Dual Source Blending

[wez]

I'd like to be able to use "Dual Source Blending" with wgpu, so that I can perform sub-pixel-AA with alpha blending.

I can see some mention of whether the Metal backend supports this in the code, but I don't see anything more than this so far.
I think that what I'd like to see for this is for BlendFactor to have enum variants representing the second source, and a way to reference that in the shader program.

Is this a lot of work to plumb in and get implemented?

For my purposes, I'm most interested in using wgpu as a way to run against Metal and DX backends as I already have an OpenGL implementation of my renderer.

[kvark]

This is desired as a feature. See also - gpuweb/gpuweb#391 upstream.

[freqmod]

Hi, i did an attempt at this over at https://github.com/freqmod/wgpu/tree/alt_blend / https://github.com/freqmod/naga/tree/alt_blend . Currently it is working for metal and vulkan.
I guess the API / shader language could be discussed, but at least it is a starting point.

I can submit a PR and help getting this merged, if there are interest to accept the current API or work to make a suitable one.

For vulkan the feature must be enabled for the functionality to work without warnings:

            features: wgpu::Features::default().union(wgpu::Features::BLEND_FUNC_EXTENDED),

Attached is a simple attempt at a fragment shader for rgb subpixel font rendering. It is still a bit wrong, but it shows how to write to the secondary output for blending (inspired by description in https://github.com/servo/webrender/blob/master/webrender/doc/text-rendering.md ),

struct Output {
    @location(0) color: vec4<f32>,
};

@group(0)@binding(1)
var atlas_sampler: sampler;

@group(0)@binding(2)
var atlas_texture: texture_2d<f32>;

@group(0)@binding(3)
var palette_sampler: sampler;

@group(0)@binding(4)
var palette_texture: texture_2d<f32>;

struct VertexOutput {
    @location(0) t_position: vec2<f32>,
    @location(1) c_position: vec2<f32>,
    @builtin(position) position: vec4<f32>,
};

struct FragmentOutput{
    @location(0) color: vec4<f32>,
    @location(0,1) mask: vec4<f32>,
}
@fragment
fn main(vo: VertexOutput) -> FragmentOutput {
    var font_col = textureSample(atlas_texture, atlas_sampler, vo.t_position);
    var palette_col = textureSample(palette_texture, palette_sampler, vec2<f32>(vo.c_position.x, 1.0));
    var a = font_col.x + font_col.y + font_col.z;
    var color = vec4(
        font_col.b * palette_col.b,
        font_col.g * palette_col.g,
        font_col.r * palette_col.r,
        a * palette_col.a);
    var mask = vec4(1.0,1.0,1.0,1.0);
    return FragmentOutput(color, mask);
}

and the target descriptor:

        fragment: Some(wgpu::FragmentState {
            module: text_fs_module,
            entry_point: "main",
            targets: &[Some(wgpu::ColorTargetState {
                format: wgpu::TextureFormat::Bgra8UnormSrgb,
                blend: Some(wgpu::BlendState {
                    color: wgpu::BlendComponent {
                        src_factor: wgpu::BlendFactor::Src,
                        dst_factor: wgpu::BlendFactor::OneMinusSrc1,
                        operation: wgpu::BlendOperation::Add,
                    },
                    alpha: wgpu::BlendComponent {
                        src_factor: wgpu::BlendFactor::Src,
                        dst_factor: wgpu::BlendFactor::OneMinusSrc1Alpha,
                        operation: wgpu::BlendOperation::Add,
                    },
                }),
                write_mask: wgpu::ColorWrites::ALL,
            })],
        }),

[teoxoy]

PRs as a next step would be great! I have a couple of comments but it would be easier to leave them on a PR.

[freqmod]

PRs as a next step would be great! I have a couple of comments but it would be easier to leave them on a PR.

Great. See
gfx-rs/naga#2427
and
#4022