vello/piet-gpu/shader/clip_reduce.comp

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

// The reduce pass for clip stack processing.

// The primary input is a sequence of path ids representing paths to
// push, with a special value of ~0 to represent pop.

// For each path, the bounding box is found in the anno stream
// (anno_alloc), though this may change.

// Output is a stack monoid reduction for the partition. The Bic
// is stored in the BicBuf, and the stack slice in StackBuf.

// Note: for this shader, only pushes are represented in the stack
// monoid reduction output, so we don't have to worry about the
// interpretation of pops.

#version 450
#extension GL_GOOGLE_include_directive : enable

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

#define LG_WG_SIZE (7 + LG_WG_FACTOR)
#define WG_SIZE (1 << LG_WG_SIZE)
#define PARTITION_SIZE WG_SIZE

layout(local_size_x = WG_SIZE) in;

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

// The intermediate state for clip processing.
struct ClipEl {
    // index of parent node
    uint parent_ix;
    // bounding box
    vec4 bbox;
};

// The bicyclic monoid
struct Bic {
    uint a;
    uint b;
};

Bic bic_combine(Bic x, Bic y) {
    uint m = min(x.b, y.a);
    return Bic(x.a + y.a - m, x.b + y.b - m);
}

shared Bic sh_bic[WG_SIZE];
shared uint sh_parent[WG_SIZE];
shared uint sh_path_ix[WG_SIZE];
shared vec4 sh_bbox[WG_SIZE];

// Load path's bbox from bbox (as written by pathseg).
vec4 load_path_bbox(uint path_ix) {
    uint base = (conf.path_bbox_alloc.offset >> 2) + 6 * path_ix;
    float bbox_l = float(memory[base]) - 32768.0;
    float bbox_t = float(memory[base + 1]) - 32768.0;
    float bbox_r = float(memory[base + 2]) - 32768.0;
    float bbox_b = float(memory[base + 3]) - 32768.0;
    vec4 bbox = vec4(bbox_l, bbox_t, bbox_r, bbox_b);
    return bbox;
}

vec4 bbox_intersect(vec4 a, vec4 b) {
    return vec4(max(a.xy, b.xy), min(a.zw, b.zw));
}

void store_bic(uint ix, Bic bic) {
    uint base = (conf.clip_bic_alloc.offset >> 2) + 2 * ix;
    memory[base] = bic.a;
    memory[base + 1] = bic.b;
}

void store_clip_el(uint ix, ClipEl el) {
    uint base = (conf.clip_stack_alloc.offset >> 2) + 5 * ix;
    memory[base] = el.parent_ix;
    memory[base + 1] = floatBitsToUint(el.bbox.x);
    memory[base + 2] = floatBitsToUint(el.bbox.y);
    memory[base + 3] = floatBitsToUint(el.bbox.z);
    memory[base + 4] = floatBitsToUint(el.bbox.w);
}

void main() {
    uint th = gl_LocalInvocationID.x;
    uint inp = memory[(conf.clip_alloc.offset >> 2) + gl_GlobalInvocationID.x];
    bool is_push = int(inp) >= 0;
    // reverse scan of bicyclic semigroup
    Bic bic = Bic(1 - uint(is_push), uint(is_push));
    sh_bic[gl_LocalInvocationID.x] = bic;
    for (uint i = 0; i < LG_WG_SIZE; i++) {
        barrier();
        if (th + (1u << i) < WG_SIZE) {
            Bic other = sh_bic[gl_LocalInvocationID.x + (1u << i)];
            bic = bic_combine(bic, other);
        }
        barrier();
        sh_bic[th] = bic;
    }
    if (th == 0) {
        store_bic(gl_WorkGroupID.x, bic);
    }
    barrier();
    uint size = sh_bic[0].b;
    bic = Bic(0, 0);
    if (th + 1 < WG_SIZE) {
        bic = sh_bic[th + 1];
    }
    if (is_push && bic.a == 0) {
        uint local_ix = size - bic.b - 1;
        sh_parent[local_ix] = th;
        sh_path_ix[local_ix] = inp;
    }
    barrier();
    // Do forward scan of bounding box intersection
    vec4 bbox;
    uint path_ix;
    if (th < size) {
        path_ix = sh_path_ix[th];
        bbox = load_path_bbox(path_ix);
    }
    // Not necessary if depth is bounded by wg size
#if 0
    for (uint i = 0; i < LG_WG_SIZE; i++) {
        // We gate so we never access uninit data, but it might
        // be more efficient to avoid the conditionals.
        if (th < size) {
            sh_bbox[th] = bbox;
        }
        barrier();
        if (th < size && th >= (1u << i)) {
            bbox = bbox_intersect(sh_bbox[th - (1u << i)], bbox);
        }
        barrier();
    }
#endif
    if (th < size) {
        uint parent_ix = sh_parent[th] + gl_WorkGroupID.x * PARTITION_SIZE;
        ClipEl el = ClipEl(parent_ix, bbox);
        store_clip_el(gl_GlobalInvocationID.x, el);
    }
}