vello/piet-gpu/shader/elements.comp

// The element processing stage, first in the pipeline.
//
// This stage is primarily about applying transforms and computing bounding
// boxes. It is organized as a scan over the input elements, producing
// annotated output elements.

#version 450
#extension GL_GOOGLE_include_directive : enable

#define N_ROWS 4
#define WG_SIZE 32
#define LG_WG_SIZE 5
#define PARTITION_SIZE (WG_SIZE * N_ROWS)

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

layout(set = 0, binding = 0) readonly buffer SceneBuf {
    uint[] scene;
};

// It would be better to use the Vulkan memory model than
// "volatile" but shooting for compatibility here rather
// than doing things right.
layout(set = 0, binding = 1) volatile buffer StateBuf {
    uint[] state;
};

// The annotated results are stored here.
layout(set = 0, binding = 2) buffer AnnotatedBuf {
    uint[] annotated;
};

#include "scene.h"
#include "state.h"
#include "annotated.h"

#define StateBuf_stride (8 + 2 * State_size)

StateRef state_aggregate_ref(uint partition_ix) {
    return StateRef(12 + partition_ix * StateBuf_stride);
}

StateRef state_prefix_ref(uint partition_ix) {
    return StateRef(12 + partition_ix * StateBuf_stride + State_size);
}

uint state_flag_index(uint partition_ix) {
    return 1 + partition_ix * (StateBuf_stride / 4);
}

// These correspond to X, A, P respectively in the prefix sum paper.
#define FLAG_NOT_READY 0
#define FLAG_AGGREGATE_READY 1
#define FLAG_PREFIX_READY 2

#define FLAG_SET_LINEWIDTH 1
#define FLAG_SET_BBOX 2
#define FLAG_RESET_BBOX 4

// This is almost like a monoid (the interaction between transformation and
// bounding boxes is approximate)
State combine_state(State a, State b) {
    State c;
    c.bbox.x = min(a.mat.x * b.bbox.x, a.mat.x * b.bbox.z) + min(a.mat.z * b.bbox.y, a.mat.z * b.bbox.w) + a.translate.x;
    c.bbox.y = min(a.mat.y * b.bbox.x, a.mat.y * b.bbox.z) + min(a.mat.w * b.bbox.y, a.mat.w * b.bbox.w) + a.translate.y;
    c.bbox.z = max(a.mat.x * b.bbox.x, a.mat.x * b.bbox.z) + max(a.mat.z * b.bbox.y, a.mat.z * b.bbox.w) + a.translate.x;
    c.bbox.w = max(a.mat.y * b.bbox.x, a.mat.y * b.bbox.z) + max(a.mat.w * b.bbox.y, a.mat.w * b.bbox.w) + a.translate.y;
    if ((a.flags & FLAG_RESET_BBOX) == 0 && b.bbox.z <= b.bbox.x && b.bbox.w <= b.bbox.y) {
        c.bbox = a.bbox;
    } else if ((a.flags & FLAG_RESET_BBOX) == 0 && (b.flags & FLAG_SET_BBOX) == 0 &&
        (a.bbox.z > a.bbox.x || a.bbox.w > a.bbox.y))
    {
        c.bbox.xy = min(a.bbox.xy, c.bbox.xy);
        c.bbox.zw = max(a.bbox.zw, c.bbox.zw);
    }
    // It would be more concise to cast to matrix types; ah well.
    c.mat.x = a.mat.x * b.mat.x + a.mat.z * b.mat.y;
    c.mat.y = a.mat.y * b.mat.x + a.mat.w * b.mat.y;
    c.mat.z = a.mat.x * b.mat.z + a.mat.z * b.mat.w;
    c.mat.w = a.mat.y * b.mat.z + a.mat.w * b.mat.w;
    c.translate.x = a.mat.x * b.translate.x + a.mat.z * b.translate.y + a.translate.x;
    c.translate.y = a.mat.y * b.translate.x + a.mat.w * b.translate.y + a.translate.y;
    c.linewidth = (b.flags & FLAG_SET_LINEWIDTH) == 0 ? a.linewidth : b.linewidth;
    c.flags = (a.flags & (FLAG_SET_LINEWIDTH | FLAG_SET_BBOX)) | b.flags;
    c.flags |= (a.flags & FLAG_RESET_BBOX) >> 1;
    return c;
}

State map_element(ElementRef ref, inout bool is_fill) {
    // TODO: it would *probably* be more efficient to make the memory read patterns less
    // divergent, though it would be more wasted memory.
    uint tag = Element_tag(ref);
    State c;
    c.bbox = vec4(0.0, 0.0, 0.0, 0.0);
    c.mat = vec4(1.0, 0.0, 0.0, 1.0);
    c.translate = vec2(0.0, 0.0);
    c.linewidth = 1.0; // TODO should be 0.0
    c.flags = 0;
    is_fill = false;
    switch (tag) {
    case Element_FillLine:
    case Element_StrokeLine:
        LineSeg line = Element_FillLine_read(ref);
        c.bbox.xy = min(line.p0, line.p1);
        c.bbox.zw = max(line.p0, line.p1);
        break;
    case Element_Quad:
        QuadSeg quad = Element_Quad_read(ref);
        c.bbox.xy = min(min(quad.p0, quad.p1), quad.p2);
        c.bbox.zw = max(max(quad.p0, quad.p1), quad.p2);
        break;
    case Element_Cubic:
        CubicSeg cubic = Element_Cubic_read(ref);
        c.bbox.xy = min(min(cubic.p0, cubic.p1), min(cubic.p2, cubic.p3));
        c.bbox.zw = max(max(cubic.p0, cubic.p1), max(cubic.p2, cubic.p3));
        break;
    case Element_Fill:
        is_fill = true;
        // fall-through
    case Element_Stroke:
        c.flags = FLAG_RESET_BBOX;
        break;
    case Element_SetLineWidth:
        SetLineWidth lw = Element_SetLineWidth_read(ref);
        c.linewidth = lw.width;
        c.flags = FLAG_SET_LINEWIDTH;
        break;
    case Element_Transform:
        Transform t = Element_Transform_read(ref);
        c.mat = t.mat;
        c.translate = t.translate;
        break;
    }
    return c;
}

// Get the bounding box of a circle transformed by the matrix into an ellipse.
vec2 get_linewidth(State st) {
    // See https://www.iquilezles.org/www/articles/ellipses/ellipses.htm
    return 0.5 * st.linewidth * vec2(length(st.mat.xz), length(st.mat.yw));
}

// We should be able to use an array of structs but the NV shader compiler
// doesn't seem to like it :/
//shared State sh_state[WG_SIZE];
shared vec4 sh_mat[WG_SIZE];
shared vec2 sh_translate[WG_SIZE];
shared vec4 sh_bbox[WG_SIZE];
shared float sh_width[WG_SIZE];
shared uint sh_flags[WG_SIZE];

shared uint sh_min_fill;

shared uint sh_tile_ix;
shared State sh_prefix;

void main() {
    State th_state[N_ROWS];
    // Determine partition to process by atomic counter (described in Section
    // 4.4 of prefix sum paper).
    if (gl_LocalInvocationID.x == 0) {
        sh_tile_ix = atomicAdd(state[0], 1);
        sh_min_fill = ~0;
    }
    barrier();
    uint tile_ix = sh_tile_ix;

    uint ix = tile_ix * PARTITION_SIZE + gl_LocalInvocationID.x * N_ROWS;
    ElementRef ref = ElementRef(ix * Element_size);

    bool is_fill;
    uint my_min_fill = ~0;
    th_state[0] = map_element(ref, is_fill);
    if (is_fill) my_min_fill = ix;
    for (uint i = 1; i < N_ROWS; i++) {
        // discussion question: would it be faster to load using more coherent patterns
        // into thread memory? This is kinda strided.
        th_state[i] = combine_state(th_state[i - 1], map_element(Element_index(ref, i), is_fill));
        if (is_fill && my_min_fill == ~0) {
            my_min_fill = ix + i;
        }
    }
    atomicMin(sh_min_fill, my_min_fill);
    State agg = th_state[N_ROWS - 1];
    sh_mat[gl_LocalInvocationID.x] = agg.mat;
    sh_translate[gl_LocalInvocationID.x] = agg.translate;
    sh_bbox[gl_LocalInvocationID.x] = agg.bbox;
    sh_width[gl_LocalInvocationID.x] = agg.linewidth;
    sh_flags[gl_LocalInvocationID.x] = agg.flags;
    for (uint i = 0; i < LG_WG_SIZE; i++) {
        barrier();
        if (gl_LocalInvocationID.x >= (1 << i)) {
            State other;
            uint ix = gl_LocalInvocationID.x - (1 << i);
            other.mat = sh_mat[ix];
            other.translate = sh_translate[ix];
            other.bbox = sh_bbox[ix];
            other.linewidth = sh_width[ix];
            other.flags = sh_flags[ix];
            agg = combine_state(other, agg);
        }
        barrier();
        sh_mat[gl_LocalInvocationID.x] = agg.mat;
        sh_translate[gl_LocalInvocationID.x] = agg.translate;
        sh_bbox[gl_LocalInvocationID.x] = agg.bbox;
        sh_width[gl_LocalInvocationID.x] = agg.linewidth;
        sh_flags[gl_LocalInvocationID.x] = agg.flags;
    }

    State exclusive;
    exclusive.bbox = vec4(0.0, 0.0, 0.0, 0.0);
    exclusive.mat = vec4(1.0, 0.0, 0.0, 1.0);
    exclusive.translate = vec2(0.0, 0.0);
    exclusive.linewidth = 1.0; //TODO should be 0.0
    exclusive.flags = 0;

    // Publish aggregate for this partition
    if (gl_LocalInvocationID.x == WG_SIZE - 1) {
        // Note: with memory model, we'd want to generate the atomic store version of this.
        State_write(state_aggregate_ref(tile_ix), agg);
        uint flag = FLAG_AGGREGATE_READY;
        memoryBarrierBuffer();
        if (tile_ix == 0) {
            State_write(state_prefix_ref(tile_ix), agg);
            flag = FLAG_PREFIX_READY;
        }
        state[state_flag_index(tile_ix)] = flag;
        if (tile_ix != 0) {
            // step 4 of paper: decoupled lookback
            uint look_back_ix = tile_ix - 1;
            while (true) {
                flag = state[state_flag_index(look_back_ix)];
                if (flag == FLAG_PREFIX_READY) {
                    State their_prefix = State_read(state_prefix_ref(look_back_ix));
                    exclusive = combine_state(their_prefix, exclusive);
                    break;
                } else if (flag == FLAG_AGGREGATE_READY) {
                    State their_agg = State_read(state_aggregate_ref(look_back_ix));
                    exclusive = combine_state(their_agg, exclusive);
                    look_back_ix--;
                }
                // else spin
            }

            // step 5 of paper: compute inclusive prefix
            State inclusive_prefix = combine_state(exclusive, agg);
            sh_prefix = exclusive;
            State_write(state_prefix_ref(tile_ix), inclusive_prefix);
            memoryBarrierBuffer();
            flag = FLAG_PREFIX_READY;
            state[state_flag_index(tile_ix)] = flag;
        }
    }
    barrier();
    my_min_fill = sh_min_fill;
    if (tile_ix != 0) {
        exclusive = sh_prefix;
    }

    State row = exclusive;
    if (gl_LocalInvocationID.x > 0) {
        uint ix = gl_LocalInvocationID.x - 1;
        State other;
        other.mat = sh_mat[ix];
        other.translate = sh_translate[ix];
        other.bbox = sh_bbox[ix];
        other.linewidth = sh_width[ix];
        other.flags = sh_flags[ix];
        row = combine_state(row, other);
    }
    if (my_min_fill == ~0 && gl_LocalInvocationID.x == 0) {
        state[state_flag_index(tile_ix) + 1] = 0x7f800000; // infinity
    }
    for (uint i = 0; i < N_ROWS; i++) {
        State st = combine_state(row, th_state[i]);
        if (my_min_fill == ix + i) {
            state[state_flag_index(tile_ix) + 1] = floatBitsToUint(st.bbox.z);
        }
        // We write the state now for development purposes, but the
        // actual goal is to write transformed and annotated elements.
        //State_write(StateRef((ix + i) * State_size), st);

        // Here we read again from the original scene. There may be
        // gains to be had from stashing in shared memory or possibly
        // registers (though register pressure is an issue).
        ElementRef this_ref = Element_index(ref, i);
        AnnotatedRef out_ref = AnnotatedRef((ix + i) * Annotated_size);
        uint tag = Element_tag(this_ref);
        switch (tag) {
        case Element_FillLine:
        case Element_StrokeLine:
            LineSeg line = Element_StrokeLine_read(this_ref);
            AnnoStrokeLineSeg anno_line;
            anno_line.p0 = st.mat.xy * line.p0.x + st.mat.zw * line.p0.y + st.translate;
            anno_line.p1 = st.mat.xy * line.p1.x + st.mat.zw * line.p1.y + st.translate;
            if (tag == Element_StrokeLine) {
                anno_line.stroke = get_linewidth(st);
            } else {
                anno_line.stroke = vec2(0.0);
            }
            // We do encoding a bit by hand to minimize divergence. Another approach
            // would be to have a fill/stroke bool.
            uint out_tag = tag == Element_FillLine ? Annotated_FillLine : Annotated_StrokeLine;
            annotated[out_ref.offset >> 2] = out_tag;
            AnnoStrokeLineSeg_write(AnnoStrokeLineSegRef(out_ref.offset + 4), anno_line);
            break;
        case Element_Stroke:
            Stroke stroke = Element_Stroke_read(this_ref);
            AnnoStroke anno_stroke;
            anno_stroke.rgba_color = stroke.rgba_color;
            vec2 lw = get_linewidth(st);
            anno_stroke.bbox = st.bbox + vec4(-lw, lw);
            anno_stroke.linewidth = st.linewidth * sqrt(st.mat.x * st.mat.w - st.mat.y * st.mat.z);
            Annotated_Stroke_write(out_ref, anno_stroke);
            break;
        case Element_Fill:
            Fill fill = Element_Fill_read(this_ref);
            AnnoFill anno_fill;
            anno_fill.rgba_color = fill.rgba_color;
            anno_fill.bbox = st.bbox;
            Annotated_Fill_write(out_ref, anno_fill);
            break;
        default:
            Annotated_Nop_write(out_ref);
            break;
        }
    }
}