src/gpu/ganesh/tessellate/StrokeTessellator.cpp

/*
 * Copyright 2022 Google LLC
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "src/gpu/ganesh/tessellate/StrokeTessellator.h"

#include "src/core/SkGeometry.h"
#include "src/core/SkPathPriv.h"
#include "src/gpu/ganesh/GrMeshDrawTarget.h"
#include "src/gpu/ganesh/GrOpFlushState.h"
#include "src/gpu/ganesh/GrResourceProvider.h"
#include "src/gpu/ganesh/tessellate/VertexChunkPatchAllocator.h"
#include "src/gpu/tessellate/PatchWriter.h"
#include "src/gpu/tessellate/StrokeIterator.h"
#include "src/gpu/tessellate/WangsFormula.h"

namespace skgpu::v1 {

namespace {

using namespace skgpu::tess;

using StrokeWriter = PatchWriter<VertexChunkPatchAllocator,
                                 Required<PatchAttribs::kJoinControlPoint>,
                                 Optional<PatchAttribs::kStrokeParams>,
                                 Optional<PatchAttribs::kColor>,
                                 Optional<PatchAttribs::kWideColorIfEnabled>,
                                 Optional<PatchAttribs::kExplicitCurveType>,
                                 ReplicateLineEndPoints,
                                 TrackJoinControlPoints>;

void write_fixed_count_patches(StrokeWriter&& patchWriter,
                               const SkMatrix& shaderMatrix,
                               StrokeTessellator::PathStrokeList* pathStrokeList) {
    // The vector xform approximates how the control points are transformed by the shader to
    // more accurately compute how many *parametric* segments are needed.
    // getMaxScale() returns -1 if it can't compute a scale factor (e.g. perspective), taking the
    // absolute value automatically converts that to an identity scale factor for our purposes.
    patchWriter.setShaderTransform(wangs_formula::VectorXform{shaderMatrix},
                                   std::abs(shaderMatrix.getMaxScale()));
    if (!(patchWriter.attribs() & PatchAttribs::kStrokeParams)) {
        // Strokes are static. Calculate tolerances once.
        patchWriter.updateUniformStrokeParams(pathStrokeList->fStroke);
    }

    for (auto* pathStroke = pathStrokeList; pathStroke; pathStroke = pathStroke->fNext) {
        const SkStrokeRec& stroke = pathStroke->fStroke;
        if (patchWriter.attribs() & PatchAttribs::kStrokeParams) {
            // Strokes are dynamic. Calculate tolerances every time.
            patchWriter.updateStrokeParamsAttrib(stroke);
        }
        if (patchWriter.attribs() & PatchAttribs::kColor) {
            patchWriter.updateColorAttrib(pathStroke->fColor);
        }

        StrokeIterator strokeIter(pathStroke->fPath, &pathStroke->fStroke, &shaderMatrix);
        while (strokeIter.next()) {
            using Verb = StrokeIterator::Verb;
            const SkPoint* p = strokeIter.pts();
            int numChops;
            switch (strokeIter.verb()) {
                case Verb::kContourFinished:
                    patchWriter.writeDeferredStrokePatch();
                    break;
                case Verb::kCircle:
                    // Round cap or else an empty stroke that is specified to be drawn as a circle.
                    patchWriter.writeCircle(p[0]);
                    [[fallthrough]];
                case Verb::kMoveWithinContour:
                    // A regular kMove invalidates the previous control point; the stroke iterator
                    // tells us a new value to use.
                    patchWriter.updateJoinControlPointAttrib(p[0]);
                    break;
                case Verb::kLine:
                    patchWriter.writeLine(p[0], p[1]);
                    break;
                case Verb::kQuad:
                    if (ConicHasCusp(p)) {
                        // The cusp is always at the midtandent.
                        SkPoint cusp = SkEvalQuadAt(p, SkFindQuadMidTangent(p));
                        patchWriter.writeCircle(cusp);
                        // A quad can only have a cusp if it's flat with a 180-degree turnaround.
                        patchWriter.writeLine(p[0], cusp);
                        patchWriter.writeLine(cusp, p[2]);
                    } else {
                        patchWriter.writeQuadratic(p);
                    }
                    break;
                case Verb::kConic:
                    if (ConicHasCusp(p)) {
                        // The cusp is always at the midtandent.
                        SkConic conic(p, strokeIter.w());
                        SkPoint cusp = conic.evalAt(conic.findMidTangent());
                        patchWriter.writeCircle(cusp);
                        // A conic can only have a cusp if it's flat with a 180-degree turnaround.
                        patchWriter.writeLine(p[0], cusp);
                        patchWriter.writeLine(cusp, p[2]);
                    } else {
                        patchWriter.writeConic(p, strokeIter.w());
                    }
                    break;
                case Verb::kCubic:
                    SkPoint chops[10];
                    float T[2];
                    bool areCusps;
                    numChops = FindCubicConvex180Chops(p, T, &areCusps);
                    if (numChops == 0) {
                        patchWriter.writeCubic(p);
                    } else if (numChops == 1) {
                        SkChopCubicAt(p, chops, T[0]);
                        if (areCusps) {
                            patchWriter.writeCircle(chops[3]);
                            // In a perfect world, these 3 points would be be equal after chopping
                            // on a cusp.
                            chops[2] = chops[4] = chops[3];
                        }
                        patchWriter.writeCubic(chops);
                        patchWriter.writeCubic(chops + 3);
                    } else {
                        SkASSERT(numChops == 2);
                        SkChopCubicAt(p, chops, T[0], T[1]);
                        if (areCusps) {
                            patchWriter.writeCircle(chops[3]);
                            patchWriter.writeCircle(chops[6]);
                            // Two cusps are only possible if it's a flat line with two 180-degree
                            // turnarounds.
                            patchWriter.writeLine(chops[0], chops[3]);
                            patchWriter.writeLine(chops[3], chops[6]);
                            patchWriter.writeLine(chops[6], chops[9]);
                        } else {
                            patchWriter.writeCubic(chops);
                            patchWriter.writeCubic(chops + 3);
                            patchWriter.writeCubic(chops + 6);
                        }
                    }
                    break;
            }
        }
    }
}

}  // namespace


SKGPU_DECLARE_STATIC_UNIQUE_KEY(gVertexIDFallbackBufferKey);

void StrokeTessellator::prepare(GrMeshDrawTarget* target,
                                const SkMatrix& shaderMatrix,
                                PathStrokeList* pathStrokeList,
                                int totalCombinedStrokeVerbCnt) {
    LinearTolerances worstCase;
    const int preallocCount = FixedCountStrokes::PreallocCount(totalCombinedStrokeVerbCnt);
    StrokeWriter patchWriter{fAttribs, &worstCase,  target, &fVertexChunkArray, preallocCount};

    write_fixed_count_patches(std::move(patchWriter), shaderMatrix, pathStrokeList);
    fVertexCount = FixedCountStrokes::VertexCount(worstCase);

    if (!target->caps().shaderCaps()->fVertexIDSupport) {
        // Our shader won't be able to use sk_VertexID. Bind a fallback vertex buffer with the IDs
        // in it instead.
        fVertexCount = std::min(fVertexCount, 2 * FixedCountStrokes::kMaxEdgesNoVertexIDs);

        SKGPU_DEFINE_STATIC_UNIQUE_KEY(gVertexIDFallbackBufferKey);

        fVertexBufferIfNoIDSupport = target->resourceProvider()->findOrMakeStaticBuffer(
                GrGpuBufferType::kVertex,
                FixedCountStrokes::VertexBufferSize(),
                gVertexIDFallbackBufferKey,
                FixedCountStrokes::WriteVertexBuffer);
    }
}

void StrokeTessellator::draw(GrOpFlushState* flushState) const {
    if (fVertexChunkArray.empty() || fVertexCount <= 0) {
        return;
    }
    if (!flushState->caps().shaderCaps()->fVertexIDSupport &&
        !fVertexBufferIfNoIDSupport) {
        return;
    }
    for (const auto& instanceChunk : fVertexChunkArray) {
        flushState->bindBuffers(nullptr, instanceChunk.fBuffer, fVertexBufferIfNoIDSupport);
        flushState->drawInstanced(instanceChunk.fCount,
                                  instanceChunk.fBase,
                                  fVertexCount,
                                  0);
    }
}

}  // namespace skgpu::v1