src/effects/SkGradientShader.cpp

/* libs/graphics/effects/SkGradientShader.cpp
**
** Copyright 2006, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License"); 
** you may not use this file except in compliance with the License. 
** You may obtain a copy of the License at 
**
**     http://www.apache.org/licenses/LICENSE-2.0 
**
** Unless required by applicable law or agreed to in writing, software 
** distributed under the License is distributed on an "AS IS" BASIS, 
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
** See the License for the specific language governing permissions and 
** limitations under the License.
*/

#include "SkGradientShader.h"
#include "SkColorPriv.h"
#include "SkUnitMapper.h"
#include "SkUtils.h"

/*
    ToDo

    - not sure we still need the full Rec struct, now that we're using a cache
    - detect const-alpha (but not opaque) in getFlags()
*/

/* dither seems to look better, but not stuningly yet, and it slows us down a little
    so its not on by default yet.
*/
#define TEST_GRADIENT_DITHER

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

typedef SkFixed (*TileProc)(SkFixed);

static SkFixed clamp_tileproc(SkFixed x) {
    return SkClampMax(x, 0xFFFF);
}

static SkFixed repeat_tileproc(SkFixed x) {
    return x & 0xFFFF;
}

static inline SkFixed mirror_tileproc(SkFixed x) {
    int s = x << 15 >> 31;
    return (x ^ s) & 0xFFFF;
}

static const TileProc gTileProcs[] = {
    clamp_tileproc,
    repeat_tileproc,
    mirror_tileproc
};

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

static inline int repeat_6bits(int x) {
    return x & 63;
}

static inline int mirror_6bits(int x) {
#ifdef SK_CPU_HAS_CONDITIONAL_INSTR
    if (x & 64)
        x = ~x;
    return x & 63;
#else
    int s = x << 25 >> 31;
    return (x ^ s) & 63;
#endif
}

static inline int repeat_8bits(int x) {
    return x & 0xFF;
}

static inline int mirror_8bits(int x) {
#ifdef SK_CPU_HAS_CONDITIONAL_INSTR
    if (x & 256) {
        x = ~x;
    }
    return x & 255;
#else
    int s = x << 23 >> 31;
    return (x ^ s) & 0xFF;
#endif
}

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

class Gradient_Shader : public SkShader {
public:
    Gradient_Shader(const SkColor colors[], const SkScalar pos[],
                int colorCount, SkShader::TileMode mode, SkUnitMapper* mapper);
    virtual ~Gradient_Shader();

    // overrides
    virtual bool setContext(const SkBitmap&, const SkPaint&, const SkMatrix&);
    virtual uint32_t getFlags() { return fFlags; }

protected:
    Gradient_Shader(SkFlattenableReadBuffer& );
    SkUnitMapper* fMapper;
    SkMatrix    fPtsToUnit;     // set by subclass
    SkMatrix    fDstToIndex;
    SkMatrix::MapXYProc fDstToIndexProc;
    SkPMColor*  fARGB32;
    TileMode    fTileMode;
    TileProc    fTileProc;
    int         fColorCount;
    uint8_t     fDstToIndexClass;
    uint8_t     fFlags;

    struct Rec {
        SkFixed     fPos;   // 0...1
        uint32_t    fScale; // (1 << 24) / range
    };
    Rec*        fRecs;

    enum {
        kCache16Bits    = 6,    // seems like enough for visual accuracy
        kCache16Count   = 1 << kCache16Bits,
        kCache32Bits    = 8,    // pretty much should always be 8
        kCache32Count   = 1 << kCache32Bits
    };
    virtual void flatten(SkFlattenableWriteBuffer& );
    const uint16_t*     getCache16();
    const SkPMColor*    getCache32();

    // called when we kill our cached colors (to be rebuilt later on demand)
    virtual void onCacheReset()  = 0;

private:
    enum {
        kColorStorageCount = 4, // more than this many colors, and we'll use sk_malloc for the space

        kStorageSize = kColorStorageCount * (sizeof(SkColor) + sizeof(SkPMColor) + sizeof(Rec))
    };
    SkColor     fStorage[(kStorageSize + 3) >> 2];
    SkColor*    fOrigColors;

    uint16_t*   fCache16;   // working ptr. If this is NULL, we need to recompute the cache values
    SkPMColor*  fCache32;   // working ptr. If this is NULL, we need to recompute the cache values

    uint16_t*   fCache16Storage;    // storage for fCache16, allocated on demand
    SkPMColor*  fCache32Storage;    // storage for fCache32, allocated on demand
    unsigned    fCacheAlpha;        // the alpha value we used when we computed the cache. larger than 8bits so we can store uninitialized value

    typedef SkShader INHERITED;
};

static inline unsigned scalarToU16(SkScalar x) {
    SkASSERT(x >= 0 && x <= SK_Scalar1);

#ifdef SK_SCALAR_IS_FLOAT
    return (unsigned)(x * 0xFFFF);
#else
    return x - (x >> 16);   // probably should be x - (x > 0x7FFF) but that is slower
#endif
}

Gradient_Shader::Gradient_Shader(const SkColor colors[], const SkScalar pos[],
             int colorCount, SkShader::TileMode mode, SkUnitMapper* mapper) {
    SkASSERT(colorCount > 1);

    fCacheAlpha = 256;  // init to a value that paint.getAlpha() can't return

    fMapper = mapper;
    mapper->safeRef();

    SkASSERT((unsigned)mode < SkShader::kTileModeCount);
    SkASSERT(SkShader::kTileModeCount == SK_ARRAY_COUNT(gTileProcs));
    fTileMode = mode;
    fTileProc = gTileProcs[mode];
    
    fCache16 = fCache16Storage = NULL;
    fCache32 = fCache32Storage = NULL;

    /*  Note: we let the caller skip the first and/or last position.
        i.e. pos[0] = 0.3, pos[1] = 0.7
        In these cases, we insert dummy entries to ensure that the final data
        will be bracketed by [0, 1].
        i.e. our_pos[0] = 0, our_pos[1] = 0.3, our_pos[2] = 0.7, our_pos[3] = 1

        Thus colorCount (the caller's value, and fColorCount (our value) may
        differ by up to 2. In the above example:
            colorCount = 2
            fColorCount = 4
     */
    fColorCount = colorCount;
    // check if we need to add in dummy start and/or end position/colors
    bool dummyFirst = false;
    bool dummyLast = false;
    if (pos) {
        dummyFirst = pos[0] != 0;
        dummyLast = pos[colorCount - 1] != SK_Scalar1;
        fColorCount += dummyFirst + dummyLast;
    }

    if (fColorCount > kColorStorageCount) {
        size_t size = sizeof(SkColor) + sizeof(SkPMColor) + sizeof(Rec);
        fOrigColors = reinterpret_cast<SkColor*>(
                                        sk_malloc_throw(size * fColorCount));
    }
    else {
        fOrigColors = fStorage;
    }

    // Now copy over the colors, adding the dummies as needed
    {
        SkColor* origColors = fOrigColors;
        if (dummyFirst) {
            *origColors++ = colors[0];
        }
        memcpy(origColors, colors, colorCount * sizeof(SkColor));
        if (dummyLast) {
            origColors += colorCount;
            *origColors = colors[colorCount - 1];
        }
    }

    // our premul colors point to the 2nd half of the array
    // these are assigned each time in setContext
    fARGB32 = fOrigColors + fColorCount;
    fRecs = (Rec*)(fARGB32 + fColorCount);
    if (fColorCount > 2) {
        Rec* recs = fRecs;
        recs->fPos = 0;
        //  recs->fScale = 0; // unused;
        recs += 1;
        if (pos) {
            /*  We need to convert the user's array of relative positions into
                fixed-point positions and scale factors. We need these results
                to be strictly monotonic (no two values equal or out of order).
                Hence this complex loop that just jams a zero for the scale
                value if it sees a segment out of order, and it assures that
                we start at 0 and end at 1.0
            */
            SkFixed prev = 0;
            int startIndex = dummyFirst ? 0 : 1;
            int count = colorCount + dummyLast;
            for (int i = startIndex; i < count; i++) {
                // force the last value to be 1.0
                SkFixed curr;
                if (i == colorCount) {  // we're really at the dummyLast
                    curr = SK_Fixed1;
                } else {
                    curr = SkScalarToFixed(pos[i]);
                }
                // pin curr withing range
                if (curr < 0) {
                    curr = 0;
                } else if (curr > SK_Fixed1) {
                    curr = SK_Fixed1;
                }
                recs->fPos = curr;
                if (curr > prev) {
                    recs->fScale = (1 << 24) / (curr - prev);
                } else {
                    recs->fScale = 0; // ignore this segment
                }
                // get ready for the next value
                prev = curr;
                recs += 1;
            }
        } else {    // assume even distribution
            SkFixed dp = SK_Fixed1 / (colorCount - 1);
            SkFixed p = dp;
            SkFixed scale = (colorCount - 1) << 8;  // (1 << 24) / dp
            for (int i = 1; i < colorCount; i++) {
                recs->fPos   = p;
                recs->fScale = scale;
                recs += 1;
                p += dp;
            }
        }
    }
    fFlags = 0;
}

Gradient_Shader::Gradient_Shader(SkFlattenableReadBuffer& buffer) :
    INHERITED(buffer) {
    fCacheAlpha = 256;

    fMapper = static_cast<SkUnitMapper*>(buffer.readFlattenable());

    fCache16 = fCache16Storage = NULL;
    fCache32 = fCache32Storage = NULL;

    int colorCount = fColorCount = buffer.readU32();
    if (colorCount > kColorStorageCount) {
        size_t size = sizeof(SkColor) + sizeof(SkPMColor) + sizeof(Rec);
        fOrigColors = (SkColor*)sk_malloc_throw(size * colorCount);
    } else {
        fOrigColors = fStorage;
    }
    buffer.read(fOrigColors, colorCount * sizeof(SkColor));
    fARGB32 = fOrigColors + colorCount;

    fTileMode = (TileMode)buffer.readU8();
    fTileProc = gTileProcs[fTileMode];
    fRecs = (Rec*)(fARGB32 + colorCount);
    if (colorCount > 2) {
        Rec* recs = fRecs;
        recs[0].fPos = 0;
        for (int i = 1; i < colorCount; i++) {
            recs[i].fPos = buffer.readS32();
            recs[i].fScale = buffer.readU32();
        }
    }
    buffer.read(&fPtsToUnit, sizeof(SkMatrix));
    fFlags = 0;
}

Gradient_Shader::~Gradient_Shader() {
    if (fCache16Storage) {
        sk_free(fCache16Storage);
    }
    if (fCache32Storage) {
        sk_free(fCache32Storage);
    }
    if (fOrigColors != fStorage) {
        sk_free(fOrigColors);
    }
    fMapper->safeUnref();
}

void Gradient_Shader::flatten(SkFlattenableWriteBuffer& buffer) {
    this->INHERITED::flatten(buffer);
    buffer.writeFlattenable(fMapper);
    buffer.write32(fColorCount);
    buffer.writeMul4(fOrigColors, fColorCount * sizeof(SkColor));
    buffer.write8(fTileMode);
    if (fColorCount > 2) {
        Rec* recs = fRecs;
        for (int i = 1; i < fColorCount; i++) {
            buffer.write32(recs[i].fPos);
            buffer.write32(recs[i].fScale);
        }
    }
    buffer.writeMul4(&fPtsToUnit, sizeof(SkMatrix));
}

bool Gradient_Shader::setContext(const SkBitmap& device,
                                 const SkPaint& paint,
                                 const SkMatrix& matrix) {
    if (!this->INHERITED::setContext(device, paint, matrix)) {
        return false;
    }

    const SkMatrix& inverse = this->getTotalInverse();

    if (!fDstToIndex.setConcat(fPtsToUnit, inverse)) {
        return false;
    }

    fDstToIndexProc = fDstToIndex.getMapXYProc();
    fDstToIndexClass = (uint8_t)SkShader::ComputeMatrixClass(fDstToIndex);

    // now convert our colors in to PMColors
    unsigned paintAlpha = this->getPaintAlpha();
    unsigned colorAlpha = 0xFF;

    for (int i = 0; i < fColorCount; i++) {
        SkColor src = fOrigColors[i];
        unsigned sa = SkColorGetA(src);
        colorAlpha &= sa;
        
        // now modulate it by the paint for our resulting ARGB32 array
        sa = SkMulDiv255Round(sa, paintAlpha);
        fARGB32[i] = SkPreMultiplyARGB(sa, SkColorGetR(src), SkColorGetG(src),
                                       SkColorGetB(src));
    }

    fFlags = this->INHERITED::getFlags();
    if ((colorAlpha & paintAlpha) == 0xFF) {
        fFlags |= kOpaqueAlpha_Flag;
    }
    // we can do span16 as long as our individual colors are opaque,
    // regardless of the paint's alpha
    if (0xFF == colorAlpha) {
        fFlags |= kHasSpan16_Flag;
    }

    // if the new alpha differs from the previous time we were called, inval our cache
    // this will trigger the cache to be rebuilt.
    // we don't care about the first time, since the cache ptrs will already be NULL
    if (fCacheAlpha != paintAlpha) {
        fCache16 = NULL;                // inval the cache
        fCache32 = NULL;                // inval the cache
        fCacheAlpha = paintAlpha;       // record the new alpha
        // inform our subclasses
        this->onCacheReset();
    }
    return true;
}

static inline int blend8(int a, int b, int scale) {
    SkASSERT(a == SkToU8(a));
    SkASSERT(b == SkToU8(b));
    SkASSERT(scale >= 0 && scale <= 256);
    return a + ((b - a) * scale >> 8);
}

static inline uint32_t dot8_blend_packed32(uint32_t s0, uint32_t s1,
                                           int blend) {
#if 0
    int a = blend8(SkGetPackedA32(s0), SkGetPackedA32(s1), blend);
    int r = blend8(SkGetPackedR32(s0), SkGetPackedR32(s1), blend);
    int g = blend8(SkGetPackedG32(s0), SkGetPackedG32(s1), blend);
    int b = blend8(SkGetPackedB32(s0), SkGetPackedB32(s1), blend);

    return SkPackARGB32(a, r, g, b);
#else
    int otherBlend = 256 - blend;

#if 0
    U32 t0 = (((s0 & 0xFF00FF) * blend + (s1 & 0xFF00FF) * otherBlend) >> 8) & 0xFF00FF;
    U32 t1 = (((s0 >> 8) & 0xFF00FF) * blend + ((s1 >> 8) & 0xFF00FF) * otherBlend) & 0xFF00FF00;
    SkASSERT((t0 & t1) == 0);
    return t0 | t1;
#else
    return  ((((s0 & 0xFF00FF) * blend + (s1 & 0xFF00FF) * otherBlend) >> 8) & 0xFF00FF) |
            ((((s0 >> 8) & 0xFF00FF) * blend + ((s1 >> 8) & 0xFF00FF) * otherBlend) & 0xFF00FF00);
#endif

#endif
}

#define Fixed_To_Dot8(x)        (((x) + 0x80) >> 8)

/** We take the original colors, not our premultiplied PMColors, since we can
    build a 16bit table as long as the original colors are opaque, even if the
    paint specifies a non-opaque alpha.
*/
static void build_16bit_cache(uint16_t cache[], SkColor c0, SkColor c1,
                              int count) {
    SkASSERT(count > 1);
    SkASSERT(SkColorGetA(c0) == 0xFF);
    SkASSERT(SkColorGetA(c1) == 0xFF);

    SkFixed r = SkColorGetR(c0);
    SkFixed g = SkColorGetG(c0);
    SkFixed b = SkColorGetB(c0);

    SkFixed dr = SkIntToFixed(SkColorGetR(c1) - r) / (count - 1);
    SkFixed dg = SkIntToFixed(SkColorGetG(c1) - g) / (count - 1);
    SkFixed db = SkIntToFixed(SkColorGetB(c1) - b) / (count - 1);

    r = SkIntToFixed(r) + 0x8000;
    g = SkIntToFixed(g) + 0x8000;
    b = SkIntToFixed(b) + 0x8000;

    do {
        unsigned rr = r >> 16;
        unsigned gg = g >> 16;
        unsigned bb = b >> 16;
        cache[0] = SkPackRGB16(SkR32ToR16(rr), SkG32ToG16(gg), SkB32ToB16(bb));
        cache[64] = SkDitherPack888ToRGB16(rr, gg, bb);
        cache += 1;
        r += dr;
        g += dg;
        b += db;
    } while (--count != 0);
}

static void build_32bit_cache(SkPMColor cache[], SkPMColor c0, SkPMColor c1,
                              int count) {
    SkASSERT(count > 1);

    SkFixed a = SkGetPackedA32(c0);
    SkFixed r = SkGetPackedR32(c0);
    SkFixed g = SkGetPackedG32(c0);
    SkFixed b = SkGetPackedB32(c0);

    SkFixed da = SkIntToFixed(SkGetPackedA32(c1) - a) / (count - 1);
    SkFixed dr = SkIntToFixed(SkGetPackedR32(c1) - r) / (count - 1);
    SkFixed dg = SkIntToFixed(SkGetPackedG32(c1) - g) / (count - 1);
    SkFixed db = SkIntToFixed(SkGetPackedB32(c1) - b) / (count - 1);

    a = SkIntToFixed(a) + 0x8000;
    r = SkIntToFixed(r) + 0x8000;
    g = SkIntToFixed(g) + 0x8000;
    b = SkIntToFixed(b) + 0x8000;

    do {
        *cache++ = SkPackARGB32(a >> 16, r >> 16, g >> 16, b >> 16);
        a += da;
        r += dr;
        g += dg;
        b += db;
    } while (--count != 0);
}

static inline int SkFixedToFFFF(SkFixed x) {
    SkASSERT((unsigned)x <= SK_Fixed1);
    return x - (x >> 16);
}

static inline U16CPU dot6to16(unsigned x) {
    SkASSERT(x < 64);
    return (x << 10) | (x << 4) | (x >> 2);
}

const uint16_t* Gradient_Shader::getCache16() {
    if (fCache16 == NULL) {
        if (fCache16Storage == NULL) // set the storage and our working ptr
#ifdef TEST_GRADIENT_DITHER
            fCache16Storage = (uint16_t*)sk_malloc_throw(sizeof(uint16_t) * kCache16Count * 2);
#else
            fCache16Storage = (uint16_t*)sk_malloc_throw(sizeof(uint16_t) * kCache16Count);
#endif
        fCache16 = fCache16Storage;
        if (fColorCount == 2) {
            build_16bit_cache(fCache16, fOrigColors[0], fOrigColors[1], kCache16Count);
        } else {
            Rec* rec = fRecs;
            int prevIndex = 0;
            for (int i = 1; i < fColorCount; i++) {
                int nextIndex = SkFixedToFFFF(rec[i].fPos) >> (16 - kCache16Bits);
                SkASSERT(nextIndex < kCache16Count);

                if (nextIndex > prevIndex)
                    build_16bit_cache(fCache16 + prevIndex, fOrigColors[i-1], fOrigColors[i], nextIndex - prevIndex + 1);
                prevIndex = nextIndex;
            }
            SkASSERT(prevIndex == kCache16Count - 1);
        }

        if (fMapper) {
#ifdef TEST_GRADIENT_DITHER
            fCache16Storage = (uint16_t*)sk_malloc_throw(sizeof(uint16_t) * kCache16Count * 2);
#else
            fCache16Storage = (uint16_t*)sk_malloc_throw(sizeof(uint16_t) * kCache16Count);
#endif
            uint16_t* linear = fCache16;         // just computed linear data
            uint16_t* mapped = fCache16Storage;  // storage for mapped data
            SkUnitMapper* map = fMapper;
            for (int i = 0; i < 64; i++) {
                int index = map->mapUnit16(dot6to16(i)) >> 10;
                mapped[i] = linear[index];
#ifdef TEST_GRADIENT_DITHER
                mapped[i + 64] = linear[index + 64];
#endif
            }
            sk_free(fCache16);
            fCache16 = fCache16Storage;
        }
    }
    return fCache16;
}

const SkPMColor* Gradient_Shader::getCache32() {
    if (fCache32 == NULL) {
        if (fCache32Storage == NULL) // set the storage and our working ptr
            fCache32Storage = (SkPMColor*)sk_malloc_throw(sizeof(SkPMColor) * kCache32Count);

        fCache32 = fCache32Storage;
        if (fColorCount == 2) {
            build_32bit_cache(fCache32, fARGB32[0], fARGB32[1], kCache32Count);
        } else {
            Rec* rec = fRecs;
            int prevIndex = 0;
            for (int i = 1; i < fColorCount; i++) {
                int nextIndex = SkFixedToFFFF(rec[i].fPos) >> (16 - kCache32Bits);
                SkASSERT(nextIndex < kCache32Count);

                if (nextIndex > prevIndex)
                    build_32bit_cache(fCache32 + prevIndex, fARGB32[i-1], fARGB32[i], nextIndex - prevIndex + 1);
                prevIndex = nextIndex;
            }
            SkASSERT(prevIndex == kCache32Count - 1);
        }

        if (fMapper) {
            fCache32Storage = (SkPMColor*)sk_malloc_throw(sizeof(SkPMColor) * kCache32Count);
            SkPMColor* linear = fCache32;           // just computed linear data
            SkPMColor* mapped = fCache32Storage;    // storage for mapped data
            SkUnitMapper* map = fMapper;
            for (int i = 0; i < 256; i++) {
                mapped[i] = linear[map->mapUnit16((i << 8) | i) >> 8];
            }
            sk_free(fCache32);
            fCache32 = fCache32Storage;
        }
    }
    return fCache32;
}

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

static void pts_to_unit_matrix(const SkPoint pts[2], SkMatrix* matrix) {
    SkVector    vec = pts[1] - pts[0];
    SkScalar    mag = vec.length();
    SkScalar    inv = mag ? SkScalarInvert(mag) : 0;

    vec.scale(inv);
    matrix->setSinCos(-vec.fY, vec.fX, pts[0].fX, pts[0].fY);
    matrix->postTranslate(-pts[0].fX, -pts[0].fY);
    matrix->postScale(inv, inv);
}

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

class Linear_Gradient : public Gradient_Shader {
public:
    Linear_Gradient(const SkPoint pts[2],
                    const SkColor colors[], const SkScalar pos[], int colorCount,
                    SkShader::TileMode mode, SkUnitMapper* mapper)
        : Gradient_Shader(colors, pos, colorCount, mode, mapper)
    {
        fCachedBitmap = NULL;
        pts_to_unit_matrix(pts, &fPtsToUnit);
    }
    virtual ~Linear_Gradient() {
        if (fCachedBitmap) {
            SkDELETE(fCachedBitmap);
        }
    }

    virtual void shadeSpan(int x, int y, SkPMColor dstC[], int count);
    virtual void shadeSpan16(int x, int y, uint16_t dstC[], int count);
    virtual bool asABitmap(SkBitmap*, SkMatrix*, TileMode*);
    virtual void onCacheReset() {
        if (fCachedBitmap) {
            SkDELETE(fCachedBitmap);
            fCachedBitmap = NULL;
        }
    }

    static SkFlattenable* CreateProc(SkFlattenableReadBuffer& buffer) { 
        return SkNEW_ARGS(Linear_Gradient, (buffer));
    }

protected:
    Linear_Gradient(SkFlattenableReadBuffer& buffer) : Gradient_Shader(buffer) {
        fCachedBitmap = NULL;
    }
    virtual Factory getFactory() { return CreateProc; }

private:
    SkBitmap* fCachedBitmap;    // allocated on demand

    typedef Gradient_Shader INHERITED;
};

//  Return true if fx, fx+dx, fx+2*dx, ... is always in range
static inline bool no_need_for_clamp(int fx, int dx, int count)
{
    SkASSERT(count > 0);
    return (unsigned)((fx | (fx + (count - 1) * dx)) >> 8) <= 0xFF;
}

void Linear_Gradient::shadeSpan(int x, int y, SkPMColor dstC[], int count)
{
    SkASSERT(count > 0);

    SkPoint             srcPt;
    SkMatrix::MapXYProc dstProc = fDstToIndexProc;
    TileProc            proc = fTileProc;
    const SkPMColor*    cache = this->getCache32();

    if (fDstToIndexClass != kPerspective_MatrixClass) {
        dstProc(fDstToIndex, SkIntToScalar(x), SkIntToScalar(y), &srcPt);
        SkFixed dx, fx = SkScalarToFixed(srcPt.fX);
        // preround fx by half the amount we throw away
        fx += 1 << 7;

        if (fDstToIndexClass == kFixedStepInX_MatrixClass) {
            SkFixed dxStorage[1];
            (void)fDstToIndex.fixedStepInX(SkIntToScalar(y), dxStorage, NULL);
            dx = dxStorage[0];
        } else {
            SkASSERT(fDstToIndexClass == kLinear_MatrixClass);
            dx = SkScalarToFixed(fDstToIndex.getScaleX());
        }

        if (SkFixedNearlyZero(dx)) {
            // we're a vertical gradient, so no change in a span
            unsigned fi = proc(fx);
            SkASSERT(fi <= 0xFFFF);
            sk_memset32(dstC, cache[fi >> (16 - kCache32Bits)], count);
        } else if (proc == clamp_tileproc) {
#if 0
            if (no_need_for_clamp(fx, dx, count))
            {
                unsigned fi;
                while ((count -= 4) >= 0)
                {
                    fi = fx >> 8; SkASSERT(fi <= 0xFF); fx += dx; *dstC++ = cache[fi];
                    fi = fx >> 8; SkASSERT(fi <= 0xFF); fx += dx; *dstC++ = cache[fi];
                    fi = fx >> 8; SkASSERT(fi <= 0xFF); fx += dx; *dstC++ = cache[fi];
                    fi = fx >> 8; SkASSERT(fi <= 0xFF); fx += dx; *dstC++ = cache[fi];
                }
                SkASSERT(count <= -1 && count >= -4);
                count += 4;
                while (--count >= 0)
                {
                    fi = fx >> 8;
                    SkASSERT(fi <= 0xFF);
                    fx += dx;
                    *dstC++ = cache[fi];
                }
            }
            else
#endif
                do {
                    unsigned fi = SkClampMax(fx >> 8, 0xFF);
                    SkASSERT(fi <= 0xFF);
                    fx += dx;
                    *dstC++ = cache[fi];
                } while (--count != 0);
        } else if (proc == mirror_tileproc) {
            do {
                unsigned fi = mirror_8bits(fx >> 8);
                SkASSERT(fi <= 0xFF);
                fx += dx;
                *dstC++ = cache[fi];
            } while (--count != 0);
        } else {
            SkASSERT(proc == repeat_tileproc);
            do {
                unsigned fi = repeat_8bits(fx >> 8);
                SkASSERT(fi <= 0xFF);
                fx += dx;
                *dstC++ = cache[fi];
            } while (--count != 0);
        }
    } else {
        SkScalar    dstX = SkIntToScalar(x);
        SkScalar    dstY = SkIntToScalar(y);
        do {
            dstProc(fDstToIndex, dstX, dstY, &srcPt);
            unsigned fi = proc(SkScalarToFixed(srcPt.fX));
            SkASSERT(fi <= 0xFFFF);
            *dstC++ = cache[fi >> (16 - kCache32Bits)];
            dstX += SK_Scalar1;
        } while (--count != 0);
    }
}

bool Linear_Gradient::asABitmap(SkBitmap* bitmap, SkMatrix* matrix,
                                TileMode xy[]) {
    // we cache our "bitmap", so it's generationID will be const on subsequent
    // calls to asABitmap
    if (NULL == fCachedBitmap) {
        fCachedBitmap = SkNEW(SkBitmap);
        fCachedBitmap->setConfig(SkBitmap::kARGB_8888_Config, kCache32Count, 1);
        fCachedBitmap->setPixels((void*)this->getCache32(), NULL);
    }

    if (bitmap) {
        *bitmap = *fCachedBitmap;
    }
    if (matrix) {
        matrix->setScale(SkIntToScalar(kCache32Count), SK_Scalar1);
        matrix->preConcat(fPtsToUnit);
    }
    if (xy) {
        xy[0] = fTileMode;
        xy[1] = kClamp_TileMode;
    }
    return true;
}

#ifdef TEST_GRADIENT_DITHER
static void dither_memset16(uint16_t dst[], uint16_t value, uint16_t other, int count)
{
    if (reinterpret_cast<uintptr_t>(dst) & 2)
    {
        *dst++ = value;
        count -= 1;
        SkTSwap(value, other);
    }

    sk_memset32((uint32_t*)dst, (value << 16) | other, count >> 1);
    
    if (count & 1)
        dst[count - 1] = value;
}
#endif

void Linear_Gradient::shadeSpan16(int x, int y, uint16_t dstC[], int count)
{
    SkASSERT(count > 0);

    SkPoint             srcPt;
    SkMatrix::MapXYProc dstProc = fDstToIndexProc;
    TileProc            proc = fTileProc;
    const uint16_t*     cache = this->getCache16();
#ifdef TEST_GRADIENT_DITHER
    int                 toggle = ((x ^ y) & 1) << kCache16Bits;
#endif

    if (fDstToIndexClass != kPerspective_MatrixClass) {
        dstProc(fDstToIndex, SkIntToScalar(x), SkIntToScalar(y), &srcPt);
        SkFixed dx, fx = SkScalarToFixed(srcPt.fX);
        // preround fx by half the amount we throw away
        fx += 1 << 7;

        if (fDstToIndexClass == kFixedStepInX_MatrixClass) {
            SkFixed dxStorage[1];
            (void)fDstToIndex.fixedStepInX(SkIntToScalar(y), dxStorage, NULL);
            dx = dxStorage[0];
        } else {
            SkASSERT(fDstToIndexClass == kLinear_MatrixClass);
            dx = SkScalarToFixed(fDstToIndex.getScaleX());
        }

        if (SkFixedNearlyZero(dx)) {
            // we're a vertical gradient, so no change in a span
            unsigned fi = proc(fx) >> 10;
            SkASSERT(fi <= 63);
#ifdef TEST_GRADIENT_DITHER
            dither_memset16(dstC, cache[toggle + fi], cache[(toggle ^ (1 << kCache16Bits)) + fi], count);
#else
            sk_memset16(dstC, cache[fi], count);
#endif
        } else if (proc == clamp_tileproc) {
            do {
                unsigned fi = SkClampMax(fx >> 10, 63);
                SkASSERT(fi <= 63);
                fx += dx;
#ifdef TEST_GRADIENT_DITHER
                *dstC++ = cache[toggle + fi];
                toggle ^= (1 << kCache16Bits);
#else
                *dstC++ = cache[fi];
#endif
            } while (--count != 0);
        } else if (proc == mirror_tileproc) {
            do {
                unsigned fi = mirror_6bits(fx >> 10);
                SkASSERT(fi <= 0x3F);
                fx += dx;
#ifdef TEST_GRADIENT_DITHER
                *dstC++ = cache[toggle + fi];
                toggle ^= (1 << kCache16Bits);
#else
                *dstC++ = cache[fi];
#endif
            } while (--count != 0);
        } else {
            SkASSERT(proc == repeat_tileproc);
            do {
                unsigned fi = repeat_6bits(fx >> 10);
                SkASSERT(fi <= 0x3F);
                fx += dx;
#ifdef TEST_GRADIENT_DITHER
                *dstC++ = cache[toggle + fi];
                toggle ^= (1 << kCache16Bits);
#else
                *dstC++ = cache[fi];
#endif
            } while (--count != 0);
        }
    } else {
        SkScalar    dstX = SkIntToScalar(x);
        SkScalar    dstY = SkIntToScalar(y);
        do {
            dstProc(fDstToIndex, dstX, dstY, &srcPt);
            unsigned fi = proc(SkScalarToFixed(srcPt.fX));
            SkASSERT(fi <= 0xFFFF);

            int index = fi >> (16 - kCache16Bits);
#ifdef TEST_GRADIENT_DITHER
            *dstC++ = cache[toggle + index];
            toggle ^= (1 << kCache16Bits);
#else
            *dstC++ = cache[index];
#endif

            dstX += SK_Scalar1;
        } while (--count != 0);
    }
}

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

#define kSQRT_TABLE_BITS    11
#define kSQRT_TABLE_SIZE    (1 << kSQRT_TABLE_BITS)

#include "SkRadialGradient_Table.h"

#if defined(SK_BUILD_FOR_WIN32) && defined(SK_DEBUG)

#include <stdio.h>

void SkRadialGradient_BuildTable()
{
    // build it 0..127 x 0..127, so we use 2^15 - 1 in the numerator for our "fixed" table

    FILE* file = ::fopen("SkRadialGradient_Table.h", "w");
    SkASSERT(file);
    ::fprintf(file, "static const uint8_t gSqrt8Table[] = {\n");

    for (int i = 0; i < kSQRT_TABLE_SIZE; i++)
    {
        if ((i & 15) == 0)
            ::fprintf(file, "\t");

        uint8_t value = SkToU8(SkFixedSqrt(i * SK_Fixed1 / kSQRT_TABLE_SIZE) >> 8);

        ::fprintf(file, "0x%02X", value);
        if (i < kSQRT_TABLE_SIZE-1)
            ::fprintf(file, ", ");
        if ((i & 15) == 15)
            ::fprintf(file, "\n");
    }
    ::fprintf(file, "};\n");
    ::fclose(file);
}

#endif


static void rad_to_unit_matrix(const SkPoint& center, SkScalar radius, SkMatrix* matrix)
{
    SkScalar    inv = SkScalarInvert(radius);

    matrix->setTranslate(-center.fX, -center.fY);
    matrix->postScale(inv, inv);
}

class Radial_Gradient : public Gradient_Shader {
public:
    Radial_Gradient(const SkPoint& center, SkScalar radius,
                    const SkColor colors[], const SkScalar pos[], int colorCount,
                    SkShader::TileMode mode, SkUnitMapper* mapper)
        : Gradient_Shader(colors, pos, colorCount, mode, mapper)
    {
        // make sure our table is insync with our current #define for kSQRT_TABLE_SIZE
        SkASSERT(sizeof(gSqrt8Table) == kSQRT_TABLE_SIZE);

        rad_to_unit_matrix(center, radius, &fPtsToUnit);
    }
    virtual void shadeSpan(int x, int y, SkPMColor dstC[], int count)
    {
        SkASSERT(count > 0);

        SkPoint             srcPt;
        SkMatrix::MapXYProc dstProc = fDstToIndexProc;
        TileProc            proc = fTileProc;
        const SkPMColor*    cache = this->getCache32();

        if (fDstToIndexClass != kPerspective_MatrixClass)
        {
            dstProc(fDstToIndex, SkIntToScalar(x), SkIntToScalar(y), &srcPt);
            SkFixed dx, fx = SkScalarToFixed(srcPt.fX);
            SkFixed dy, fy = SkScalarToFixed(srcPt.fY);

            if (fDstToIndexClass == kFixedStepInX_MatrixClass)
            {
                SkFixed storage[2];
                (void)fDstToIndex.fixedStepInX(SkIntToScalar(y), &storage[0], &storage[1]);
                dx = storage[0];
                dy = storage[1];
            }
            else
            {
                SkASSERT(fDstToIndexClass == kLinear_MatrixClass);
                dx = SkScalarToFixed(fDstToIndex.getScaleX());
                dy = SkScalarToFixed(fDstToIndex.getSkewY());
            }

            if (proc == clamp_tileproc)
            {
                const uint8_t* sqrt_table = gSqrt8Table;
                fx >>= 1;
                dx >>= 1;
                fy >>= 1;
                dy >>= 1;
                do {
                    unsigned xx = SkPin32(fx, -0xFFFF >> 1, 0xFFFF >> 1);
                    unsigned fi = SkPin32(fy, -0xFFFF >> 1, 0xFFFF >> 1);
                    fi = (xx * xx + fi * fi) >> (14 + 16 - kSQRT_TABLE_BITS);
                    fi = SkFastMin32(fi, 0xFFFF >> (16 - kSQRT_TABLE_BITS));
                    *dstC++ = cache[sqrt_table[fi] >> (8 - kCache32Bits)];
                    fx += dx;
                    fy += dy;
                } while (--count != 0);
            }
            else if (proc == mirror_tileproc)
            {
                do {
                    SkFixed dist = SkFixedSqrt(SkFixedSquare(fx) + SkFixedSquare(fy));
                    unsigned fi = mirror_tileproc(dist);
                    SkASSERT(fi <= 0xFFFF);
                    *dstC++ = cache[fi >> (16 - kCache32Bits)];
                    fx += dx;
                    fy += dy;
                } while (--count != 0);
            }
            else
            {
                SkASSERT(proc == repeat_tileproc);
                do {
                    SkFixed dist = SkFixedSqrt(SkFixedSquare(fx) + SkFixedSquare(fy));
                    unsigned fi = repeat_tileproc(dist);
                    SkASSERT(fi <= 0xFFFF);
                    *dstC++ = cache[fi >> (16 - kCache32Bits)];
                    fx += dx;
                    fy += dy;
                } while (--count != 0);
            }
        }
        else    // perspective case
        {
            SkScalar dstX = SkIntToScalar(x);
            SkScalar dstY = SkIntToScalar(y);
            do {
                dstProc(fDstToIndex, dstX, dstY, &srcPt);
                unsigned fi = proc(SkScalarToFixed(srcPt.length()));
                SkASSERT(fi <= 0xFFFF);
                *dstC++ = cache[fi >> (16 - kCache32Bits)];
                dstX += SK_Scalar1;
            } while (--count != 0);
        }
    }
    virtual void shadeSpan16(int x, int y, uint16_t dstC[], int count)
    {
        SkASSERT(count > 0);

        SkPoint             srcPt;
        SkMatrix::MapXYProc dstProc = fDstToIndexProc;
        TileProc            proc = fTileProc;
        const uint16_t*     cache = this->getCache16();
#ifdef TEST_GRADIENT_DITHER
        int                 toggle = ((x ^ y) & 1) << kCache16Bits;
#endif

        if (fDstToIndexClass != kPerspective_MatrixClass)
        {
            dstProc(fDstToIndex, SkIntToScalar(x), SkIntToScalar(y), &srcPt);
            SkFixed dx, fx = SkScalarToFixed(srcPt.fX);
            SkFixed dy, fy = SkScalarToFixed(srcPt.fY);

            if (fDstToIndexClass == kFixedStepInX_MatrixClass)
            {
                SkFixed storage[2];
                (void)fDstToIndex.fixedStepInX(SkIntToScalar(y), &storage[0], &storage[1]);
                dx = storage[0];
                dy = storage[1];
            }
            else
            {
                SkASSERT(fDstToIndexClass == kLinear_MatrixClass);
                dx = SkScalarToFixed(fDstToIndex.getScaleX());
                dy = SkScalarToFixed(fDstToIndex.getSkewY());
            }

            if (proc == clamp_tileproc)
            {
                const uint8_t* sqrt_table = gSqrt8Table;

                /* knock these down so we can pin against +- 0x7FFF, which is an immediate load,
                    rather than 0xFFFF which is slower. This is a compromise, since it reduces our
                    precision, but that appears to be visually OK. If we decide this is OK for
                    all of our cases, we could (it seems) put this scale-down into fDstToIndex,
                    to avoid having to do these extra shifts each time.
                */
                fx >>= 1;
                dx >>= 1;
                fy >>= 1;
                dy >>= 1;
                if (dy == 0)    // might perform this check for the other modes, but the win will be a smaller % of the total
                {
                    fy = SkPin32(fy, -0xFFFF >> 1, 0xFFFF >> 1);
                    fy *= fy;
                    do {
                        unsigned xx = SkPin32(fx, -0xFFFF >> 1, 0xFFFF >> 1);
                        unsigned fi = (xx * xx + fy) >> (14 + 16 - kSQRT_TABLE_BITS);
                        fi = SkFastMin32(fi, 0xFFFF >> (16 - kSQRT_TABLE_BITS));
                        fx += dx;
#ifdef TEST_GRADIENT_DITHER
                        *dstC++ = cache[toggle + (sqrt_table[fi] >> (8 - kCache16Bits))];
                        toggle ^= (1 << kCache16Bits);
#else
                        *dstC++ = cache[sqrt_table[fi] >> (8 - kCache16Bits)];
#endif
                    } while (--count != 0);
                }
                else
                {
                    do {
                        unsigned xx = SkPin32(fx, -0xFFFF >> 1, 0xFFFF >> 1);
                        unsigned fi = SkPin32(fy, -0xFFFF >> 1, 0xFFFF >> 1);
                        fi = (xx * xx + fi * fi) >> (14 + 16 - kSQRT_TABLE_BITS);
                        fi = SkFastMin32(fi, 0xFFFF >> (16 - kSQRT_TABLE_BITS));
                        fx += dx;
                        fy += dy;
#ifdef TEST_GRADIENT_DITHER
                        *dstC++ = cache[toggle + (sqrt_table[fi] >> (8 - kCache16Bits))];
                        toggle ^= (1 << kCache16Bits);
#else
                        *dstC++ = cache[sqrt_table[fi] >> (8 - kCache16Bits)];
#endif
                    } while (--count != 0);
                }
            }
            else if (proc == mirror_tileproc)
            {
                do {
                    SkFixed dist = SkFixedSqrt(SkFixedSquare(fx) + SkFixedSquare(fy));
                    unsigned fi = mirror_tileproc(dist);
                    SkASSERT(fi <= 0xFFFF);
                    fx += dx;
                    fy += dy;
#ifdef TEST_GRADIENT_DITHER
                    *dstC++ = cache[toggle + (fi >> (16 - kCache16Bits))];
                    toggle ^= (1 << kCache16Bits);
#else
                    *dstC++ = cache[fi >> (16 - kCache16Bits)];
#endif
                } while (--count != 0);
            }
            else
            {
                SkASSERT(proc == repeat_tileproc);
                do {
                    SkFixed dist = SkFixedSqrt(SkFixedSquare(fx) + SkFixedSquare(fy));
                    unsigned fi = repeat_tileproc(dist);
                    SkASSERT(fi <= 0xFFFF);
                    fx += dx;
                    fy += dy;
#ifdef TEST_GRADIENT_DITHER
                    *dstC++ = cache[toggle + (fi >> (16 - kCache16Bits))];
                    toggle ^= (1 << kCache16Bits);
#else
                    *dstC++ = cache[fi >> (16 - kCache16Bits)];
#endif
                } while (--count != 0);
            }
        }
        else    // perspective case
        {
            SkScalar dstX = SkIntToScalar(x);
            SkScalar dstY = SkIntToScalar(y);
            do {
                dstProc(fDstToIndex, dstX, dstY, &srcPt);
                unsigned fi = proc(SkScalarToFixed(srcPt.length()));
                SkASSERT(fi <= 0xFFFF);

                int index = fi >> (16 - kCache16Bits);
#ifdef TEST_GRADIENT_DITHER
                *dstC++ = cache[toggle + index];
                toggle ^= (1 << kCache16Bits);
#else
                *dstC++ = cache[index];
#endif

                dstX += SK_Scalar1;
            } while (--count != 0);
        }
    }

    static SkFlattenable* CreateProc(SkFlattenableReadBuffer& buffer) { 
        return SkNEW_ARGS(Radial_Gradient, (buffer));
    }

protected:
    Radial_Gradient(SkFlattenableReadBuffer& buffer) : Gradient_Shader(buffer) {};
    virtual Factory getFactory() { return CreateProc; }
    virtual void onCacheReset() {}

private:
    typedef Gradient_Shader INHERITED;
};

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

class Sweep_Gradient : public Gradient_Shader {
public:
    Sweep_Gradient(SkScalar cx, SkScalar cy, const SkColor colors[],
                   const SkScalar pos[], int count, SkUnitMapper* mapper)
    : Gradient_Shader(colors, pos, count, SkShader::kClamp_TileMode, mapper)
    {
        fPtsToUnit.setTranslate(-cx, -cy);
    }
    virtual void shadeSpan(int x, int y, SkPMColor dstC[], int count);
    virtual void shadeSpan16(int x, int y, uint16_t dstC[], int count);
    
    static SkFlattenable* CreateProc(SkFlattenableReadBuffer& buffer) {
        return SkNEW_ARGS(Sweep_Gradient, (buffer));
    }

protected:
    Sweep_Gradient(SkFlattenableReadBuffer& buffer) : Gradient_Shader(buffer) {}
    virtual Factory getFactory() { return CreateProc; }
    virtual void onCacheReset() {}

private:
    typedef Gradient_Shader INHERITED;
};

#ifdef COMPUTE_SWEEP_TABLE
#define PI  3.14159265
static bool gSweepTableReady;
static uint8_t gSweepTable[65];

/*  Our table stores precomputed values for atan: [0...1] -> [0..PI/4]
    We scale the results to [0..32]
*/
static const uint8_t* build_sweep_table()
{
    if (!gSweepTableReady)
    {
        const int N = 65;
        const double DENOM = N - 1;
        
        for (int i = 0; i < N; i++)
        {
            double arg = i / DENOM;
            double v = atan(arg);
            int iv = (int)round(v * DENOM * 2 / PI);
//            printf("[%d] atan(%g) = %g %d\n", i, arg, v, iv);
            printf("%d, ", iv);
            gSweepTable[i] = iv;
        }
        gSweepTableReady = true;
    }
    return gSweepTable;
}
#else
static const uint8_t gSweepTable[] = {
    0, 1, 1, 2, 3, 3, 4, 4, 5, 6, 6, 7, 8, 8, 9, 9,
    10, 11, 11, 12, 12, 13, 13, 14, 15, 15, 16, 16, 17, 17, 18, 18,
    19, 19, 20, 20, 21, 21, 22, 22, 23, 23, 24, 24, 25, 25, 25, 26,
    26, 27, 27, 27, 28, 28, 29, 29, 29, 30, 30, 30, 31, 31, 31, 32,
    32
};
static const uint8_t* build_sweep_table() { return gSweepTable; }
#endif

// divide numer/denom, with a bias of 6bits. Assumes numer <= denom
// and denom != 0. Since our table is 6bits big (+1), this is a nice fit.
// Same as (but faster than) SkFixedDiv(numer, denom) >> 10

//unsigned div_64(int numer, int denom);
static unsigned div_64(int numer, int denom)
{
    SkASSERT(numer <= denom);
    SkASSERT(numer > 0);
    SkASSERT(denom > 0);
        
    int nbits = SkCLZ(numer);
    int dbits = SkCLZ(denom);
    int bits = 6 - nbits + dbits;
    SkASSERT(bits <= 6);
    
    if (bits < 0)   // detect underflow
        return 0;

    denom <<= dbits - 1;
    numer <<= nbits - 1;

    unsigned result = 0;

    // do the first one
    if ((numer -= denom) >= 0)
        result = 1;
    else
        numer += denom;
    
    // Now fall into our switch statement if there are more bits to compute
    if (bits > 0)
    {
        // make room for the rest of the answer bits
        result <<= bits;
        switch (bits) {
        case 6:
            if ((numer = (numer << 1) - denom) >= 0)
                result |= 32;
            else
                numer += denom;
        case 5:
            if ((numer = (numer << 1) - denom) >= 0)
                result |= 16;
            else
                numer += denom;
        case 4:
            if ((numer = (numer << 1) - denom) >= 0)
                result |= 8;
            else
                numer += denom;
        case 3:
            if ((numer = (numer << 1) - denom) >= 0)
                result |= 4;
            else
                numer += denom;
        case 2:
            if ((numer = (numer << 1) - denom) >= 0)
                result |= 2;
            else
                numer += denom;
        case 1:
        default:    // not strictly need, but makes GCC make better ARM code
            if ((numer = (numer << 1) - denom) >= 0)
                result |= 1;
            else
                numer += denom;
        }
    }
    return result;
}

// Given x,y in the first quadrant, return 0..63 for the angle [0..90]
static unsigned atan_0_90(SkFixed y, SkFixed x)
{
#ifdef SK_DEBUG
    {
        static bool gOnce;
        if (!gOnce)
        {
            gOnce = true;
            SkASSERT(div_64(55, 55) == 64);
            SkASSERT(div_64(128, 256) == 32);
            SkASSERT(div_64(2326528, 4685824) == 31);
            SkASSERT(div_64(753664, 5210112) == 9);
            SkASSERT(div_64(229376, 4882432) == 3);
            SkASSERT(div_64(2, 64) == 2);
            SkASSERT(div_64(1, 64) == 1);
            // test that we handle underflow correctly
            SkASSERT(div_64(12345, 0x54321234) == 0);
        }
    }
#endif

    SkASSERT(y > 0 && x > 0);
    const uint8_t* table = build_sweep_table();

    unsigned result;
    bool swap = (x < y);
    if (swap)
    {
        // first part of the atan(v) = PI/2 - atan(1/v) identity
        // since our div_64 and table want v <= 1, where v = y/x
        SkTSwap<SkFixed>(x, y);
    }

    result = div_64(y, x);
    
#ifdef SK_DEBUG
    {
        unsigned result2 = SkDivBits(y, x, 6);
        SkASSERT(result2 == result ||
                 (result == 1 && result2 == 0));
    }
#endif

    SkASSERT(result < SK_ARRAY_COUNT(gSweepTable));
    result = table[result];

    if (swap)
    {
        // complete the atan(v) = PI/2 - atan(1/v) identity
        result = 64 - result;
        // pin to 63
        result -= result >> 6;
    }

    SkASSERT(result <= 63);
    return result;
}

//  returns angle in a circle [0..2PI) -> [0..255]
static unsigned SkATan2_255(SkFixed y, SkFixed x)
{
    if (x == 0)
    {
        if (y == 0)
            return 0;
        return y < 0 ? 192 : 64;
    }
    if (y == 0)
        return x < 0 ? 128 : 0;
    
    /*  Find the right quadrant for x,y
        Since atan_0_90 only handles the first quadrant, we rotate x,y
        appropriately before calling it, and then add the right amount
        to account for the real quadrant.
        quadrant 0 : add 0                  | x > 0 && y > 0
        quadrant 1 : add 64 (90 degrees)    | x < 0 && y > 0
        quadrant 2 : add 128 (180 degrees)  | x < 0 && y < 0
        quadrant 3 : add 192 (270 degrees)  | x > 0 && y < 0
        
        map x<0 to (1 << 6)
        map y<0 to (3 << 6)
        add = map_x ^ map_y
    */
    int xsign = x >> 31;
    int ysign = y >> 31;
    int add = ((-xsign) ^ (ysign & 3)) << 6;

#ifdef SK_DEBUG
    if (0 == add)
        SkASSERT(x > 0 && y > 0);
    else if (64 == add)
        SkASSERT(x < 0 && y > 0);
    else if (128 == add)
        SkASSERT(x < 0 && y < 0);
    else if (192 == add)
        SkASSERT(x > 0 && y < 0);
    else
        SkASSERT(!"bad value for add");
#endif
    
    /*  This ^ trick makes x, y positive, and the swap<> handles quadrants
        where we need to rotate x,y by 90 or -90
    */
    x = (x ^ xsign) - xsign;
    y = (y ^ ysign) - ysign;
    if (add & 64)               // quads 1 or 3 need to swap x,y
        SkTSwap<SkFixed>(x, y);

    unsigned result = add + atan_0_90(y, x);
    SkASSERT(result < 256);
    return result;
}

void Sweep_Gradient::shadeSpan(int x, int y, SkPMColor dstC[], int count)
{
    SkMatrix::MapXYProc proc = fDstToIndexProc;
    const SkMatrix&     matrix = fDstToIndex;
    const SkPMColor*    cache = this->getCache32();
    SkPoint             srcPt;
    
    if (fDstToIndexClass != kPerspective_MatrixClass)
    {
        proc(matrix, SkIntToScalar(x) + SK_ScalarHalf,
                     SkIntToScalar(y) + SK_ScalarHalf, &srcPt);
        SkFixed dx, fx = SkScalarToFixed(srcPt.fX);
        SkFixed dy, fy = SkScalarToFixed(srcPt.fY);
        
        if (fDstToIndexClass == kFixedStepInX_MatrixClass)
        {
            SkFixed storage[2];
            (void)matrix.fixedStepInX(SkIntToScalar(y) + SK_ScalarHalf,
                                      &storage[0], &storage[1]);
            dx = storage[0];
            dy = storage[1];
        }
        else
        {
            SkASSERT(fDstToIndexClass == kLinear_MatrixClass);
            dx = SkScalarToFixed(matrix.getScaleX());
            dy = SkScalarToFixed(matrix.getSkewY());
        }
        
        for (; count > 0; --count)
        {
            *dstC++ = cache[SkATan2_255(fy, fx)];
            fx += dx;
            fy += dy;
        }
    }
    else    // perspective case
    {
        for (int stop = x + count; x < stop; x++)
        {
            proc(matrix, SkIntToScalar(x) + SK_ScalarHalf,
                 SkIntToScalar(y) + SK_ScalarHalf, &srcPt);
            
            int index = SkATan2_255(SkScalarToFixed(srcPt.fY),
                                    SkScalarToFixed(srcPt.fX));
            *dstC++ = cache[index];
        }
    }
}

void Sweep_Gradient::shadeSpan16(int x, int y, uint16_t dstC[], int count)
{
    SkMatrix::MapXYProc proc = fDstToIndexProc;
    const SkMatrix&     matrix = fDstToIndex;
    const uint16_t*     cache = this->getCache16();
    int                 toggle = ((x ^ y) & 1) << kCache16Bits;
    SkPoint             srcPt;

    if (fDstToIndexClass != kPerspective_MatrixClass)
    {
        proc(matrix, SkIntToScalar(x) + SK_ScalarHalf,
                     SkIntToScalar(y) + SK_ScalarHalf, &srcPt);
        SkFixed dx, fx = SkScalarToFixed(srcPt.fX);
        SkFixed dy, fy = SkScalarToFixed(srcPt.fY);
        
        if (fDstToIndexClass == kFixedStepInX_MatrixClass)
        {
            SkFixed storage[2];
            (void)matrix.fixedStepInX(SkIntToScalar(y) + SK_ScalarHalf,
                                      &storage[0], &storage[1]);
            dx = storage[0];
            dy = storage[1];
        }
        else
        {
            SkASSERT(fDstToIndexClass == kLinear_MatrixClass);
            dx = SkScalarToFixed(matrix.getScaleX());
            dy = SkScalarToFixed(matrix.getSkewY());
        }
        
        for (; count > 0; --count)
        {
            int index = SkATan2_255(fy, fx) >> (8 - kCache16Bits);
            *dstC++ = cache[toggle + index];
            toggle ^= (1 << kCache16Bits);
            fx += dx;
            fy += dy;
        }
    }
    else    // perspective case
    {
        for (int stop = x + count; x < stop; x++)
        {
            proc(matrix, SkIntToScalar(x) + SK_ScalarHalf,
                         SkIntToScalar(y) + SK_ScalarHalf, &srcPt);
            
            int index = SkATan2_255(SkScalarToFixed(srcPt.fY),
                                    SkScalarToFixed(srcPt.fX));
            index >>= (8 - kCache16Bits);
            *dstC++ = cache[toggle + index];
            toggle ^= (1 << kCache16Bits);
        }
    }
}

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

// assumes colors is SkColor* and pos is SkScalar*
#define EXPAND_1_COLOR(count)               \
    SkColor tmp[2];                         \
    do {                                    \
        if (1 == count) {                   \
            tmp[0] = tmp[1] = colors[0];    \
            colors = tmp;                   \
            pos = NULL;                     \
            count = 2;                      \
        }                                   \
    } while (0)

SkShader* SkGradientShader::CreateLinear(   const SkPoint pts[2],
                                            const SkColor colors[], const SkScalar pos[], int colorCount,
                                            SkShader::TileMode mode, SkUnitMapper* mapper)
{
    if (NULL == pts || NULL == colors || colorCount < 1) {
        return NULL;
    }
    EXPAND_1_COLOR(colorCount);

    SkScalar posStorage[2];
    if (colorCount == 2 && pos == NULL) {
        posStorage[0] = SK_Scalar1/4;
        posStorage[1] = 3*SK_Scalar1/4;
        pos = posStorage;
    }

    return SkNEW_ARGS(Linear_Gradient, (pts, colors, pos, colorCount, mode, mapper));
}

SkShader* SkGradientShader::CreateRadial(   const SkPoint& center, SkScalar radius,
                                            const SkColor colors[], const SkScalar pos[], int colorCount,
                                            SkShader::TileMode mode, SkUnitMapper* mapper)
{
    if (radius <= 0 || NULL == colors || colorCount < 1) {
        return NULL;
    }
    EXPAND_1_COLOR(colorCount);

    return SkNEW_ARGS(Radial_Gradient, (center, radius, colors, pos, colorCount, mode, mapper));
}

SkShader* SkGradientShader::CreateSweep(SkScalar cx, SkScalar cy,
                                        const SkColor colors[],
                                        const SkScalar pos[],
                                        int count, SkUnitMapper* mapper)
{
    if (NULL == colors || count < 1) {
        return NULL;
    }
    EXPAND_1_COLOR(count);

    return SkNEW_ARGS(Sweep_Gradient, (cx, cy, colors, pos, count, mapper));
}

static SkFlattenable::Registrar gLinearGradientReg("Linear_Gradient",
                                                   Linear_Gradient::CreateProc);

static SkFlattenable::Registrar gRadialGradientReg("Radial_Gradient",
                                                   Radial_Gradient::CreateProc);

static SkFlattenable::Registrar gSweepGradientReg("Sweep_Gradient",
                                                   Sweep_Gradient::CreateProc);