src/core/SkBitmapProcState.cpp

/*
 * Copyright 2011 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "include/core/SkImageEncoder.h"
#include "include/core/SkPaint.h"
#include "include/core/SkShader.h"
#include "include/private/SkColorData.h"
#include "include/private/SkMacros.h"
#include "include/private/SkTPin.h"
#include "src/core/SkBitmapCache.h"
#include "src/core/SkBitmapProcState.h"
#include "src/core/SkMipmap.h"
#include "src/core/SkMipmapAccessor.h"
#include "src/core/SkOpts.h"
#include "src/core/SkResourceCache.h"

// One-stop-shop shader for,
//   - nearest-neighbor sampling (_nofilter_),
//   - clamp tiling in X and Y both (Clamp_),
//   - with at most a scale and translate matrix (_DX_),
//   - and no extra alpha applied (_opaque_),
//   - sampling from 8888 (_S32_) and drawing to 8888 (_S32_).
static void Clamp_S32_opaque_D32_nofilter_DX_shaderproc(const void* sIn, int x, int y,
                                                        SkPMColor* dst, int count) {
    const SkBitmapProcState& s = *static_cast<const SkBitmapProcState*>(sIn);
    SkASSERT(s.fInvMatrix.isScaleTranslate());
    SkASSERT(s.fAlphaScale == 256);

    const unsigned maxX = s.fPixmap.width() - 1;
    SkFractionalInt fx;
    int dstY;
    {
        const SkBitmapProcStateAutoMapper mapper(s, x, y);
        const unsigned maxY = s.fPixmap.height() - 1;
        dstY = SkTPin<int>(mapper.intY(), 0, maxY);
        fx = mapper.fractionalIntX();
    }

    const SkPMColor* src = s.fPixmap.addr32(0, dstY);
    const SkFractionalInt dx = s.fInvSxFractionalInt;

    // Check if we're safely inside [0...maxX] so no need to clamp each computed index.
    //
    if ((uint64_t)SkFractionalIntToInt(fx) <= maxX &&
        (uint64_t)SkFractionalIntToInt(fx + dx * (count - 1)) <= maxX)
    {
        int count4 = count >> 2;
        for (int i = 0; i < count4; ++i) {
            SkPMColor src0 = src[SkFractionalIntToInt(fx)]; fx += dx;
            SkPMColor src1 = src[SkFractionalIntToInt(fx)]; fx += dx;
            SkPMColor src2 = src[SkFractionalIntToInt(fx)]; fx += dx;
            SkPMColor src3 = src[SkFractionalIntToInt(fx)]; fx += dx;
            dst[0] = src0;
            dst[1] = src1;
            dst[2] = src2;
            dst[3] = src3;
            dst += 4;
        }
        for (int i = (count4 << 2); i < count; ++i) {
            unsigned index = SkFractionalIntToInt(fx);
            SkASSERT(index <= maxX);
            *dst++ = src[index];
            fx += dx;
        }
    } else {
        for (int i = 0; i < count; ++i) {
            dst[i] = src[SkTPin<int>(SkFractionalIntToInt(fx), 0, maxX)];
            fx += dx;
        }
    }
}

static void S32_alpha_D32_nofilter_DX(const SkBitmapProcState& s,
                                      const uint32_t* xy, int count, SkPMColor* colors) {
    SkASSERT(count > 0 && colors != nullptr);
    SkASSERT(s.fInvMatrix.isScaleTranslate());
    SkASSERT(!s.fBilerp);
    SkASSERT(4 == s.fPixmap.info().bytesPerPixel());
    SkASSERT(s.fAlphaScale <= 256);

    // xy is a 32-bit y-coordinate, followed by 16-bit x-coordinates.
    unsigned y = *xy++;
    SkASSERT(y < (unsigned)s.fPixmap.height());

    auto row = (const SkPMColor*)( (const char*)s.fPixmap.addr() + y * s.fPixmap.rowBytes() );

    if (1 == s.fPixmap.width()) {
        sk_memset32(colors, SkAlphaMulQ(row[0], s.fAlphaScale), count);
        return;
    }

    // Step 4 xs == 2 uint32_t at a time.
    while (count >= 4) {
        uint32_t x01 = *xy++,
                 x23 = *xy++;

        SkPMColor p0 = row[UNPACK_PRIMARY_SHORT  (x01)];
        SkPMColor p1 = row[UNPACK_SECONDARY_SHORT(x01)];
        SkPMColor p2 = row[UNPACK_PRIMARY_SHORT  (x23)];
        SkPMColor p3 = row[UNPACK_SECONDARY_SHORT(x23)];

        *colors++ = SkAlphaMulQ(p0, s.fAlphaScale);
        *colors++ = SkAlphaMulQ(p1, s.fAlphaScale);
        *colors++ = SkAlphaMulQ(p2, s.fAlphaScale);
        *colors++ = SkAlphaMulQ(p3, s.fAlphaScale);

        count -= 4;
    }

    // Step 1 x == 1 uint16_t at a time.
    auto x = (const uint16_t*)xy;
    while (count --> 0) {
        *colors++ = SkAlphaMulQ(row[*x++], s.fAlphaScale);
    }
}

static void S32_alpha_D32_nofilter_DXDY(const SkBitmapProcState& s,
                                        const uint32_t* xy, int count, SkPMColor* colors) {
    SkASSERT(count > 0 && colors != nullptr);
    SkASSERT(!s.fBilerp);
    SkASSERT(4 == s.fPixmap.info().bytesPerPixel());
    SkASSERT(s.fAlphaScale <= 256);

    auto src = (const char*)s.fPixmap.addr();
    size_t rb = s.fPixmap.rowBytes();

    while (count --> 0) {
        uint32_t XY = *xy++,
                 x  = XY & 0xffff,
                 y  = XY >> 16;
        SkASSERT(x < (unsigned)s.fPixmap.width ());
        SkASSERT(y < (unsigned)s.fPixmap.height());
        *colors++ = ((const SkPMColor*)(src + y*rb))[x];
    }
}

SkBitmapProcState::SkBitmapProcState(const SkImage_Base* image, SkTileMode tmx, SkTileMode tmy)
    : fImage(image)
    , fTileModeX(tmx)
    , fTileModeY(tmy)
{}

// true iff the matrix has a scale and no more than an optional translate.
static bool matrix_only_scale_translate(const SkMatrix& m) {
    return (m.getType() & ~SkMatrix::kTranslate_Mask) == SkMatrix::kScale_Mask;
}

/**
 *  For the purposes of drawing bitmaps, if a matrix is "almost" translate
 *  go ahead and treat it as if it were, so that subsequent code can go fast.
 */
static bool just_trans_general(const SkMatrix& matrix) {
    SkASSERT(matrix_only_scale_translate(matrix));

    const SkScalar tol = SK_Scalar1 / 32768;

    return SkScalarNearlyZero(matrix[SkMatrix::kMScaleX] - SK_Scalar1, tol)
        && SkScalarNearlyZero(matrix[SkMatrix::kMScaleY] - SK_Scalar1, tol);
}

/**
 *  Determine if the matrix can be treated as integral-only-translate,
 *  for the purpose of filtering.
 */
static bool just_trans_integral(const SkMatrix& m) {
    static constexpr SkScalar tol = SK_Scalar1 / 256;

    return m.getType() <= SkMatrix::kTranslate_Mask
        && SkScalarNearlyEqual(m.getTranslateX(), SkScalarRoundToScalar(m.getTranslateX()), tol)
        && SkScalarNearlyEqual(m.getTranslateY(), SkScalarRoundToScalar(m.getTranslateY()), tol);
}

static bool valid_for_filtering(unsigned dimension) {
    // for filtering, width and height must fit in 14bits, since we use steal
    // 2 bits from each to store our 4bit subpixel data
    return (dimension & ~0x3FFF) == 0;
}

bool SkBitmapProcState::init(const SkMatrix& inv, SkAlpha paintAlpha,
                             const SkSamplingOptions& sampling) {
    SkASSERT(!inv.hasPerspective());
    SkASSERT(SkOpts::S32_alpha_D32_filter_DXDY || inv.isScaleTranslate());
    SkASSERT(!sampling.useCubic);
    SkASSERT(sampling.mipmap != SkMipmapMode::kLinear);

    fPixmap.reset();
    fInvMatrix = inv;
    fBilerp = false;

    auto* access = SkMipmapAccessor::Make(&fAlloc, (const SkImage*)fImage, inv, sampling.mipmap);
    if (!access) {
        return false;
    }
    std::tie(fPixmap, fInvMatrix) = access->level();

    fPaintAlpha = paintAlpha;
    fBilerp = sampling.filter == SkFilterMode::kLinear;
    SkASSERT(fPixmap.addr());

    bool integral_translate_only = just_trans_integral(fInvMatrix);
    if (!integral_translate_only) {
        // Most of the scanline procs deal with "unit" texture coordinates, as this
        // makes it easy to perform tiling modes (repeat = (x & 0xFFFF)). To generate
        // those, we divide the matrix by its dimensions here.
        //
        // We don't do this if we're either trivial (can ignore the matrix) or clamping
        // in both X and Y since clamping to width,height is just as easy as to 0xFFFF.

        if (fTileModeX != SkTileMode::kClamp || fTileModeY != SkTileMode::kClamp) {
            SkMatrixPriv::PostIDiv(&fInvMatrix, fPixmap.width(), fPixmap.height());
        }

        // Now that all possible changes to the matrix have taken place, check
        // to see if we're really close to a no-scale matrix.  If so, explicitly
        // set it to be so.  Subsequent code may inspect this matrix to choose
        // a faster path in this case.

        // This code will only execute if the matrix has some scale component;
        // if it's already pure translate then we won't do this inversion.

        if (matrix_only_scale_translate(fInvMatrix)) {
            SkMatrix forward;
            if (fInvMatrix.invert(&forward) && just_trans_general(forward)) {
                fInvMatrix.setTranslate(-forward.getTranslateX(), -forward.getTranslateY());
            }
        }

        // Recompute the flag after matrix adjustments.
        integral_translate_only = just_trans_integral(fInvMatrix);
    }

    if (fBilerp &&
        (!valid_for_filtering(fPixmap.width() | fPixmap.height()) || integral_translate_only)) {
        fBilerp = false;
    }

    return true;
}

/*
 *  Analyze filter-quality and matrix, and decide how to implement that.
 *
 *  In general, we cascade down the request level [ High ... None ]
 *  - for a given level, if we can fulfill it, fine, else
 *    - else we downgrade to the next lower level and try again.
 *  We can always fulfill requests for Low and None
 *  - sometimes we will "ignore" Low and give None, but this is likely a legacy perf hack
 *    and may be removed.
 */
bool SkBitmapProcState::chooseProcs() {
    SkASSERT(!fInvMatrix.hasPerspective());
    SkASSERT(SkOpts::S32_alpha_D32_filter_DXDY || fInvMatrix.isScaleTranslate());
    SkASSERT(fPixmap.colorType() == kN32_SkColorType);
    SkASSERT(fPixmap.alphaType() == kPremul_SkAlphaType ||
             fPixmap.alphaType() == kOpaque_SkAlphaType);

    SkASSERT(fTileModeX != SkTileMode::kDecal);

    fInvProc            = SkMatrixPriv::GetMapXYProc(fInvMatrix);
    fInvSxFractionalInt = SkScalarToFractionalInt(fInvMatrix.getScaleX());
    fInvKyFractionalInt = SkScalarToFractionalInt(fInvMatrix.getSkewY ());

    fAlphaScale = SkAlpha255To256(fPaintAlpha);

    bool translate_only = (fInvMatrix.getType() & ~SkMatrix::kTranslate_Mask) == 0;
    fMatrixProc = this->chooseMatrixProc(translate_only);
    SkASSERT(fMatrixProc);

    if (fInvMatrix.isScaleTranslate()) {
        fSampleProc32 = fBilerp ? SkOpts::S32_alpha_D32_filter_DX   : S32_alpha_D32_nofilter_DX  ;
    } else {
        fSampleProc32 = fBilerp ? SkOpts::S32_alpha_D32_filter_DXDY : S32_alpha_D32_nofilter_DXDY;
    }
    SkASSERT(fSampleProc32);

    // our special-case shaderprocs
    // TODO: move this one into chooseShaderProc32() or pull all that in here.
    if (fAlphaScale == 256
            && !fBilerp
            && SkTileMode::kClamp == fTileModeX
            && SkTileMode::kClamp == fTileModeY
            && fInvMatrix.isScaleTranslate()) {
        fShaderProc32 = Clamp_S32_opaque_D32_nofilter_DX_shaderproc;
    } else {
        fShaderProc32 = this->chooseShaderProc32();
    }

    return true;
}

static void Clamp_S32_D32_nofilter_trans_shaderproc(const void* sIn,
                                                    int x, int y,
                                                    SkPMColor* colors,
                                                    int count) {
    const SkBitmapProcState& s = *static_cast<const SkBitmapProcState*>(sIn);
    SkASSERT(s.fInvMatrix.isTranslate());
    SkASSERT(count > 0 && colors != nullptr);
    SkASSERT(!s.fBilerp);

    const int maxX = s.fPixmap.width() - 1;
    const int maxY = s.fPixmap.height() - 1;
    int ix = s.fFilterOneX + x;
    int iy = SkTPin(s.fFilterOneY + y, 0, maxY);
    const SkPMColor* row = s.fPixmap.addr32(0, iy);

    // clamp to the left
    if (ix < 0) {
        int n = std::min(-ix, count);
        sk_memset32(colors, row[0], n);
        count -= n;
        if (0 == count) {
            return;
        }
        colors += n;
        SkASSERT(-ix == n);
        ix = 0;
    }
    // copy the middle
    if (ix <= maxX) {
        int n = std::min(maxX - ix + 1, count);
        memcpy(colors, row + ix, n * sizeof(SkPMColor));
        count -= n;
        if (0 == count) {
            return;
        }
        colors += n;
    }
    SkASSERT(count > 0);
    // clamp to the right
    sk_memset32(colors, row[maxX], count);
}

static inline int sk_int_mod(int x, int n) {
    SkASSERT(n > 0);
    if ((unsigned)x >= (unsigned)n) {
        if (x < 0) {
            x = n + ~(~x % n);
        } else {
            x = x % n;
        }
    }
    return x;
}

static inline int sk_int_mirror(int x, int n) {
    x = sk_int_mod(x, 2 * n);
    if (x >= n) {
        x = n + ~(x - n);
    }
    return x;
}

static void Repeat_S32_D32_nofilter_trans_shaderproc(const void* sIn,
                                                     int x, int y,
                                                     SkPMColor* colors,
                                                     int count) {
    const SkBitmapProcState& s = *static_cast<const SkBitmapProcState*>(sIn);
    SkASSERT(s.fInvMatrix.isTranslate());
    SkASSERT(count > 0 && colors != nullptr);
    SkASSERT(!s.fBilerp);

    const int stopX = s.fPixmap.width();
    const int stopY = s.fPixmap.height();
    int ix = s.fFilterOneX + x;
    int iy = sk_int_mod(s.fFilterOneY + y, stopY);
    const SkPMColor* row = s.fPixmap.addr32(0, iy);

    ix = sk_int_mod(ix, stopX);
    for (;;) {
        int n = std::min(stopX - ix, count);
        memcpy(colors, row + ix, n * sizeof(SkPMColor));
        count -= n;
        if (0 == count) {
            return;
        }
        colors += n;
        ix = 0;
    }
}

static inline void filter_32_alpha(unsigned t,
                                   SkPMColor color0,
                                   SkPMColor color1,
                                   SkPMColor* dstColor,
                                   unsigned alphaScale) {
    SkASSERT((unsigned)t <= 0xF);
    SkASSERT(alphaScale <= 256);

    const uint32_t mask = 0xFF00FF;

    int scale = 256 - 16*t;
    uint32_t lo = (color0 & mask) * scale;
    uint32_t hi = ((color0 >> 8) & mask) * scale;

    scale = 16*t;
    lo += (color1 & mask) * scale;
    hi += ((color1 >> 8) & mask) * scale;

    // TODO: if (alphaScale < 256) ...
    lo = ((lo >> 8) & mask) * alphaScale;
    hi = ((hi >> 8) & mask) * alphaScale;

    *dstColor = ((lo >> 8) & mask) | (hi & ~mask);
}

static void S32_D32_constX_shaderproc(const void* sIn,
                                      int x, int y,
                                      SkPMColor* colors,
                                      int count) {
    const SkBitmapProcState& s = *static_cast<const SkBitmapProcState*>(sIn);
    SkASSERT(s.fInvMatrix.isScaleTranslate());
    SkASSERT(count > 0 && colors != nullptr);
    SkASSERT(1 == s.fPixmap.width());

    int iY0;
    int iY1   SK_INIT_TO_AVOID_WARNING;
    int iSubY SK_INIT_TO_AVOID_WARNING;

    if (s.fBilerp) {
        SkBitmapProcState::MatrixProc mproc = s.getMatrixProc();
        uint32_t xy[2];

        mproc(s, xy, 1, x, y);

        iY0 = xy[0] >> 18;
        iY1 = xy[0] & 0x3FFF;
        iSubY = (xy[0] >> 14) & 0xF;
    } else {
        int yTemp;

        if (s.fInvMatrix.isTranslate()) {
            yTemp = s.fFilterOneY + y;
        } else{
            const SkBitmapProcStateAutoMapper mapper(s, x, y);

            // When the matrix has a scale component the setup code in
            // chooseProcs multiples the inverse matrix by the inverse of the
            // bitmap's width and height. Since this method is going to do
            // its own tiling and sampling we need to undo that here.
            if (SkTileMode::kClamp != s.fTileModeX || SkTileMode::kClamp != s.fTileModeY) {
                yTemp = SkFractionalIntToInt(mapper.fractionalIntY() * s.fPixmap.height());
            } else {
                yTemp = mapper.intY();
            }
        }

        const int stopY = s.fPixmap.height();
        switch (s.fTileModeY) {
            case SkTileMode::kClamp:
                iY0 = SkTPin(yTemp, 0, stopY-1);
                break;
            case SkTileMode::kRepeat:
                iY0 = sk_int_mod(yTemp, stopY);
                break;
            case SkTileMode::kMirror:
            default:
                iY0 = sk_int_mirror(yTemp, stopY);
                break;
        }

#ifdef SK_DEBUG
        {
            const SkBitmapProcStateAutoMapper mapper(s, x, y);
            int iY2;

            if (!s.fInvMatrix.isTranslate() &&
                (SkTileMode::kClamp != s.fTileModeX || SkTileMode::kClamp != s.fTileModeY)) {
                iY2 = SkFractionalIntToInt(mapper.fractionalIntY() * s.fPixmap.height());
            } else {
                iY2 = mapper.intY();
            }

            switch (s.fTileModeY) {
            case SkTileMode::kClamp:
                iY2 = SkTPin(iY2, 0, stopY-1);
                break;
            case SkTileMode::kRepeat:
                iY2 = sk_int_mod(iY2, stopY);
                break;
            case SkTileMode::kMirror:
            default:
                iY2 = sk_int_mirror(iY2, stopY);
                break;
            }

            SkASSERT(iY0 == iY2);
        }
#endif
    }

    const SkPMColor* row0 = s.fPixmap.addr32(0, iY0);
    SkPMColor color;

    if (s.fBilerp) {
        const SkPMColor* row1 = s.fPixmap.addr32(0, iY1);
        filter_32_alpha(iSubY, *row0, *row1, &color, s.fAlphaScale);
    } else {
        if (s.fAlphaScale < 256) {
            color = SkAlphaMulQ(*row0, s.fAlphaScale);
        } else {
            color = *row0;
        }
    }

    sk_memset32(colors, color, count);
}

static void DoNothing_shaderproc(const void*, int x, int y,
                                 SkPMColor* colors, int count) {
    // if we get called, the matrix is too tricky, so we just draw nothing
    sk_memset32(colors, 0, count);
}

bool SkBitmapProcState::setupForTranslate() {
    SkPoint pt;
    const SkBitmapProcStateAutoMapper mapper(*this, 0, 0, &pt);

    /*
     *  if the translate is larger than our ints, we can get random results, or
     *  worse, we might get 0x80000000, which wreaks havoc on us, since we can't
     *  negate it.
     */
    const SkScalar too_big = SkIntToScalar(1 << 30);
    if (SkScalarAbs(pt.fX) > too_big || SkScalarAbs(pt.fY) > too_big) {
        return false;
    }

    // Since we know we're not filtered, we re-purpose these fields allow
    // us to go from device -> src coordinates w/ just an integer add,
    // rather than running through the inverse-matrix
    fFilterOneX = mapper.intX();
    fFilterOneY = mapper.intY();

    return true;
}

SkBitmapProcState::ShaderProc32 SkBitmapProcState::chooseShaderProc32() {

    if (kN32_SkColorType != fPixmap.colorType()) {
        return nullptr;
    }

    if (1 == fPixmap.width() && fInvMatrix.isScaleTranslate()) {
        if (!fBilerp && fInvMatrix.isTranslate() && !this->setupForTranslate()) {
            return DoNothing_shaderproc;
        }
        return S32_D32_constX_shaderproc;
    }

    if (fAlphaScale < 256) {
        return nullptr;
    }
    if (!fInvMatrix.isTranslate()) {
        return nullptr;
    }
    if (fBilerp) {
        return nullptr;
    }

    SkTileMode tx = fTileModeX;
    SkTileMode ty = fTileModeY;

    if (SkTileMode::kClamp == tx && SkTileMode::kClamp == ty) {
        if (this->setupForTranslate()) {
            return Clamp_S32_D32_nofilter_trans_shaderproc;
        }
        return DoNothing_shaderproc;
    }
    if (SkTileMode::kRepeat == tx && SkTileMode::kRepeat == ty) {
        if (this->setupForTranslate()) {
            return Repeat_S32_D32_nofilter_trans_shaderproc;
        }
        return DoNothing_shaderproc;
    }
    return nullptr;
}

#ifdef SK_DEBUG

static void check_scale_nofilter(uint32_t bitmapXY[], int count,
                                 unsigned mx, unsigned my) {
    unsigned y = *bitmapXY++;
    SkASSERT(y < my);

    const uint16_t* xptr = reinterpret_cast<const uint16_t*>(bitmapXY);
    for (int i = 0; i < count; ++i) {
        SkASSERT(xptr[i] < mx);
    }
}

static void check_scale_filter(uint32_t bitmapXY[], int count,
                                 unsigned mx, unsigned my) {
    uint32_t YY = *bitmapXY++;
    unsigned y0 = YY >> 18;
    unsigned y1 = YY & 0x3FFF;
    SkASSERT(y0 < my);
    SkASSERT(y1 < my);

    for (int i = 0; i < count; ++i) {
        uint32_t XX = bitmapXY[i];
        unsigned x0 = XX >> 18;
        unsigned x1 = XX & 0x3FFF;
        SkASSERT(x0 < mx);
        SkASSERT(x1 < mx);
    }
}

static void check_affine_nofilter(uint32_t bitmapXY[], int count, unsigned mx, unsigned my) {
    for (int i = 0; i < count; ++i) {
        uint32_t XY = bitmapXY[i];
        unsigned x = XY & 0xFFFF;
        unsigned y = XY >> 16;
        SkASSERT(x < mx);
        SkASSERT(y < my);
    }
}

static void check_affine_filter(uint32_t bitmapXY[], int count, unsigned mx, unsigned my) {
    for (int i = 0; i < count; ++i) {
        uint32_t YY = *bitmapXY++;
        unsigned y0 = YY >> 18;
        unsigned y1 = YY & 0x3FFF;
        SkASSERT(y0 < my);
        SkASSERT(y1 < my);

        uint32_t XX = *bitmapXY++;
        unsigned x0 = XX >> 18;
        unsigned x1 = XX & 0x3FFF;
        SkASSERT(x0 < mx);
        SkASSERT(x1 < mx);
    }
}

void SkBitmapProcState::DebugMatrixProc(const SkBitmapProcState& state,
                                        uint32_t bitmapXY[], int count,
                                        int x, int y) {
    SkASSERT(bitmapXY);
    SkASSERT(count > 0);

    state.fMatrixProc(state, bitmapXY, count, x, y);

    void (*proc)(uint32_t bitmapXY[], int count, unsigned mx, unsigned my);

    if (state.fInvMatrix.isScaleTranslate()) {
        proc = state.fBilerp ? check_scale_filter : check_scale_nofilter;
    } else {
        proc = state.fBilerp ? check_affine_filter : check_affine_nofilter;
    }

    proc(bitmapXY, count, state.fPixmap.width(), state.fPixmap.height());
}

SkBitmapProcState::MatrixProc SkBitmapProcState::getMatrixProc() const {
    return DebugMatrixProc;
}

#endif

/*
    The storage requirements for the different matrix procs are as follows,
    where each X or Y is 2 bytes, and N is the number of pixels/elements:

    scale/translate     nofilter      Y(4bytes) + N * X
    affine/perspective  nofilter      N * (X Y)
    scale/translate     filter        Y Y + N * (X X)
    affine              filter        N * (Y Y X X)
 */
int SkBitmapProcState::maxCountForBufferSize(size_t bufferSize) const {
    int32_t size = static_cast<int32_t>(bufferSize);

    size &= ~3; // only care about 4-byte aligned chunks
    if (fInvMatrix.isScaleTranslate()) {
        size -= 4;   // the shared Y (or YY) coordinate
        if (size < 0) {
            size = 0;
        }
        size >>= 1;
    } else {
        size >>= 2;
    }

    if (fBilerp) {
        size >>= 1;
    }

    return size;
}