vello/piet-gpu/shader/coarse.comp

// The coarse rasterizer stage of the pipeline.

#version 450
#extension GL_GOOGLE_include_directive : enable

#include "setup.h"

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

layout(set = 0, binding = 0) buffer AnnotatedBuf {
    uint[] annotated;
};

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

layout(set = 0, binding = 2) buffer AllocBuf {
    uint alloc;
};

layout(set = 0, binding = 3) buffer PtclBuf {
    uint[] ptcl;
};

#include "annotated.h"
#include "bins.h"
#include "ptcl.h"

#define N_RINGBUF 512

shared uint sh_elements[N_RINGBUF];
shared uint sh_chunk[N_WG];
shared uint sh_chunk_next[N_WG];
shared uint sh_chunk_n[N_WG];
shared uint sh_min_buf;
// Some of these are kept in shared memory to ease register
// pressure, but it could go either way.
shared uint sh_first_el[N_WG];
shared uint sh_selected_n;
shared uint sh_elements_ref;

shared uint sh_bitmaps[N_SLICE][N_TILE];

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

void main() {
    // Could use either linear or 2d layouts for both dispatch and
    // invocations within the workgroup. We'll use variables to abstract.
    uint bin_ix = N_TILE_X * gl_WorkGroupID.y + gl_WorkGroupID.x;
    // Top left coordinates of this bin.
    vec2 xy0 = vec2(N_TILE_X * TILE_WIDTH_PX * gl_WorkGroupID.x, N_TILE_Y * TILE_HEIGHT_PX * gl_WorkGroupID.y);
    uint th_ix = gl_LocalInvocationID.x;
    uint wr_ix = 0;
    uint rd_ix = 0;
    uint first_el;
    if (th_ix < N_WG) {
        uint start_chunk = (bin_ix * N_WG + th_ix) * BIN_INITIAL_ALLOC;
        sh_chunk[th_ix] = start_chunk;
        BinChunk chunk = BinChunk_read(BinChunkRef(start_chunk));
        sh_chunk_n[th_ix] = chunk.n;
        sh_chunk_next[th_ix] = chunk.next.offset;
        sh_first_el[th_ix] = chunk.n > 0 ?
            BinInstance_read(BinInstanceRef(start_chunk + BinChunk_size)).element_ix : ~0;
    }
    uint probe = 0; // for debugging
    do {
        for (uint i = 0; i < N_SLICE; i++) {
            sh_bitmaps[i][th_ix] = 0;
        }

        while (wr_ix - rd_ix <= N_TILE) {
            // Choose segment with least element.
            uint my_min;
            if (th_ix < N_WG) {
                if (th_ix == 0) {
                    sh_selected_n = 0;
                    sh_min_buf = ~1;
                }
            }
            barrier();
            // Tempting to do this with subgroups, but atomic should be good enough.
            my_min = sh_first_el[th_ix];
            if (th_ix < N_WG) {
                atomicMin(sh_min_buf, my_min);
            }
            barrier();
            if (th_ix < N_WG) {
                if (sh_first_el[th_ix] == sh_min_buf) {
                    sh_elements_ref = sh_chunk[th_ix] + BinChunk_size;
                    uint selected_n = sh_chunk_n[th_ix];
                    sh_selected_n = selected_n;
                    uint next_chunk = sh_chunk_next[th_ix];
                    if (next_chunk == 0) {
                        sh_first_el[th_ix] = ~0;
                    } else {
                        sh_chunk[th_ix] = next_chunk;
                        BinChunk chunk = BinChunk_read(BinChunkRef(next_chunk));
                        sh_chunk_n[th_ix] = chunk.n;
                        sh_chunk_next[th_ix] = chunk.next.offset;
                        sh_first_el[th_ix] = BinInstance_read(
                            BinInstanceRef(next_chunk + BinChunk_size)).element_ix;
                    }
                }
            }
            barrier();
            uint chunk_n = sh_selected_n;
            if (chunk_n == 0) {
                // All chunks consumed
                break;
            }
            BinInstanceRef inst_ref = BinInstanceRef(sh_elements_ref);
            if (th_ix < chunk_n) {
                uint el = BinInstance_read(BinInstance_index(inst_ref, th_ix)).element_ix;
                sh_elements[(wr_ix + th_ix) % N_RINGBUF] = el;
            }
            wr_ix += chunk_n;
        }

        // We've done the merge and filled the buffer.

        // Read one element, compute coverage.
        uint tag = Annotated_Nop;
        AnnotatedRef ref;
        if (th_ix + rd_ix < wr_ix) {
            uint element_ix = sh_elements[(rd_ix + th_ix) % N_RINGBUF];
            ref = AnnotatedRef(element_ix * Annotated_size);
            tag = Annotated_tag(ref);
        }

        int x0 = 0, y0 = 0, x1 = 0, y1 = 0;
        switch (tag) {
        case Annotated_Line:
            AnnoLineSeg line = Annotated_Line_read(ref);
            x0 = int(floor((min(line.p0.x, line.p1.x) - line.stroke.x - xy0.x) * SX));
            y0 = int(floor((min(line.p0.y, line.p1.y) - line.stroke.y - xy0.y) * SY));
            x1 = int(ceil((max(line.p0.x, line.p1.x) + line.stroke.x - xy0.x) * SX));
            y1 = int(ceil((max(line.p0.y, line.p1.y) + line.stroke.y - xy0.y) * SY));
            break;
        case Annotated_Fill:
        case Annotated_Stroke:
            // Note: we take advantage of the fact that fills and strokes
            // have compatible layout.
            AnnoFill fill = Annotated_Fill_read(ref);
            x0 = int(floor((fill.bbox.x - xy0.x) * SX));
            y0 = int(floor((fill.bbox.y - xy0.y) * SY));
            x1 = int(ceil((fill.bbox.z - xy0.x) * SX));
            y1 = int(ceil((fill.bbox.w - xy0.y) * SY));
            break;
        }
        // At this point, we run an iterator over the coverage area,
        // trying to keep divergence low.
        // Right now, it's just a bbox, but we'll get finer with
        // segments.
        x0 = clamp(x0, 0, N_TILE_X);
        x1 = clamp(x1, x0, N_TILE_X);
        y0 = clamp(y0, 0, N_TILE_Y);
        y1 = clamp(y1, y0, N_TILE_Y);
        // This loop draws a rectangle to the coverage bitmasks. For
        // line segments, draw more precisely.
        if (x0 == x1) y1 = y0;
        int x = x0, y = y0;
        uint my_slice = th_ix / 32;
        uint my_mask = 1 << (th_ix & 31);
        while (y < y1) {
            atomicOr(sh_bitmaps[my_slice][y * N_TILE_X + x], my_mask);
            x++;
            if (x == x1) {
                x = x0;
                y++;
            }
        }

        // Output elements for this tile, based on bitmaps.
        uint slice_ix = 0;
        uint bitmap = sh_bitmaps[0][th_ix];
        while (true) {
            if (bitmap == 0) {
                slice_ix++;
                if (slice_ix == N_SLICE) {
                    break;
                }
                bitmap = sh_bitmaps[slice_ix][th_ix];
                if (bitmap == 0) {
                    continue;
                }
            }
            uint element_ref_ix = slice_ix * 32 + findLSB(bitmap);
            uint element_ix = sh_elements[(rd_ix + element_ref_ix) % N_RINGBUF];

            // At this point, we read the element again from global memory.
            // If that turns out to be expensive, maybe we can pack it into
            // shared memory (or perhaps just the tag).
            probe += 1;

            // clear LSB
            bitmap &= bitmap - 1;
        }

        rd_ix += N_TILE;
    } while (wr_ix > rd_ix);
    ptcl[bin_ix * N_TILE + th_ix] = probe;
}