vello/piet-gpu/shader/backdrop.comp

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

// Propagation of tile backdrop for filling.
//
// Each thread reads one path element and calculates the row and column counts of spanned tiles
// based on the bounding box.
// The row count then goes through a prefix sum to redistribute and load-balance the work across the workgroup.
// In the following step, the workgroup loops over the corresponding tile rows per element in parallel.
// For each row the per tile backdrop will be read, as calculated in the previous coarse path segment kernel,
// and propagated from the left to the right (prefix summed).
//
// Output state:
//  - Each path element has an array of tiles covering the whole path based on boundig box
//  - Each tile per path element contains the 'backdrop' and a list of subdivided path segments

#version 450
#extension GL_GOOGLE_include_directive : enable

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

#define LG_BACKDROP_WG (7 + LG_WG_FACTOR)
#define BACKDROP_WG (1 << LG_BACKDROP_WG)
#ifndef BACKDROP_DIST_FACTOR
// Some paths (those covering a large area) can generate a lot of backdrop tiles; BACKDROP_DIST_FACTOR defines how much
// additional threads should we spawn for parallel row processing. The additional threads does not participate in the
// earlier stages (calculating the tile counts) but does work in the final prefix sum stage which has a lot more
// parallelism.

// This feature is opt-in: one variant is compiled with the following default, while the other variant is compiled with
// a larger BACKDROP_DIST_FACTOR, which is used on GPUs supporting a larger workgroup size to improve performance.
#define BACKDROP_DIST_FACTOR 1
#endif

layout(local_size_x = BACKDROP_WG, local_size_y = BACKDROP_DIST_FACTOR) in;

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

#include "tile.h"

shared uint sh_row_count[BACKDROP_WG];
shared Alloc sh_row_alloc[BACKDROP_WG];
shared uint sh_row_width[BACKDROP_WG];

void main() {
    if (!check_deps(STAGE_BINNING | STAGE_TILE_ALLOC | STAGE_PATH_COARSE)) {
        return;
    }

    uint th_ix = gl_LocalInvocationIndex;
    uint element_ix = gl_GlobalInvocationID.x;

    // Work assignment: 1 thread : 1 path element
    uint row_count = 0;
    if (gl_LocalInvocationID.y == 0) {
        if (element_ix < conf.n_elements) {
            // Possible TODO: it's not necessary to process backdrops of stroked paths.
            // We had logic for that but took it out because it used the Annotated struct.
            PathRef path_ref = PathRef(conf.tile_alloc.offset + element_ix * Path_size);
            Path path = Path_read(conf.tile_alloc, path_ref);
            sh_row_width[th_ix] = path.bbox.z - path.bbox.x;
            row_count = path.bbox.w - path.bbox.y;
            // Paths that don't cross tile top edges don't have backdrops.
            // Don't apply the optimization to paths that may cross the y = 0
            // top edge, but clipped to 1 row.
            if (row_count == 1 && path.bbox.y > 0) {
                // Note: this can probably be expanded to width = 2 as
                // long as it doesn't cross the left edge.
                row_count = 0;
            }
            Alloc path_alloc = new_alloc(
                path.tiles.offset, (path.bbox.z - path.bbox.x) * (path.bbox.w - path.bbox.y) * Tile_size, true);
            sh_row_alloc[th_ix] = path_alloc;
        }
        sh_row_count[th_ix] = row_count;
    }

    // Prefix sum of sh_row_count
    for (uint i = 0; i < LG_BACKDROP_WG; i++) {
        barrier();
        if (gl_LocalInvocationID.y == 0 && th_ix >= (1u << i)) {
            row_count += sh_row_count[th_ix - (1u << i)];
        }
        barrier();
        if (gl_LocalInvocationID.y == 0) {
            sh_row_count[th_ix] = row_count;
        }
    }
    barrier();
    // Work assignment: 1 thread : 1 path element row
    uint total_rows = sh_row_count[BACKDROP_WG - 1];
    for (uint row = th_ix; row < total_rows; row += BACKDROP_WG * BACKDROP_DIST_FACTOR) {
        // Binary search to find element
        uint el_ix = 0;
        for (uint i = 0; i < LG_BACKDROP_WG; i++) {
            uint probe = el_ix + (uint(BACKDROP_WG / 2) >> i);
            if (row >= sh_row_count[probe - 1]) {
                el_ix = probe;
            }
        }
        uint width = sh_row_width[el_ix];
        if (width > 0) {
            // Process one row sequentially
            // Read backdrop value per tile and prefix sum it
            Alloc tiles_alloc = sh_row_alloc[el_ix];
            uint seq_ix = row - (el_ix > 0 ? sh_row_count[el_ix - 1] : 0);
            uint tile_el_ix = (tiles_alloc.offset >> 2) + 1 + seq_ix * 2 * width;
            uint sum = read_mem(tiles_alloc, tile_el_ix);
            for (uint x = 1; x < width; x++) {
                tile_el_ix += 2;
                sum += read_mem(tiles_alloc, tile_el_ix);
                write_mem(tiles_alloc, tile_el_ix, sum);
            }
        }
    }
}