vello/piet-gpu/shader/kernel4.comp

// SPDX-License-Identifier: Apache-2.0 OR MIT OR Unlicense

// This is "kernel 4" in a 4-kernel pipeline. It renders the commands
// in the per-tile command list to 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

#include "mem.h"
#include "setup.h"

#define CHUNK_X 2
#define CHUNK_Y 4
#define CHUNK CHUNK_X * CHUNK_Y
#define CHUNK_DX (TILE_WIDTH_PX / CHUNK_X)
#define CHUNK_DY (TILE_HEIGHT_PX / CHUNK_Y)
layout(local_size_x = CHUNK_DX, local_size_y = CHUNK_DY) in;

layout(set = 0, binding = 1) restrict readonly buffer ConfigBuf {
    Config conf;
};

#ifdef GRAY
layout(r8, set = 0, binding = 2) uniform restrict writeonly image2D image;
#else
layout(rgba8, set = 0, binding = 2) uniform restrict writeonly image2D image;
#endif

layout(rgba8, set = 0, binding = 3) uniform restrict readonly image2D image_atlas;

layout(rgba8, set = 0, binding = 4) uniform restrict readonly image2D gradients;

#include "ptcl.h"
#include "tile.h"

#define MAX_BLEND_STACK 128
mediump vec3 tosRGB(mediump vec3 rgb) {
    bvec3 cutoff = greaterThanEqual(rgb, vec3(0.0031308));
    mediump vec3 below = vec3(12.92)*rgb;
    mediump vec3 above = vec3(1.055)*pow(rgb, vec3(0.41666)) - vec3(0.055);
    return mix(below, above, cutoff);
}

mediump vec3 fromsRGB(mediump vec3 srgb) {
    // Formula from EXT_sRGB.
    bvec3 cutoff = greaterThanEqual(srgb, vec3(0.04045));
    mediump vec3 below = srgb/vec3(12.92);
    mediump vec3 above = pow((srgb + vec3(0.055))/vec3(1.055), vec3(2.4));
    return mix(below, above, cutoff);
}

// unpacksRGB unpacks a color in the sRGB color space to a vec4 in the linear color
// space.
mediump vec4 unpacksRGB(uint srgba) {
    mediump vec4 color = unpackUnorm4x8(srgba).wzyx;
    return vec4(fromsRGB(color.rgb), color.a);
}

// packsRGB packs a color in the linear color space into its 8-bit sRGB equivalent.
uint packsRGB(mediump vec4 rgba) {
    rgba = vec4(tosRGB(rgba.rgb), rgba.a);
    return packUnorm4x8(rgba.wzyx);
}

uvec2 chunk_offset(uint i) {
    return uvec2(i % CHUNK_X * CHUNK_DX, i / CHUNK_X * CHUNK_DY);
}

mediump vec4[CHUNK] fillImage(uvec2 xy, CmdImage cmd_img) {
    mediump vec4 rgba[CHUNK];
    for (uint i = 0; i < CHUNK; i++) {
        ivec2 uv = ivec2(xy + chunk_offset(i)) + cmd_img.offset;
        mediump vec4 fg_rgba;
        fg_rgba = imageLoad(image_atlas, uv);
        fg_rgba.rgb = fromsRGB(fg_rgba.rgb);
        rgba[i] = fg_rgba;
    }
    return rgba;
}

void main() {
    uint tile_ix = gl_WorkGroupID.y * conf.width_in_tiles + gl_WorkGroupID.x;
    Alloc cmd_alloc = slice_mem(conf.ptcl_alloc, tile_ix * PTCL_INITIAL_ALLOC, PTCL_INITIAL_ALLOC);
    CmdRef cmd_ref = CmdRef(cmd_alloc.offset);

    uvec2 xy_uint = uvec2(gl_LocalInvocationID.x + TILE_WIDTH_PX * gl_WorkGroupID.x, gl_LocalInvocationID.y + TILE_HEIGHT_PX * gl_WorkGroupID.y);
    vec2 xy = vec2(xy_uint);
    mediump vec4 rgba[CHUNK];
    uint blend_stack[MAX_BLEND_STACK][CHUNK];
    mediump float blend_alpha_stack[MAX_BLEND_STACK][CHUNK];
    for (uint i = 0; i < CHUNK; i++) {
        rgba[i] = vec4(0.0);
    }

    mediump float area[CHUNK];
    uint clip_depth = 0;
    bool mem_ok = mem_error == NO_ERROR;
    while (mem_ok) {
        uint tag = Cmd_tag(cmd_alloc, cmd_ref).tag;
        if (tag == Cmd_End) {
            break;
        }
        switch (tag) {
        case Cmd_Stroke:
            // Calculate distance field from all the line segments in this tile.
            CmdStroke stroke = Cmd_Stroke_read(cmd_alloc, cmd_ref);
            mediump float df[CHUNK];
            for (uint k = 0; k < CHUNK; k++) df[k] = 1e9;
            TileSegRef tile_seg_ref = TileSegRef(stroke.tile_ref);
            do {
                TileSeg seg = TileSeg_read(new_alloc(tile_seg_ref.offset, TileSeg_size, mem_ok), tile_seg_ref);
                vec2 line_vec = seg.vector;
                for (uint k = 0; k < CHUNK; k++) {
                    vec2 dpos = xy + vec2(0.5, 0.5) - seg.origin;
                    dpos += vec2(chunk_offset(k));
                    float t = clamp(dot(line_vec, dpos) / dot(line_vec, line_vec), 0.0, 1.0);
                    df[k] = min(df[k], length(line_vec * t - dpos));
                }
                tile_seg_ref = seg.next;
            } while (tile_seg_ref.offset != 0);
            for (uint k = 0; k < CHUNK; k++) {
                area[k] = clamp(stroke.half_width + 0.5 - df[k], 0.0, 1.0);
            }
            cmd_ref.offset += 4 + CmdStroke_size;
            break;
        case Cmd_Fill:
            CmdFill fill = Cmd_Fill_read(cmd_alloc, cmd_ref);
            for (uint k = 0; k < CHUNK; k++) area[k] = float(fill.backdrop);
            tile_seg_ref = TileSegRef(fill.tile_ref);
            // Calculate coverage based on backdrop + coverage of each line segment
            do {
                TileSeg seg = TileSeg_read(new_alloc(tile_seg_ref.offset, TileSeg_size, mem_ok), tile_seg_ref);
                for (uint k = 0; k < CHUNK; k++) {
                    vec2 my_xy = xy + vec2(chunk_offset(k));
                    vec2 start = seg.origin - my_xy;
                    vec2 end = start + seg.vector;
                    vec2 window = clamp(vec2(start.y, end.y), 0.0, 1.0);
                    if (window.x != window.y) {
                        vec2 t = (window - start.y) / seg.vector.y;
                        vec2 xs = vec2(mix(start.x, end.x, t.x), mix(start.x, end.x, t.y));
                        float xmin = min(min(xs.x, xs.y), 1.0) - 1e-6;
                        float xmax = max(xs.x, xs.y);
                        float b = min(xmax, 1.0);
                        float c = max(b, 0.0);
                        float d = max(xmin, 0.0);
                        float a = (b + 0.5 * (d * d - c * c) - xmin) / (xmax - xmin);
                        area[k] += a * (window.x - window.y);
                    }
                    area[k] += sign(seg.vector.x) * clamp(my_xy.y - seg.y_edge + 1.0, 0.0, 1.0);
                }
                tile_seg_ref = seg.next;
            } while (tile_seg_ref.offset != 0);
            for (uint k = 0; k < CHUNK; k++) {
                area[k] = min(abs(area[k]), 1.0);
            }
            cmd_ref.offset += 4 + CmdFill_size;
            break;
        case Cmd_Solid:
            for (uint k = 0; k < CHUNK; k++) {
                area[k] = 1.0;
            }
            cmd_ref.offset += 4;
            break;
        case Cmd_Alpha:
            CmdAlpha alpha = Cmd_Alpha_read(cmd_alloc, cmd_ref);
            for (uint k = 0; k < CHUNK; k++) {
                area[k] = alpha.alpha;
            }
            cmd_ref.offset += 4 + CmdAlpha_size;
            break;
        case Cmd_Color:
            CmdColor color = Cmd_Color_read(cmd_alloc, cmd_ref);
            mediump vec4 fg = unpacksRGB(color.rgba_color);
            for (uint k = 0; k < CHUNK; k++) {
                mediump vec4 fg_k = fg * area[k];
                rgba[k] = rgba[k] * (1.0 - fg_k.a) + fg_k;
            }
            cmd_ref.offset += 4 + CmdColor_size;
            break;
        case Cmd_LinGrad:
            CmdLinGrad lin = Cmd_LinGrad_read(cmd_alloc, cmd_ref);
            float d = lin.line_x * float(xy.x) + lin.line_y * float(xy.y) + lin.line_c;
            for (uint k = 0; k < CHUNK; k++) {
                vec2 chunk_xy = vec2(chunk_offset(k));
                float my_d = d + lin.line_x * chunk_xy.x + lin.line_y * chunk_xy.y;
                int x = int(round(clamp(my_d, 0.0, 1.0) * float(GRADIENT_WIDTH - 1)));
                mediump vec4 fg_rgba = imageLoad(gradients, ivec2(x, int(lin.index)));
                fg_rgba.rgb = fromsRGB(fg_rgba.rgb);
                rgba[k] = fg_rgba;
            }
            cmd_ref.offset += 4 + CmdLinGrad_size;
            break;
        case Cmd_Image:
            CmdImage fill_img = Cmd_Image_read(cmd_alloc, cmd_ref);
            mediump vec4 img[CHUNK] = fillImage(xy_uint, fill_img);
            for (uint k = 0; k < CHUNK; k++) {
                mediump vec4 fg_k = img[k] * area[k];
                rgba[k] = rgba[k] * (1.0 - fg_k.a) + fg_k;
            }
            cmd_ref.offset += 4 + CmdImage_size;
            break;
        case Cmd_BeginClip:
            for (uint k = 0; k < CHUNK; k++) {
                // We reject any inputs that might overflow in render_ctx.rs.
                // The following is a sanity check so we don't corrupt memory should there be malformed inputs.
                uint d = min(clip_depth, MAX_BLEND_STACK - 1);
                blend_stack[d][k] = packsRGB(vec4(rgba[k]));
                blend_alpha_stack[d][k] = clamp(abs(area[k]), 0.0, 1.0);
                rgba[k] = vec4(0.0);
            }
            clip_depth++;
            cmd_ref.offset += 4;
            break;
        case Cmd_EndClip:
            clip_depth--;
            for (uint k = 0; k < CHUNK; k++) {
                uint d = min(clip_depth, MAX_BLEND_STACK - 1);
                mediump vec4 bg = unpacksRGB(blend_stack[d][k]);
                mediump vec4 fg = rgba[k] * area[k] * blend_alpha_stack[d][k];
                rgba[k] = bg * (1.0 - fg.a) + fg;
            }
            cmd_ref.offset += 4;
            break;
        case Cmd_Jump:
            cmd_ref = CmdRef(Cmd_Jump_read(cmd_alloc, cmd_ref).new_ref);
            cmd_alloc.offset = cmd_ref.offset;
            break;
        }
    }

    for (uint i = 0; i < CHUNK; i++) {
#ifdef GRAY
        // Just store the alpha value; later we can specialize this kernel more to avoid
        // computing unneeded RGB colors.
        imageStore(image, ivec2(xy_uint + chunk_offset(i)), vec4(rgba[i].a));
#else
        imageStore(image, ivec2(xy_uint + chunk_offset(i)), vec4(tosRGB(rgba[i].rgb), rgba[i].a));
#endif
    }
}