src/effects/SkDiscretePathEffect.cpp

/*
 * Copyright 2006 The Android Open Source Project
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */


#include "include/core/SkPathMeasure.h"
#include "include/core/SkStrokeRec.h"
#include "include/effects/SkDiscretePathEffect.h"
#include "include/private/SkFixed.h"
#include "src/core/SkPathEffectBase.h"
#include "src/core/SkPointPriv.h"
#include "src/core/SkReadBuffer.h"
#include "src/core/SkWriteBuffer.h"

/** \class LCGRandom

    Utility class that implements pseudo random 32bit numbers using a fast
    linear equation. Unlike rand(), this class holds its own seed (initially
    set to 0), so that multiple instances can be used with no side-effects.

    Copied from the original implementation of SkRandom. Only contains the
    methods used by SkDiscretePathEffect::filterPath, with methods that were
    not called directly moved to private.
*/
class LCGRandom {
public:
    LCGRandom(uint32_t seed) : fSeed(seed) {}

    /** Return the next pseudo random number expressed as a SkScalar
        in the range [-SK_Scalar1..SK_Scalar1).
    */
    SkScalar nextSScalar1() { return SkFixedToScalar(this->nextSFixed1()); }

private:
    /** Return the next pseudo random number as an unsigned 32bit value.
    */
    uint32_t nextU() { uint32_t r = fSeed * kMul + kAdd; fSeed = r; return r; }

    /** Return the next pseudo random number as a signed 32bit value.
     */
    int32_t nextS() { return (int32_t)this->nextU(); }

    /** Return the next pseudo random number expressed as a signed SkFixed
     in the range [-SK_Fixed1..SK_Fixed1).
     */
    SkFixed nextSFixed1() { return this->nextS() >> 15; }

    //  See "Numerical Recipes in C", 1992 page 284 for these constants
    enum {
        kMul = 1664525,
        kAdd = 1013904223
    };
    uint32_t fSeed;
};

static void Perterb(SkPoint* p, const SkVector& tangent, SkScalar scale) {
    SkVector normal = tangent;
    SkPointPriv::RotateCCW(&normal);
    normal.setLength(scale);
    *p += normal;
}

class SK_API SkDiscretePathEffectImpl : public SkPathEffectBase {
public:
    SkDiscretePathEffectImpl(SkScalar segLength, SkScalar deviation, uint32_t seedAssist)
        : fSegLength(segLength), fPerterb(deviation), fSeedAssist(seedAssist)
    {
        SkASSERT(SkScalarIsFinite(segLength));
        SkASSERT(SkScalarIsFinite(deviation));
        SkASSERT(segLength > SK_ScalarNearlyZero);
    }

    bool onFilterPath(SkPath* dst, const SkPath& src, SkStrokeRec* rec,
                      const SkRect*) const override {
        bool doFill = rec->isFillStyle();

        SkPathMeasure   meas(src, doFill);

        /* Caller may supply their own seed assist, which by default is 0 */
        uint32_t seed = fSeedAssist ^ SkScalarRoundToInt(meas.getLength());

        LCGRandom   rand(seed ^ ((seed << 16) | (seed >> 16)));
        SkScalar    scale = fPerterb;
        SkPoint     p;
        SkVector    v;

        do {
            SkScalar    length = meas.getLength();
    #if defined(SK_BUILD_FOR_FUZZER)
            if (length > 1000) {
                return false;
            }
    #endif

            if (fSegLength * (2 + doFill) > length) {
                meas.getSegment(0, length, dst, true);  // to short for us to mangle
            } else {
                int         n = SkScalarRoundToInt(length / fSegLength);
                constexpr int kMaxReasonableIterations = 100000;
                n = std::min(n, kMaxReasonableIterations);
                SkScalar    delta = length / n;
                SkScalar    distance = 0;

                if (meas.isClosed()) {
                    n -= 1;
                    distance += delta/2;
                }

                if (meas.getPosTan(distance, &p, &v)) {
                    Perterb(&p, v, rand.nextSScalar1() * scale);
                    dst->moveTo(p);
                }
                while (--n >= 0) {
                    distance += delta;
                    if (meas.getPosTan(distance, &p, &v)) {
                        Perterb(&p, v, rand.nextSScalar1() * scale);
                        dst->lineTo(p);
                    }
                }
                if (meas.isClosed()) {
                    dst->close();
                }
            }
        } while (meas.nextContour());
        return true;
    }

    bool computeFastBounds(SkRect* bounds) const override {
        if (bounds) {
            SkScalar maxOutset = SkScalarAbs(fPerterb);
            bounds->outset(maxOutset, maxOutset);
        }
        return true;
    }

    static sk_sp<SkFlattenable> CreateProc(SkReadBuffer& buffer) {
        SkScalar segLength = buffer.readScalar();
        SkScalar perterb = buffer.readScalar();
        uint32_t seed = buffer.readUInt();
        return SkDiscretePathEffect::Make(segLength, perterb, seed);
    }

    void flatten(SkWriteBuffer& buffer) const override {
        buffer.writeScalar(fSegLength);
        buffer.writeScalar(fPerterb);
        buffer.writeUInt(fSeedAssist);
    }

    Factory getFactory() const override { return CreateProc; }
    const char* getTypeName() const override { return "SkDiscretePathEffect"; }

private:
    const SkScalar fSegLength,
                   fPerterb;
    /* Caller-supplied 32 bit seed assist */
    const uint32_t fSeedAssist;

    using INHERITED = SkPathEffectBase;
};

//////////////////////////////////////////////////////////////////////////////////////////////////

sk_sp<SkPathEffect> SkDiscretePathEffect::Make(SkScalar segLength, SkScalar deviation,
                                               uint32_t seedAssist) {
    if (!SkScalarIsFinite(segLength) || !SkScalarIsFinite(deviation)) {
        return nullptr;
    }
    if (segLength <= SK_ScalarNearlyZero) {
        return nullptr;
    }
    return sk_sp<SkPathEffect>(new SkDiscretePathEffectImpl(segLength, deviation, seedAssist));
}

void SkDiscretePathEffect::RegisterFlattenables() {
    SkFlattenable::Register("SkDiscretePathEffect", SkDiscretePathEffectImpl::CreateProc);
}