src/gpu/ccpr/GrGSCoverageProcessor.cpp

/*
 * Copyright 2017 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "src/gpu/ccpr/GrGSCoverageProcessor.h"

#include "src/gpu/GrMesh.h"
#include "src/gpu/glsl/GrGLSLVertexGeoBuilder.h"

using InputType = GrGLSLGeometryBuilder::InputType;
using OutputType = GrGLSLGeometryBuilder::OutputType;

/**
 * This class and its subclasses implement the coverage processor with geometry shaders.
 */
class GrGSCoverageProcessor::Impl : public GrGLSLGeometryProcessor {
protected:
    Impl(std::unique_ptr<Shader> shader) : fShader(std::move(shader)) {}

    virtual bool hasCoverage(const GrGSCoverageProcessor& proc) const { return false; }

    void setData(const GrGLSLProgramDataManager& pdman, const GrPrimitiveProcessor&,
                 FPCoordTransformIter&& transformIter) final {
        this->setTransformDataHelper(SkMatrix::I(), pdman, &transformIter);
    }

    void onEmitCode(EmitArgs& args, GrGPArgs* gpArgs) final {
        const GrGSCoverageProcessor& proc = args.fGP.cast<GrGSCoverageProcessor>();

        // The vertex shader simply forwards transposed x or y values to the geometry shader.
        SkASSERT(1 == proc.numVertexAttributes());
        gpArgs->fPositionVar = proc.fInputXOrYValues.asShaderVar();

        // Geometry shader.
        GrGLSLVaryingHandler* varyingHandler = args.fVaryingHandler;
        this->emitGeometryShader(proc, varyingHandler, args.fGeomBuilder, args.fRTAdjustName);
        varyingHandler->emitAttributes(proc);
        varyingHandler->setNoPerspective();
        SkASSERT(!args.fFPCoordTransformHandler->nextCoordTransform());

        // Fragment shader.
        GrGLSLFPFragmentBuilder* f = args.fFragBuilder;
        f->codeAppendf("half coverage;");
        fShader->emitFragmentCoverageCode(f, "coverage");
        f->codeAppendf("%s = half4(coverage);", args.fOutputColor);
        f->codeAppendf("%s = half4(1);", args.fOutputCoverage);
    }

    void emitGeometryShader(
            const GrGSCoverageProcessor& proc, GrGLSLVaryingHandler* varyingHandler,
            GrGLSLGeometryBuilder* g, const char* rtAdjust) const {
        int numInputPoints = proc.numInputPoints();
        SkASSERT(3 == numInputPoints || 4 == numInputPoints);

        int inputWidth = (4 == numInputPoints || proc.hasInputWeight()) ? 4 : 3;
        const char* posValues = (4 == inputWidth) ? "sk_Position" : "sk_Position.xyz";
        g->codeAppendf("float%ix2 pts = transpose(float2x%i(sk_in[0].%s, sk_in[1].%s));",
                       inputWidth, inputWidth, posValues, posValues);

        GrShaderVar wind("wind", kHalf_GrSLType);
        g->declareGlobal(wind);
        Shader::CalcWind(proc, g, "pts", wind.c_str());
        if (PrimitiveType::kWeightedTriangles == proc.primitiveType()) {
            SkASSERT(3 == numInputPoints);
            SkASSERT(kFloat4_GrVertexAttribType == proc.fInputXOrYValues.cpuType());
            g->codeAppendf("%s *= half(sk_in[0].sk_Position.w);", wind.c_str());
        }

        SkString emitVertexFn;
        SkSTArray<3, GrShaderVar> emitArgs;
        const char* corner = emitArgs.emplace_back("corner", kFloat2_GrSLType).c_str();
        const char* bloatdir = emitArgs.emplace_back("bloatdir", kFloat2_GrSLType).c_str();
        const char* inputCoverage = nullptr;
        if (this->hasCoverage(proc)) {
            inputCoverage = emitArgs.emplace_back("coverage", kHalf_GrSLType).c_str();
        }
        const char* cornerCoverage = nullptr;
        if (Subpass::kCorners == proc.fSubpass) {
            cornerCoverage = emitArgs.emplace_back("corner_coverage", kHalf2_GrSLType).c_str();
        }
        g->emitFunction(kVoid_GrSLType, "emitVertex", emitArgs.count(), emitArgs.begin(), [&]() {
            SkString fnBody;
            fnBody.appendf("float2 vertexpos = fma(%s, float2(bloat), %s);", bloatdir, corner);
            const char* coverage = inputCoverage;
            if (!coverage) {
                if (!fShader->calculatesOwnEdgeCoverage()) {
                    // Flat edge opposite the curve. Coverages need full precision since distance
                    // to the opposite edge can be large.
                    fnBody.appendf("float coverage = dot(float3(vertexpos, 1), %s);",
                                   fEdgeDistanceEquation.c_str());
                } else {
                    // The "coverage" param should hold only the signed winding value.
                    fnBody.appendf("float coverage = 1;");
                }
                coverage = "coverage";
            }
            fnBody.appendf("%s *= %s;", coverage, wind.c_str());
            if (cornerCoverage) {
                fnBody.appendf("%s.x *= %s;", cornerCoverage, wind.c_str());
            }
            fShader->emitVaryings(varyingHandler, GrGLSLVarying::Scope::kGeoToFrag, &fnBody,
                                  "vertexpos", coverage, cornerCoverage, wind.c_str());
            g->emitVertex(&fnBody, "vertexpos", rtAdjust);
            return fnBody;
        }().c_str(), &emitVertexFn);

        float bloat = kAABloatRadius;
#ifdef SK_DEBUG
        if (proc.debugBloatEnabled()) {
            bloat *= proc.debugBloat();
        }
#endif
        g->defineConstant("bloat", bloat);

        if (!this->hasCoverage(proc) && !fShader->calculatesOwnEdgeCoverage()) {
            // Determine the amount of coverage to subtract out for the flat edge of the curve.
            g->declareGlobal(fEdgeDistanceEquation);
            g->codeAppendf("float2 p0 = pts[0], p1 = pts[%i];", numInputPoints - 1);
            g->codeAppendf("float2 n = float2(p0.y - p1.y, p1.x - p0.x);");
            g->codeAppend ("float nwidth = bloat*2 * (abs(n.x) + abs(n.y));");
            // When nwidth=0, wind must also be 0 (and coverage * wind = 0). So it doesn't matter
            // what we come up with here as long as it isn't NaN or Inf.
            g->codeAppend ("n /= (0 != nwidth) ? nwidth : 1;");
            g->codeAppendf("%s = float3(-n, dot(n, p0) - .5*sign(%s));",
                           fEdgeDistanceEquation.c_str(), wind.c_str());
        }

        this->onEmitGeometryShader(proc, g, wind, emitVertexFn.c_str());
    }

    virtual void onEmitGeometryShader(const GrGSCoverageProcessor&, GrGLSLGeometryBuilder*,
                                      const GrShaderVar& wind, const char* emitVertexFn) const = 0;

    const std::unique_ptr<Shader> fShader;
    const GrShaderVar fEdgeDistanceEquation{"edge_distance_equation", kFloat3_GrSLType};

    typedef GrGLSLGeometryProcessor INHERITED;
};

/**
 * Generates conservative rasters around a triangle and its edges, and calculates coverage ramps.
 *
 * Triangle rough outlines are drawn in two steps: (1) draw a conservative raster of the entire
 * triangle, with a coverage of +1, and (2) draw conservative rasters around each edge, with a
 * coverage ramp from -1 to 0. These edge coverage values convert jagged conservative raster edges
 * into smooth, antialiased ones.
 *
 * The final corners get touched up in a later step by TriangleCornerImpl.
 */
class GrGSCoverageProcessor::TriangleHullImpl : public GrGSCoverageProcessor::Impl {
public:
    TriangleHullImpl(std::unique_ptr<Shader> shader) : Impl(std::move(shader)) {}

    bool hasCoverage(const GrGSCoverageProcessor& proc) const override { return true; }

    void onEmitGeometryShader(const GrGSCoverageProcessor&, GrGLSLGeometryBuilder* g,
                              const GrShaderVar& wind, const char* emitVertexFn) const override {
        fShader->emitSetupCode(g, "pts");

        // Visualize the input triangle as upright and equilateral, with a flat base. Paying special
        // attention to wind, we can identify the points as top, bottom-left, and bottom-right.
        //
        // NOTE: We generate the rasters in 5 independent invocations, so each invocation designates
        // the corner it will begin with as the top.
        g->codeAppendf("int i = (%s > 0 ? sk_InvocationID : 4 - sk_InvocationID) %% 3;",
                       wind.c_str());
        g->codeAppend ("float2 top = pts[i];");
        g->codeAppendf("float2 right = pts[(i + (%s > 0 ? 1 : 2)) %% 3];", wind.c_str());
        g->codeAppendf("float2 left = pts[(i + (%s > 0 ? 2 : 1)) %% 3];", wind.c_str());

        // Determine which direction to outset the conservative raster from each of the three edges.
        g->codeAppend ("float2 leftbloat = sign(top - left);");
        g->codeAppend ("leftbloat = float2(0 != leftbloat.y ? leftbloat.y : leftbloat.x, "
                                          "0 != leftbloat.x ? -leftbloat.x : -leftbloat.y);");

        g->codeAppend ("float2 rightbloat = sign(right - top);");
        g->codeAppend ("rightbloat = float2(0 != rightbloat.y ? rightbloat.y : rightbloat.x, "
                                           "0 != rightbloat.x ? -rightbloat.x : -rightbloat.y);");

        g->codeAppend ("float2 downbloat = sign(left - right);");
        g->codeAppend ("downbloat = float2(0 != downbloat.y ? downbloat.y : downbloat.x, "
                                           "0 != downbloat.x ? -downbloat.x : -downbloat.y);");

        // The triangle's conservative raster has a coverage of +1 all around.
        g->codeAppend ("half4 coverages = half4(+1);");

        // Edges have coverage ramps.
        g->codeAppend ("if (sk_InvocationID >= 2) {"); // Are we an edge?
        Shader::CalcEdgeCoverageAtBloatVertex(g, "top", "right",
                                              "float2(+rightbloat.y, -rightbloat.x)",
                                              "coverages[0]");
        g->codeAppend (    "coverages.yzw = half3(-1, 0, -1 - coverages[0]);");
        // Reassign bloats to characterize a conservative raster around a single edge, rather than
        // the entire triangle.
        g->codeAppend (    "leftbloat = downbloat = -rightbloat;");
        g->codeAppend ("}");

        // Here we generate the conservative raster geometry. The triangle's conservative raster is
        // the convex hull of 3 pixel-size boxes centered on the input points. This translates to a
        // convex polygon with either one, two, or three vertices at each input point (depending on
        // how sharp the corner is) that we split between two invocations. Edge conservative rasters
        // are convex hulls of 2 pixel-size boxes, one at each endpoint. For more details on
        // conservative raster, see:
        // https://developer.nvidia.com/gpugems/GPUGems2/gpugems2_chapter42.html
        g->codeAppendf("bool2 left_right_notequal = notEqual(leftbloat, rightbloat);");
        g->codeAppend ("if (all(left_right_notequal)) {");
                           // The top corner will have three conservative raster vertices. Emit the
                           // middle one first to the triangle strip.
        g->codeAppendf(    "%s(top, float2(-leftbloat.y, +leftbloat.x), coverages[0]);",
                           emitVertexFn);
        g->codeAppend ("}");
        g->codeAppend ("if (any(left_right_notequal)) {");
                           // Second conservative raster vertex for the top corner.
        g->codeAppendf(    "%s(top, rightbloat, coverages[1]);", emitVertexFn);
        g->codeAppend ("}");

        // Main interior body.
        g->codeAppendf("%s(top, leftbloat, coverages[2]);", emitVertexFn);
        g->codeAppendf("%s(right, rightbloat, coverages[1]);", emitVertexFn);

        // Here the invocations diverge slightly. We can't symmetrically divide three triangle
        // points between two invocations, so each does the following:
        //
        // sk_InvocationID=0: Finishes the main interior body of the triangle hull.
        // sk_InvocationID=1: Remaining two conservative raster vertices for the third hull corner.
        // sk_InvocationID=2..4: Finish the opposite endpoint of their corresponding edge.
        g->codeAppendf("bool2 right_down_notequal = notEqual(rightbloat, downbloat);");
        g->codeAppend ("if (any(right_down_notequal) || 0 == sk_InvocationID) {");
        g->codeAppendf(    "%s((0 == sk_InvocationID) ? left : right, "
                              "(0 == sk_InvocationID) ? leftbloat : downbloat, "
                              "coverages[2]);", emitVertexFn);
        g->codeAppend ("}");
        g->codeAppend ("if (all(right_down_notequal) && 0 != sk_InvocationID) {");
        g->codeAppendf(    "%s(right, float2(-rightbloat.y, +rightbloat.x), coverages[3]);",
                           emitVertexFn);
        g->codeAppend ("}");

        // 5 invocations: 2 triangle hull invocations and 3 edges.
        g->configure(InputType::kLines, OutputType::kTriangleStrip, 6, 5);
    }
};

/**
 * Generates a conservative raster around a convex quadrilateral that encloses a cubic or quadratic.
 */
class GrGSCoverageProcessor::CurveHullImpl : public GrGSCoverageProcessor::Impl {
public:
    CurveHullImpl(std::unique_ptr<Shader> shader) : Impl(std::move(shader)) {}

    void onEmitGeometryShader(const GrGSCoverageProcessor&, GrGLSLGeometryBuilder* g,
                              const GrShaderVar& wind, const char* emitVertexFn) const override {
        const char* hullPts = "pts";
        fShader->emitSetupCode(g, "pts", &hullPts);

        // Visualize the input (convex) quadrilateral as a square. Paying special attention to wind,
        // we can identify the points by their corresponding corner.
        //
        // NOTE: We split the square down the diagonal from top-right to bottom-left, and generate
        // the hull in two independent invocations. Each invocation designates the corner it will
        // begin with as top-left.
        g->codeAppend ("int i = sk_InvocationID * 2;");
        g->codeAppendf("float2 topleft = %s[i];", hullPts);
        g->codeAppendf("float2 topright = %s[%s > 0 ? i + 1 : 3 - i];", hullPts, wind.c_str());
        g->codeAppendf("float2 bottomleft = %s[%s > 0 ? 3 - i : i + 1];", hullPts, wind.c_str());
        g->codeAppendf("float2 bottomright = %s[2 - i];", hullPts);

        // Determine how much to outset the conservative raster hull from the relevant edges.
        g->codeAppend ("float2 leftbloat = float2(topleft.y > bottomleft.y ? +1 : -1, "
                                                 "topleft.x > bottomleft.x ? -1 : +1);");
        g->codeAppend ("float2 upbloat = float2(topright.y > topleft.y ? +1 : -1, "
                                               "topright.x > topleft.x ? -1 : +1);");
        g->codeAppend ("float2 rightbloat = float2(bottomright.y > topright.y ? +1 : -1, "
                                                  "bottomright.x > topright.x ? -1 : +1);");

        // Here we generate the conservative raster geometry. It is the convex hull of 4 pixel-size
        // boxes centered on the input points, split evenly between two invocations. This translates
        // to a polygon with either one, two, or three vertices at each input point, depending on
        // how sharp the corner is. For more details on conservative raster, see:
        // https://developer.nvidia.com/gpugems/GPUGems2/gpugems2_chapter42.html
        g->codeAppendf("bool2 left_up_notequal = notEqual(leftbloat, upbloat);");
        g->codeAppend ("if (all(left_up_notequal)) {");
                           // The top-left corner will have three conservative raster vertices.
                           // Emit the middle one first to the triangle strip.
        g->codeAppendf(    "%s(topleft, float2(-leftbloat.y, leftbloat.x));", emitVertexFn);
        g->codeAppend ("}");
        g->codeAppend ("if (any(left_up_notequal)) {");
                           // Second conservative raster vertex for the top-left corner.
        g->codeAppendf(    "%s(topleft, leftbloat);", emitVertexFn);
        g->codeAppend ("}");

        // Main interior body of this invocation's half of the hull.
        g->codeAppendf("%s(topleft, upbloat);", emitVertexFn);
        g->codeAppendf("%s(bottomleft, leftbloat);", emitVertexFn);
        g->codeAppendf("%s(topright, upbloat);", emitVertexFn);

        // Remaining two conservative raster vertices for the top-right corner.
        g->codeAppendf("bool2 up_right_notequal = notEqual(upbloat, rightbloat);");
        g->codeAppend ("if (any(up_right_notequal)) {");
        g->codeAppendf(    "%s(topright, rightbloat);", emitVertexFn);
        g->codeAppend ("}");
        g->codeAppend ("if (all(up_right_notequal)) {");
        g->codeAppendf(    "%s(topright, float2(-upbloat.y, upbloat.x));", emitVertexFn);
        g->codeAppend ("}");

        g->configure(InputType::kLines, OutputType::kTriangleStrip, 7, 2);
    }
};

/**
 * Generates conservative rasters around corners (aka pixel-size boxes) and calculates
 * coverage and attenuation ramps to fix up the coverage values written by the hulls.
 */
class GrGSCoverageProcessor::CornerImpl : public GrGSCoverageProcessor::Impl {
public:
    CornerImpl(std::unique_ptr<Shader> shader) : Impl(std::move(shader)) {}

    bool hasCoverage(const GrGSCoverageProcessor& proc) const override {
        return proc.isTriangles();
    }

    void onEmitGeometryShader(const GrGSCoverageProcessor& proc, GrGLSLGeometryBuilder* g,
                              const GrShaderVar& wind, const char* emitVertexFn) const override {
        fShader->emitSetupCode(g, "pts");

        g->codeAppendf("int corneridx = sk_InvocationID;");
        if (!proc.isTriangles()) {
            g->codeAppendf("corneridx *= %i;", proc.numInputPoints() - 1);
        }

        g->codeAppendf("float2 corner = pts[corneridx];");
        g->codeAppendf("float2 left = pts[(corneridx + (%s > 0 ? %i : 1)) %% %i];",
                       wind.c_str(), proc.numInputPoints() - 1, proc.numInputPoints());
        g->codeAppendf("float2 right = pts[(corneridx + (%s > 0 ? 1 : %i)) %% %i];",
                       wind.c_str(), proc.numInputPoints() - 1, proc.numInputPoints());

        g->codeAppend ("float2 leftdir = corner - left;");
        g->codeAppend ("leftdir = (float2(0) != leftdir) ? normalize(leftdir) : float2(1, 0);");

        g->codeAppend ("float2 rightdir = right - corner;");
        g->codeAppend ("rightdir = (float2(0) != rightdir) ? normalize(rightdir) : float2(1, 0);");

        // Find "outbloat" and "crossbloat" at our corner. The outbloat points diagonally out of the
        // triangle, in the direction that should ramp to zero coverage with attenuation. The
        // crossbloat runs perpindicular to outbloat.
        g->codeAppend ("float2 outbloat = float2(leftdir.x > rightdir.x ? +1 : -1, "
                                                "leftdir.y > rightdir.y ? +1 : -1);");
        g->codeAppend ("float2 crossbloat = float2(-outbloat.y, +outbloat.x);");

        g->codeAppend ("half attenuation; {");
        Shader::CalcCornerAttenuation(g, "leftdir", "rightdir", "attenuation");
        g->codeAppend ("}");

        if (proc.isTriangles()) {
            g->codeAppend ("half2 left_coverages; {");
            Shader::CalcEdgeCoveragesAtBloatVertices(g, "left", "corner", "-outbloat",
                                                     "-crossbloat", "left_coverages");
            g->codeAppend ("}");

            g->codeAppend ("half2 right_coverages; {");
            Shader::CalcEdgeCoveragesAtBloatVertices(g, "corner", "right", "-outbloat",
                                                     "crossbloat", "right_coverages");
            g->codeAppend ("}");

            // Emit a corner box. The first coverage argument erases the values that were written
            // previously by the hull and edge geometry. The second pair are multiplied together by
            // the fragment shader. They ramp to 0 with attenuation in the direction of outbloat,
            // and linearly from left-edge coverage to right-edge coverage in the direction of
            // crossbloat.
            //
            // NOTE: Since this is not a linear mapping, it is important that the box's diagonal
            // shared edge points in the direction of outbloat.
            g->codeAppendf("%s(corner, -crossbloat, right_coverages[1] - left_coverages[1],"
                              "half2(1 + left_coverages[1], 1));",
                           emitVertexFn);

            g->codeAppendf("%s(corner, outbloat, 1 + left_coverages[0] + right_coverages[0], "
                              "half2(0, attenuation));",
                           emitVertexFn);

            g->codeAppendf("%s(corner, -outbloat, -1 - left_coverages[0] - right_coverages[0], "
                              "half2(1 + left_coverages[0] + right_coverages[0], 1));",
                           emitVertexFn);

            g->codeAppendf("%s(corner, crossbloat, left_coverages[1] - right_coverages[1],"
                              "half2(1 + right_coverages[1], 1));",
                           emitVertexFn);
        } else {
            // Curves are simpler. Setting "wind = -wind" causes the Shader to erase what it had
            // written in the previous pass hull. Then, at each vertex of the corner box, the Shader
            // will calculate the curve's local coverage value, interpolate it alongside our
            // attenuation parameter, and multiply the two together for a final coverage value.
            g->codeAppendf("%s = -%s;", wind.c_str(), wind.c_str());
            if (!fShader->calculatesOwnEdgeCoverage()) {
                g->codeAppendf("%s = -%s;",
                               fEdgeDistanceEquation.c_str(), fEdgeDistanceEquation.c_str());
            }
            g->codeAppendf("%s(corner, -crossbloat, half2(-1, 1));", emitVertexFn);
            g->codeAppendf("%s(corner, outbloat, half2(0, attenuation));",
                           emitVertexFn);
            g->codeAppendf("%s(corner, -outbloat, half2(-1, 1));", emitVertexFn);
            g->codeAppendf("%s(corner, crossbloat, half2(-1, 1));", emitVertexFn);
        }

        g->configure(InputType::kLines, OutputType::kTriangleStrip, 4, proc.isTriangles() ? 3 : 2);
    }
};

void GrGSCoverageProcessor::reset(PrimitiveType primitiveType, GrResourceProvider*) {
    fPrimitiveType = primitiveType;  // This will affect the return values for numInputPoints, etc.

    if (4 == this->numInputPoints() || this->hasInputWeight()) {
        fInputXOrYValues =
                {"x_or_y_values", kFloat4_GrVertexAttribType, kFloat4_GrSLType};
        GR_STATIC_ASSERT(sizeof(QuadPointInstance) ==
                         2 * GrVertexAttribTypeSize(kFloat4_GrVertexAttribType));
        GR_STATIC_ASSERT(offsetof(QuadPointInstance, fY) ==
                         GrVertexAttribTypeSize(kFloat4_GrVertexAttribType));
    } else {
        fInputXOrYValues =
                {"x_or_y_values", kFloat3_GrVertexAttribType, kFloat3_GrSLType};
        GR_STATIC_ASSERT(sizeof(TriPointInstance) ==
                         2 * GrVertexAttribTypeSize(kFloat3_GrVertexAttribType));
    }

    this->setVertexAttributes(&fInputXOrYValues, 1);
}

void GrGSCoverageProcessor::appendMesh(sk_sp<const GrGpuBuffer> instanceBuffer, int instanceCount,
                                       int baseInstance, SkTArray<GrMesh>* out) const {
    // We don't actually make instanced draw calls. Instead, we feed transposed x,y point values to
    // the GPU in a regular vertex array and draw kLines (see initGS). Then, each vertex invocation
    // receives either the shape's x or y values as inputs, which it forwards to the geometry
    // shader.
    GrMesh& mesh = out->emplace_back(GrPrimitiveType::kLines);
    mesh.setNonIndexedNonInstanced(instanceCount * 2);
    mesh.setVertexData(std::move(instanceBuffer), baseInstance * 2);
}

void GrGSCoverageProcessor::draw(
        GrOpFlushState* flushState, const GrPipeline& pipeline, const SkIRect scissorRects[],
        const GrMesh meshes[], int meshCount, const SkRect& drawBounds) const {
    // The geometry shader impl draws primitives in two subpasses: The first pass fills the interior
    // and does edge AA. The second pass does touch up on corner pixels.
    for (int i = 0; i < 2; ++i) {
        fSubpass = (Subpass) i;
        this->GrCCCoverageProcessor::draw(
                flushState, pipeline, scissorRects, meshes, meshCount, drawBounds);
    }
}

GrGLSLPrimitiveProcessor* GrGSCoverageProcessor::onCreateGLSLInstance(
        std::unique_ptr<Shader> shader) const {
    if (Subpass::kHulls == fSubpass) {
        return this->isTriangles()
                   ? (Impl*) new TriangleHullImpl(std::move(shader))
                   : (Impl*) new CurveHullImpl(std::move(shader));
    }
    SkASSERT(Subpass::kCorners == fSubpass);
    return new CornerImpl(std::move(shader));
}