piet-gpu/shader/tile_alloc.comp

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

// Allocation and initialization of tiles for paths.

#version 450
#extension GL_GOOGLE_include_directive : enable

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

#define LG_TILE_ALLOC_WG (7 + LG_WG_FACTOR)
#define TILE_ALLOC_WG (1 << LG_TILE_ALLOC_WG)

layout(local_size_x = TILE_ALLOC_WG, local_size_y = 1) in;

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

layout(binding = 2) readonly buffer SceneBuf {
    uint[] scene;
};

#include "drawtag.h"
#include "tile.h"

// scale factors useful for converting coordinates to tiles
#define SX (1.0 / float(TILE_WIDTH_PX))
#define SY (1.0 / float(TILE_HEIGHT_PX))

shared uint sh_tile_count[TILE_ALLOC_WG];
shared uint sh_tile_offset;

vec4 load_draw_bbox(uint draw_ix) {
    uint base = (conf.draw_bbox_alloc.offset >> 2) + 4 * draw_ix;
    float x0 = uintBitsToFloat(memory[base]);
    float y0 = uintBitsToFloat(memory[base + 1]);
    float x1 = uintBitsToFloat(memory[base + 2]);
    float y1 = uintBitsToFloat(memory[base + 3]);
    vec4 bbox = vec4(x0, y0, x1, y1);
    return bbox;
}

void main() {
    if (!check_deps(STAGE_BINNING)) {
        return;
    }
    uint th_ix = gl_LocalInvocationID.x;
    uint element_ix = gl_GlobalInvocationID.x;
    // At the moment, element_ix == path_ix. The clip-intersected bounding boxes
    // for elements (draw objects) are computed in the binning stage, but at some
    // point we'll probably want to break that correspondence. Tiles should be
    // allocated for paths, not draw objs. EndClip doesn't need an allocation.
    PathRef path_ref = PathRef(conf.tile_alloc.offset + element_ix * Path_size);
    uint drawtag_base = conf.drawtag_offset >> 2;

    uint drawtag = Drawtag_Nop;
    if (element_ix < conf.n_elements) {
        drawtag = scene[drawtag_base + element_ix];
    }
    int x0 = 0, y0 = 0, x1 = 0, y1 = 0;
    // Allocate an empty path for EndClip; at some point we'll change
    // this to be per path rather than per draw object.
    if (drawtag != Drawtag_Nop && drawtag != Drawtag_EndClip) {
        vec4 bbox = load_draw_bbox(element_ix);
        x0 = int(floor(bbox.x * SX));
        y0 = int(floor(bbox.y * SY));
        x1 = int(ceil(bbox.z * SX));
        y1 = int(ceil(bbox.w * SY));
    }
    x0 = clamp(x0, 0, int(conf.width_in_tiles));
    y0 = clamp(y0, 0, int(conf.height_in_tiles));
    x1 = clamp(x1, 0, int(conf.width_in_tiles));
    y1 = clamp(y1, 0, int(conf.height_in_tiles));

    Path path;
    path.bbox = uvec4(x0, y0, x1, y1);
    uint tile_count = (x1 - x0) * (y1 - y0);

    sh_tile_count[th_ix] = tile_count;
    uint total_tile_count = tile_count;
    // Prefix sum of sh_tile_count
    for (uint i = 0; i < LG_TILE_ALLOC_WG; i++) {
        barrier();
        if (th_ix >= (1u << i)) {
            total_tile_count += sh_tile_count[th_ix - (1u << i)];
        }
        barrier();
        sh_tile_count[th_ix] = total_tile_count;
    }
    if (th_ix == TILE_ALLOC_WG - 1) {
        sh_tile_offset = malloc_stage(total_tile_count * Tile_size, conf.mem_size, STAGE_TILE_ALLOC);
    }
    barrier();
    uint offset_start = sh_tile_offset;
    if (offset_start == MALLOC_FAILED) {
        return;
    }

    if (element_ix < conf.n_elements) {
        uint tile_subix = th_ix > 0 ? sh_tile_count[th_ix - 1] : 0;
        path.tiles = TileRef(offset_start + Tile_size * tile_subix);
        Path_write(conf.tile_alloc, path_ref, path);
    }

    // Zero out allocated tiles efficiently
    uint total_count = sh_tile_count[TILE_ALLOC_WG - 1] * (Tile_size / 4);
    uint start_ix = offset_start >> 2;
    for (uint i = th_ix; i < total_count; i += TILE_ALLOC_WG) {
        memory[start_ix + i] = 0;
    }
}