion.

I tracked down why None works and it's because the blend_enable = False bool is passed to vulkan so the other default values in the PipelineColorBlendAttachmentState don't matter.

To repro

Constant output fragment shader

        mod fs {
            vulkano_shaders::shader! {
                ty: "fragment",
                src: r"
                    #version 450

                    layout(location = 0) out vec4 f_color;

                    void main() {
                        f_color = vec4(0.5, 0.5, 0.5, 1.0);
                    }
                ",
            }
        }

Set blend in pipeline

// How pixel values are combined with the values already present in the
                    // framebuffer. The default value overwrites the old value with the new one,
                    // without any blending.
                    color_blend_state: Some(ColorBlendState::with_attachment_states(
                        subpass.num_color_attachments(),
                        ColorBlendAttachmentState {
                            blend: Some(AttachmentBlend::default()),
                            ..Default::default()
                        },
                    )),

Then you'll note that the triangle is 0.25 white when it should be 0.5 white (you can read the exact values out in renderdoc)