shader/shared/drawtag.wgsl

// Copyright 2022 the Vello Authors
// SPDX-License-Identifier: Apache-2.0 OR MIT OR Unlicense

// The DrawMonoid is computed as a prefix sum to aid in decoding
// the variable-length encoding of draw objects.
struct DrawMonoid {
    // The number of paths preceding this draw object.
    path_ix: u32,
    // The number of clip operations preceding this draw object.
    clip_ix: u32,
    // The offset of the encoded draw object in the scene (u32s).
    scene_offset: u32,
    // The offset of the associated info.
    info_offset: u32,
}

// Each draw object has a 32-bit draw tag, which is a bit-packed
// version of the draw monoid.
let DRAWTAG_NOP = 0u;
let DRAWTAG_FILL_COLOR = 0x44u;
let DRAWTAG_FILL_LIN_GRADIENT = 0x114u;
let DRAWTAG_FILL_RAD_GRADIENT = 0x29cu;
let DRAWTAG_FILL_SWEEP_GRADIENT = 0x254u;
let DRAWTAG_FILL_IMAGE = 0x248u;
let DRAWTAG_BEGIN_CLIP = 0x9u;
let DRAWTAG_END_CLIP = 0x21u;

/// The first word of each draw info stream entry contains the flags. This is not a part of the
/// draw object stream but get used after the draw objects have been reduced on the GPU.
/// 0 represents a non-zero fill. 1 represents an even-odd fill.
let DRAW_INFO_FLAGS_FILL_RULE_BIT = 1u;

fn draw_monoid_identity() -> DrawMonoid {
    return DrawMonoid();
}

fn combine_draw_monoid(a: DrawMonoid, b: DrawMonoid) -> DrawMonoid {
    var c: DrawMonoid;
    c.path_ix = a.path_ix + b.path_ix;
    c.clip_ix = a.clip_ix + b.clip_ix;
    c.scene_offset = a.scene_offset + b.scene_offset;
    c.info_offset = a.info_offset + b.info_offset;
    return c;
}

fn map_draw_tag(tag_word: u32) -> DrawMonoid {
    var c: DrawMonoid;
    c.path_ix = u32(tag_word != DRAWTAG_NOP);
    c.clip_ix = tag_word & 1u;
    c.scene_offset = (tag_word >> 2u) & 0x07u;
    c.info_offset = (tag_word >> 6u) & 0x0fu;
    return c;
}