vello/piet-gpu/shader/image.comp

// A simple kernel to create an image.

// Right now, this kernel stores the image in a buffer, but a better
// plan is to use a texture. This is because of limited support.

#version 450
#extension GL_GOOGLE_include_directive : enable

layout(local_size_x = 16, local_size_y = 16) in;

layout(set = 0, binding = 0) readonly buffer SceneBuf {
    uint[] scene;
};

layout(set = 0, binding = 1) buffer ImageBuf {
    uint[] image;
};

#include "scene.h"

// TODO: make the image size dynamic.
#define IMAGE_WIDTH 2048
#define IMAGE_HEIGHT 1535

void main() {
    uvec2 xy = gl_GlobalInvocationID.xy;
    vec2 uv = vec2(xy) * vec2(1.0 / IMAGE_WIDTH, 1.0 / IMAGE_HEIGHT);
    vec4 rgba = vec4(uv.xyy, 1.0);

    // Render the scene. Right now, every pixel traverses the scene graph,
    // which is horribly wasteful, but the goal is to get *some* output and
    // then optimize.

    SimpleGroup group = SimpleGroup_read(SimpleGroupRef(0));
    for (uint i = 0; i < group.n_items; i++) {
        // Writing this out by hand is illuminating the need for array access
        // in piet-gpu-derive. There is some support in the `_index` functions,
        // but those are only generated for structs, not for `Ref` or other
        // types.
        PietItemRef item_ref = PietItemRef(scene[(group.items.offset >> 2) + i]);
        uint tag = PietItem_tag(item_ref);
        if (tag == PietItem_Circle) {
            PietCircle = PietItem_Circle_read(item_ref);
        }
    }

    uvec4 s = uvec4(round(rgba * 255.0));
    uint rgba_packed = s.x | (s.y << 8) | (s.z << 16) | (s.w << 24);
    image[xy.y * IMAGE_WIDTH + xy.x] = rgba_packed;
    if (xy.y == 0 && xy.x < 8) {
        image[xy.x] = scene[xy.x];
    }
}