Skia深入分析3skia图片绘制的实现(2)

此篇讲图像采样 一、采样流程 在上一节里的流程图有写到，图像绘制的实际渲染发生在某个blitter的blitRect函数中，我们先看一个具体的blitRect实现。 void SkARGB32_Shader_Blitter::blitRect(int x, int y, int width, int height) { SkASSERT(x = 0 y = 0 x

此篇讲图像采样
一、采样流程
在上一节里的流程图有写到，图像绘制的实际渲染发生在某个blitter的blitRect函数中，我们先看一个具体的blitRect实现。

void SkARGB32_Shader_Blitter::blitRect(int x, int y, int width, int height) {
    SkASSERT(x >= 0 && y >= 0 &&
             x + width shadeSpan(x, y, device, width);
            span = device;
            while (--height > 0) {
                device = (uint32_t*)((char*)device + deviceRB);
                memcpy(device, span, width shadeSpan(x, y, span, width);
            SkXfermode* xfer = fXfermode;
            if (xfer) {
                do {
                    xfer->xfer32(device, span, width, NULL);
                    y += 1;
                    device = (uint32_t*)((char*)device + deviceRB);
                } while (--height > 0);
            } else {
                SkBlitRow::Proc32 proc = fProc32;
                do {
                    proc(device, span, width, 255);
                    y += 1;
                    device = (uint32_t*)((char*)device + deviceRB);
                } while (--height > 0);
            }
        }
        return;
    }

    if (fShadeDirectlyIntoDevice) {
        void* ctx;
        SkShader::Context::ShadeProc shadeProc = shaderContext->asAShadeProc(&ctx);
        if (shadeProc) {
            do {
                shadeProc(ctx, x, y, device, width);
                y += 1;
                device = (uint32_t*)((char*)device + deviceRB);
            } while (--height > 0);
        } else {
            do {
                shaderContext->shadeSpan(x, y, device, width);
                y += 1;
                device = (uint32_t*)((char*)device + deviceRB);
            } while (--height > 0);
        }
    } else {
        SkXfermode* xfer = fXfermode;
        if (xfer) {
            do {
                shaderContext->shadeSpan(x, y, span, width);
                xfer->xfer32(device, span, width, NULL);
                y += 1;
                device = (uint32_t*)((char*)device + deviceRB);
            } while (--height > 0);
        } else {
            SkBlitRow::Proc32 proc = fProc32;
            do {
                shaderContext->shadeSpan(x, y, span, width);
                proc(device, span, width, 255);
                y += 1;
                device = (uint32_t*)((char*)device + deviceRB);
            } while (--height > 0);
        }
    }
}

其中shadeSpan用来将shader中x，y坐标处的值取n个到dst的buffer中。

对于图像绘制时，它是 SkBitmapProcShader，这里是其实现：

void SkBitmapProcShader::BitmapProcShaderContext::shadeSpan(int x, int y, SkPMColor dstC[],
                                                            int count) {
    const SkBitmapProcState& state = *fState;
    if (state.getShaderProc32()) {
        state.getShaderProc32()(state, x, y, dstC, count);
        return;
    }

    uint32_t buffer[BUF_MAX + TEST_BUFFER_EXTRA];
    SkBitmapProcState::MatrixProc   mproc = state.getMatrixProc();
    SkBitmapProcState::SampleProc32 sproc = state.getSampleProc32();
    int max = state.maxCountForBufferSize(sizeof(buffer[0]) * BUF_MAX);

    SkASSERT(state.fBitmap->getPixels());
    SkASSERT(state.fBitmap->pixelRef() == NULL ||
             state.fBitmap->pixelRef()->isLocked());

    for (;;) {
        int n = count;
        if (n > max) {
            n = max;
        }
        SkASSERT(n > 0 && n  0);
        x += n;
        dstC += n;
    }
}

流程如下：
1、存在 shaderProc，直接用
2、计算一次能处理的像素数count
3、mproc计算count个坐标，sproc根据坐标值去取色
注意到之前三个函数指针：
state.getShaderProc32
mproc = state.getMatrixProc
sproc = state.getShaderProc32
这三个函数指针在一开始创建blitter时设定：

SkBlitter::Choose -> SkShader::createContext -> SkBitmapProcShader::onCreateContext -> SkBitmapProcState::chooseProcs

这是一个相当长的函数，它做的事情如下：
1、（优化步骤）在大于SkPaint::kLow_FilterLevel的质量要求下，试图做预缩放。
2、选择matrix函数：chooseMatrixProc。
3、选择sample函数：
（1）高质量：setBitmapFilterProcs
（2）kLow_FilterLevel或kNone_FilterLevel：采取flags计算的方法,根据x,y变化矩阵情况和采样要求选择函数
4、（优化步骤）在满足条件时，选取shader函数，此函数替代matrix和sample函数
5、（优化步骤）platformProcs()，进一步选择优化版本的sample函数
对于RGB565格式的目标，使用的是SkShader的 shadeSpan16 方法。shadeSpan16的代码逻辑类似，不再说明。

bool SkBitmapProcState::chooseProcs(const SkMatrix& inv, const SkPaint& paint) {
    SkASSERT(fOrigBitmap.width() && fOrigBitmap.height());

    fBitmap = NULL;
    fInvMatrix = inv;
    fFilterLevel = paint.getFilterLevel();

    SkASSERT(NULL == fScaledCacheID);

    // possiblyScaleImage will look to see if it can rescale the image as a
    // preprocess; either by scaling up to the target size, or by selecting
    // a nearby mipmap level.  If it does, it will adjust the working
    // matrix as well as the working bitmap.  It may also adjust the filter
    // quality to avoid re-filtering an already perfectly scaled image.
    if (!this->possiblyScaleImage()) {
        if (!this->lockBaseBitmap()) {
            return false;
        }
    }
    // The above logic should have always assigned fBitmap, but in case it
    // didn't, we check for that now...
    // TODO(dominikg): Ask humper@ if we can just use an SkASSERT(fBitmap)?
    if (NULL == fBitmap) {
        return false;
    }

    // If we are "still" kMedium_FilterLevel, then the request was not fulfilled by possiblyScale,
    // so we downgrade to kLow (so the rest of the sniffing code can assume that)
    if (SkPaint::kMedium_FilterLevel == fFilterLevel) {
        fFilterLevel = SkPaint::kLow_FilterLevel;
    }

    bool trivialMatrix = (fInvMatrix.getType() & ~SkMatrix::kTranslate_Mask) == 0;
    bool clampClamp = SkShader::kClamp_TileMode == fTileModeX &&
                      SkShader::kClamp_TileMode == fTileModeY;

    if (!(clampClamp || trivialMatrix)) {
        fInvMatrix.postIDiv(fOrigBitmap.width(), fOrigBitmap.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)) {
            if (clampClamp ? just_trans_clamp(forward, *fBitmap)
                            : just_trans_general(forward)) {
                SkScalar tx = -SkScalarRoundToScalar(forward.getTranslateX());
                SkScalar ty = -SkScalarRoundToScalar(forward.getTranslateY());
                fInvMatrix.setTranslate(tx, ty);
            }
        }
    }

    fInvProc        = fInvMatrix.getMapXYProc();
    fInvType        = fInvMatrix.getType();
    fInvSx          = SkScalarToFixed(fInvMatrix.getScaleX());
    fInvSxFractionalInt = SkScalarToFractionalInt(fInvMatrix.getScaleX());
    fInvKy          = SkScalarToFixed(fInvMatrix.getSkewY());
    fInvKyFractionalInt = SkScalarToFractionalInt(fInvMatrix.getSkewY());

    fAlphaScale = SkAlpha255To256(paint.getAlpha());

    fShaderProc32 = NULL;
    fShaderProc16 = NULL;
    fSampleProc32 = NULL;
    fSampleProc16 = NULL;

    // recompute the triviality of the matrix here because we may have
    // changed it!

    trivialMatrix = (fInvMatrix.getType() & ~SkMatrix::kTranslate_Mask) == 0;

    if (SkPaint::kHigh_FilterLevel == fFilterLevel) {
        // If this is still set, that means we wanted HQ sampling
        // but couldn't do it as a preprocess.  Let's try to install
        // the scanline version of the HQ sampler.  If that process fails,
        // downgrade to bilerp.

        // NOTE: Might need to be careful here in the future when we want
        // to have the platform proc have a shot at this; it's possible that
        // the chooseBitmapFilterProc will fail to install a shader but a
        // platform-specific one might succeed, so it might be premature here
        // to fall back to bilerp.  This needs thought.

        if (!this->setBitmapFilterProcs()) {
            fFilterLevel = SkPaint::kLow_FilterLevel;
        }
    }

    if (SkPaint::kLow_FilterLevel == fFilterLevel) {
        // Only try bilerp if the matrix is "interesting" and
        // the image has a suitable size.

        if (fInvType width() | fBitmap->height())) {
            fFilterLevel = SkPaint::kNone_FilterLevel;
        }
    }

    // At this point, we know exactly what kind of sampling the per-scanline
    // shader will perform.

    fMatrixProc = this->chooseMatrixProc(trivialMatrix);
    // TODO(dominikg): SkASSERT(fMatrixProc) instead? chooseMatrixProc never returns NULL.
    if (NULL == fMatrixProc) {
        return false;
    }

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

    // No need to do this if we're doing HQ sampling; if filter quality is
    // still set to HQ by the time we get here, then we must have installed
    // the shader procs above and can skip all this.

    if (fFilterLevel  SkPaint::kNone_FilterLevel) {
            index |= 4;
        }
        // bits 3,4,5 encoding the source bitmap format
        switch (fBitmap->colorType()) {
            case kN32_SkColorType:
                index |= 0;
                break;
            case kRGB_565_SkColorType:
                index |= 8;
                break;
            case kIndex_8_SkColorType:
                index |= 16;
                break;
            case kARGB_4444_SkColorType:
                index |= 24;
                break;
            case kAlpha_8_SkColorType:
                index |= 32;
                fPaintPMColor = SkPreMultiplyColor(paint.getColor());
                break;
            default:
                // TODO(dominikg): Should we ever get here? SkASSERT(false) instead?
                return false;
        }

    #if !SK_ARM_NEON_IS_ALWAYS
        static const SampleProc32 gSkBitmapProcStateSample32[] = {
            S32_opaque_D32_nofilter_DXDY,
            S32_alpha_D32_nofilter_DXDY,
            S32_opaque_D32_nofilter_DX,
            S32_alpha_D32_nofilter_DX,
            S32_opaque_D32_filter_DXDY,
            S32_alpha_D32_filter_DXDY,
            S32_opaque_D32_filter_DX,
            S32_alpha_D32_filter_DX,

            S16_opaque_D32_nofilter_DXDY,
            S16_alpha_D32_nofilter_DXDY,
            S16_opaque_D32_nofilter_DX,
            S16_alpha_D32_nofilter_DX,
            S16_opaque_D32_filter_DXDY,
            S16_alpha_D32_filter_DXDY,
            S16_opaque_D32_filter_DX,
            S16_alpha_D32_filter_DX,

            SI8_opaque_D32_nofilter_DXDY,
            SI8_alpha_D32_nofilter_DXDY,
            SI8_opaque_D32_nofilter_DX,
            SI8_alpha_D32_nofilter_DX,
            SI8_opaque_D32_filter_DXDY,
            SI8_alpha_D32_filter_DXDY,
            SI8_opaque_D32_filter_DX,
            SI8_alpha_D32_filter_DX,

            S4444_opaque_D32_nofilter_DXDY,
            S4444_alpha_D32_nofilter_DXDY,
            S4444_opaque_D32_nofilter_DX,
            S4444_alpha_D32_nofilter_DX,
            S4444_opaque_D32_filter_DXDY,
            S4444_alpha_D32_filter_DXDY,
            S4444_opaque_D32_filter_DX,
            S4444_alpha_D32_filter_DX,

            // A8 treats alpha/opaque the same (equally efficient)
            SA8_alpha_D32_nofilter_DXDY,
            SA8_alpha_D32_nofilter_DXDY,
            SA8_alpha_D32_nofilter_DX,
            SA8_alpha_D32_nofilter_DX,
            SA8_alpha_D32_filter_DXDY,
            SA8_alpha_D32_filter_DXDY,
            SA8_alpha_D32_filter_DX,
            SA8_alpha_D32_filter_DX
        };

        static const SampleProc16 gSkBitmapProcStateSample16[] = {
            S32_D16_nofilter_DXDY,
            S32_D16_nofilter_DX,
            S32_D16_filter_DXDY,
            S32_D16_filter_DX,

            S16_D16_nofilter_DXDY,
            S16_D16_nofilter_DX,
            S16_D16_filter_DXDY,
            S16_D16_filter_DX,

            SI8_D16_nofilter_DXDY,
            SI8_D16_nofilter_DX,
            SI8_D16_filter_DXDY,
            SI8_D16_filter_DX,

            // Don't support 4444 -> 565
            NULL, NULL, NULL, NULL,
            // Don't support A8 -> 565
            NULL, NULL, NULL, NULL
        };
    #endif

        fSampleProc32 = SK_ARM_NEON_WRAP(gSkBitmapProcStateSample32)[index];
        index >>= 1;    // shift away any opaque/alpha distinction
        fSampleProc16 = SK_ARM_NEON_WRAP(gSkBitmapProcStateSample16)[index];

        // our special-case shaderprocs
        if (SK_ARM_NEON_WRAP(S16_D16_filter_DX) == fSampleProc16) {
            if (clampClamp) {
                fShaderProc16 = SK_ARM_NEON_WRAP(Clamp_S16_D16_filter_DX_shaderproc);
            } else if (SkShader::kRepeat_TileMode == fTileModeX &&
                       SkShader::kRepeat_TileMode == fTileModeY) {
                fShaderProc16 = SK_ARM_NEON_WRAP(Repeat_S16_D16_filter_DX_shaderproc);
            }
        } else if (SK_ARM_NEON_WRAP(SI8_opaque_D32_filter_DX) == fSampleProc32 && clampClamp) {
            fShaderProc32 = SK_ARM_NEON_WRAP(Clamp_SI8_opaque_D32_filter_DX_shaderproc);
        }

        if (NULL == fShaderProc32) {
            fShaderProc32 = this->chooseShaderProc32();
        }
    }

    // see if our platform has any accelerated overrides
    this->platformProcs();

    return true;
}

二、MatrixProc和SampleProc
MatrixProc的使命是生成坐标集。SampleProc则根据坐标集取像素，采样合成
我们先倒过来看 sampleProc 看这个坐标集是怎么使用的：
nofilter_dx系列：

nofilter_dxdy系列：

void MAKENAME(_nofilter_DXDY)(const SkBitmapProcState& s,
        const uint32_t* SK_RESTRICT xy,
        int count, DSTTYPE* SK_RESTRICT colors) {
    for (int i = (count >> 1); i > 0; --i) {
        XY = *xy++;
        SkASSERT((XY >> 16) height() &&
                (XY & 0xFFFF) width());
        src = ((const SRCTYPE*)(srcAddr + (XY >> 16) * rb))[XY & 0xFFFF];
        *colors++ = RETURNDST(src);

        XY = *xy++;
        SkASSERT((XY >> 16) height() &&
                (XY & 0xFFFF) width());
        src = ((const SRCTYPE*)(srcAddr + (XY >> 16) * rb))[XY & 0xFFFF];
        *colors++ = RETURNDST(src);
    }
    if (count & 1) {
        XY = *xy++;
        SkASSERT((XY >> 16) height() &&
                (XY & 0xFFFF) width());
        src = ((const SRCTYPE*)(srcAddr + (XY >> 16) * rb))[XY & 0xFFFF];
        *colors++ = RETURNDST(src);
    }

}

这两个系列是直接取了x,y坐标处的图像像素
filter_dx系列：

filter_dxdy系列：

void MAKENAME(_filter_DX)(const SkBitmapProcState& s,
                          const uint32_t* SK_RESTRICT xy,
                           int count, DSTTYPE* SK_RESTRICT colors) {
    SkASSERT(count > 0 && colors != NULL);
    SkASSERT(s.fFilterLevel != SkPaint::kNone_FilterLevel);
    SkDEBUGCODE(CHECKSTATE(s);)

#ifdef PREAMBLE
    PREAMBLE(s);
#endif
    const char* SK_RESTRICT srcAddr = (const char*)s.fBitmap->getPixels();
    size_t rb = s.fBitmap->rowBytes();
    unsigned subY;
    const SRCTYPE* SK_RESTRICT row0;
    const SRCTYPE* SK_RESTRICT row1;

    // setup row ptrs and update proc_table
    {
        uint32_t XY = *xy++;
        unsigned y0 = XY >> 14;
        row0 = (const SRCTYPE*)(srcAddr + (y0 >> 4) * rb);
        row1 = (const SRCTYPE*)(srcAddr + (XY & 0x3FFF) * rb);
        subY = y0 & 0xF;
    }

    do {
        uint32_t XX = *xy++;    // x0:14 | 4 | x1:14
        unsigned x0 = XX >> 14;
        unsigned x1 = XX & 0x3FFF;
        unsigned subX = x0 & 0xF;
        x0 >>= 4;

        FILTER_PROC(subX, subY,
                    SRC_TO_FILTER(row0[x0]),
                    SRC_TO_FILTER(row0[x1]),
                    SRC_TO_FILTER(row1[x0]),
                    SRC_TO_FILTER(row1[x1]),
                    colors);
        colors += 1;

    } while (--count != 0);

#ifdef POSTAMBLE
    POSTAMBLE(s);
#endif
}
void MAKENAME(_filter_DXDY)(const SkBitmapProcState& s,
                            const uint32_t* SK_RESTRICT xy,
                            int count, DSTTYPE* SK_RESTRICT colors) {
    SkASSERT(count > 0 && colors != NULL);
    SkASSERT(s.fFilterLevel != SkPaint::kNone_FilterLevel);
    SkDEBUGCODE(CHECKSTATE(s);)

#ifdef PREAMBLE
        PREAMBLE(s);
#endif
    const char* SK_RESTRICT srcAddr = (const char*)s.fBitmap->getPixels();
    size_t rb = s.fBitmap->rowBytes();

    do {
        uint32_t data = *xy++;
        unsigned y0 = data >> 14;
        unsigned y1 = data & 0x3FFF;
        unsigned subY = y0 & 0xF;
        y0 >>= 4;

        data = *xy++;
        unsigned x0 = data >> 14;
        unsigned x1 = data & 0x3FFF;
        unsigned subX = x0 & 0xF;
        x0 >>= 4;

        const SRCTYPE* SK_RESTRICT row0 = (const SRCTYPE*)(srcAddr + y0 * rb);
        const SRCTYPE* SK_RESTRICT row1 = (const SRCTYPE*)(srcAddr + y1 * rb);

        FILTER_PROC(subX, subY,
                    SRC_TO_FILTER(row0[x0]),
                    SRC_TO_FILTER(row0[x1]),
                    SRC_TO_FILTER(row1[x0]),
                    SRC_TO_FILTER(row1[x1]),
                    colors);
        colors += 1;
    } while (--count != 0);

#ifdef POSTAMBLE
    POSTAMBLE(s);
#endif
}

将四个相邻像素取出来之后，作Filter处理

看晕了么，其实总结一下是这样：
nofilter_dx，第一个32位数表示y，其余的32位数包含两个x坐标。
nofilter_dxdy，用16位表示x，16位表示y。这种情况就是取的最近值，直接到x,y坐标处取值就可以了。
filter_dxdy系列，每个32位数分别表示X和Y坐标（14:4:14），交错排列，中间的差值部分是相差的小数扩大16倍而得的近似整数。
filter_dx系列，第一个数为Y坐标用14:4:14的方式存储，后面的数为X坐标，也用14:4:14的方式存储，前后为对应坐标，中间为放大16倍的距离，这个情况是一行之内y坐标相同（只做缩放或小数平移的情况），一样是作双线性插值。

下面我们来看matrixproc的实现，

先跟进 chooseMatrixProc的代码：

SkBitmapProcState::MatrixProc SkBitmapProcState::chooseMatrixProc(bool trivial_matrix) {
//    test_int_tileprocs();
    // check for our special case when there is no scale/affine/perspective
    if (trivial_matrix) {
        SkASSERT(SkPaint::kNone_FilterLevel == fFilterLevel);
        fIntTileProcY = choose_int_tile_proc(fTileModeY);
        switch (fTileModeX) {
            case SkShader::kClamp_TileMode:
                return clampx_nofilter_trans;
            case SkShader::kRepeat_TileMode:
                return repeatx_nofilter_trans;
            case SkShader::kMirror_TileMode:
                return mirrorx_nofilter_trans;
        }
    }

    int index = 0;
    if (fFilterLevel != SkPaint::kNone_FilterLevel) {
        index = 1;
    }
    if (fInvType & SkMatrix::kPerspective_Mask) {
        index += 4;
    } else if (fInvType & SkMatrix::kAffine_Mask) {
        index += 2;
    }

    if (SkShader::kClamp_TileMode == fTileModeX && SkShader::kClamp_TileMode == fTileModeY) {
        // clamp gets special version of filterOne
        fFilterOneX = SK_Fixed1;
        fFilterOneY = SK_Fixed1;
        return SK_ARM_NEON_WRAP(ClampX_ClampY_Procs)[index];
    }

    // all remaining procs use this form for filterOne
    fFilterOneX = SK_Fixed1 / fBitmap->width();
    fFilterOneY = SK_Fixed1 / fBitmap->height();

    if (SkShader::kRepeat_TileMode == fTileModeX && SkShader::kRepeat_TileMode == fTileModeY) {
        return SK_ARM_NEON_WRAP(RepeatX_RepeatY_Procs)[index];
    }

    fTileProcX = choose_tile_proc(fTileModeX);
    fTileProcY = choose_tile_proc(fTileModeY);
    fTileLowBitsProcX = choose_tile_lowbits_proc(fTileModeX);
    fTileLowBitsProcY = choose_tile_lowbits_proc(fTileModeY);
    return GeneralXY_Procs[index];
}

有些函数是找符号找不到的，我们注意到SkBitmapProcState.cpp 中包含了多次 SkBitmapProcState_matrix.h 头文件：

#if !SK_ARM_NEON_IS_ALWAYS
#define MAKENAME(suffix)        ClampX_ClampY ## suffix
#define TILEX_PROCF(fx, max)    SkClampMax((fx) >> 16, max)
#define TILEY_PROCF(fy, max)    SkClampMax((fy) >> 16, max)
#define TILEX_LOW_BITS(fx, max) (((fx) >> 12) & 0xF)
#define TILEY_LOW_BITS(fy, max) (((fy) >> 12) & 0xF)
#define CHECK_FOR_DECAL
#include "SkBitmapProcState_matrix.h"

头文件代码如下：

/*
 * 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 "SkMath.h"
#include "SkMathPriv.h"

#define SCALE_FILTER_NAME       MAKENAME(_filter_scale)
#define AFFINE_FILTER_NAME      MAKENAME(_filter_affine)
#define PERSP_FILTER_NAME       MAKENAME(_filter_persp)

#define PACK_FILTER_X_NAME  MAKENAME(_pack_filter_x)
#define PACK_FILTER_Y_NAME  MAKENAME(_pack_filter_y)

#ifndef PREAMBLE
    #define PREAMBLE(state)
    #define PREAMBLE_PARAM_X
    #define PREAMBLE_PARAM_Y
    #define PREAMBLE_ARG_X
    #define PREAMBLE_ARG_Y
#endif

// declare functions externally to suppress warnings.
void SCALE_FILTER_NAME(const SkBitmapProcState& s,
                              uint32_t xy[], int count, int x, int y);
void AFFINE_FILTER_NAME(const SkBitmapProcState& s,
                               uint32_t xy[], int count, int x, int y);
void PERSP_FILTER_NAME(const SkBitmapProcState& s,
                              uint32_t* SK_RESTRICT xy, int count,
                              int x, int y);

static inline uint32_t PACK_FILTER_Y_NAME(SkFixed f, unsigned max,
                                          SkFixed one PREAMBLE_PARAM_Y) {
    unsigned i = TILEY_PROCF(f, max);
    i = (i width() - 1;
    const SkFixed one = s.fFilterOneX;
    const SkFractionalInt dx = s.fInvSxFractionalInt;
    SkFractionalInt fx;

    {
        SkPoint pt;
        s.fInvProc(s.fInvMatrix, SkIntToScalar(x) + SK_ScalarHalf,
                                  SkIntToScalar(y) + SK_ScalarHalf, &pt);
        const SkFixed fy = SkScalarToFixed(pt.fY) - (s.fFilterOneY >> 1);
        const unsigned maxY = s.fBitmap->height() - 1;
        // compute our two Y values up front
        *xy++ = PACK_FILTER_Y_NAME(fy, maxY, s.fFilterOneY PREAMBLE_ARG_Y);
        // now initialize fx
        fx = SkScalarToFractionalInt(pt.fX) - (SkFixedToFractionalInt(one) >> 1);
    }

#ifdef CHECK_FOR_DECAL
    if (can_truncate_to_fixed_for_decal(fx, dx, count, maxX)) {
        decal_filter_scale(xy, SkFractionalIntToFixed(fx),
                           SkFractionalIntToFixed(dx), count);
    } else
#endif
    {
        do {
            SkFixed fixedFx = SkFractionalIntToFixed(fx);
            *xy++ = PACK_FILTER_X_NAME(fixedFx, maxX, one PREAMBLE_ARG_X);
            fx += dx;
        } while (--count != 0);
    }
}

void AFFINE_FILTER_NAME(const SkBitmapProcState& s,
                               uint32_t xy[], int count, int x, int y) {
    SkASSERT(s.fInvType & SkMatrix::kAffine_Mask);
    SkASSERT((s.fInvType & ~(SkMatrix::kTranslate_Mask |
                             SkMatrix::kScale_Mask |
                             SkMatrix::kAffine_Mask)) == 0);

    PREAMBLE(s);
    SkPoint srcPt;
    s.fInvProc(s.fInvMatrix,
               SkIntToScalar(x) + SK_ScalarHalf,
               SkIntToScalar(y) + SK_ScalarHalf, &srcPt);

    SkFixed oneX = s.fFilterOneX;
    SkFixed oneY = s.fFilterOneY;
    SkFixed fx = SkScalarToFixed(srcPt.fX) - (oneX >> 1);
    SkFixed fy = SkScalarToFixed(srcPt.fY) - (oneY >> 1);
    SkFixed dx = s.fInvSx;
    SkFixed dy = s.fInvKy;
    unsigned maxX = s.fBitmap->width() - 1;
    unsigned maxY = s.fBitmap->height() - 1;

    do {
        *xy++ = PACK_FILTER_Y_NAME(fy, maxY, oneY PREAMBLE_ARG_Y);
        fy += dy;
        *xy++ = PACK_FILTER_X_NAME(fx, maxX, oneX PREAMBLE_ARG_X);
        fx += dx;
    } while (--count != 0);
}

void PERSP_FILTER_NAME(const SkBitmapProcState& s,
                              uint32_t* SK_RESTRICT xy, int count,
                              int x, int y) {
    SkASSERT(s.fInvType & SkMatrix::kPerspective_Mask);

    PREAMBLE(s);
    unsigned maxX = s.fBitmap->width() - 1;
    unsigned maxY = s.fBitmap->height() - 1;
    SkFixed oneX = s.fFilterOneX;
    SkFixed oneY = s.fFilterOneY;

    SkPerspIter   iter(s.fInvMatrix,
                       SkIntToScalar(x) + SK_ScalarHalf,
                       SkIntToScalar(y) + SK_ScalarHalf, count);

    while ((count = iter.next()) != 0) {
        const SkFixed* SK_RESTRICT srcXY = iter.getXY();
        do {
            *xy++ = PACK_FILTER_Y_NAME(srcXY[1] - (oneY >> 1), maxY,
                                       oneY PREAMBLE_ARG_Y);
            *xy++ = PACK_FILTER_X_NAME(srcXY[0] - (oneX >> 1), maxX,
                                       oneX PREAMBLE_ARG_X);
            srcXY += 2;
        } while (--count != 0);
    }
}

#undef MAKENAME
#undef TILEX_PROCF
#undef TILEY_PROCF
#ifdef CHECK_FOR_DECAL
    #undef CHECK_FOR_DECAL
#endif

#undef SCALE_FILTER_NAME
#undef AFFINE_FILTER_NAME
#undef PERSP_FILTER_NAME

#undef PREAMBLE
#undef PREAMBLE_PARAM_X
#undef PREAMBLE_PARAM_Y
#undef PREAMBLE_ARG_X
#undef PREAMBLE_ARG_Y

#undef TILEX_LOW_BITS
#undef TILEY_LOW_BITS

然后我们就清楚了，这些函数名是用宏组合出来的。（神一般的代码。。。。。）
怎么算坐标的不详述了，主要按原理去推就可以了，坐标计算有三种模式：CLAMP（越界时限制在边界）、REPEAT（越界时从开头取起）、MIRROR（越界时取样方向倒转去取）。
sampleProc函数也是类似的方法组合出来的，不详述。


三、高级插值算法
双线性插值虽然在一般情况下够用了，但在放大图片时，效果还是不够好。需要更好的效果，可以用高级插值算法，代价是性能的大幅消耗。
高级插值算法目前在Android的Java代码处是走不进去的，不知道chromium是否用到。
几个要点：
1、在 setBitmapFilterProcs 时判断高级插值是否支持，若支持，设置 shaderProc 为 highQualityFilter32/highQualityFilter16（也就是独立计算坐标和采样像素）
2、highQualityFilter先通过变换矩阵计算原始点。
3、highQualityFilter根据 SkBitmapFilter  的采样窗口，将这个窗口中的所有点按其与原始点矩离，查询对应权重值，然后相加，得到最终像素点。
4、SkBitmapFilter 采用查表法去给出权重值，预计算由子类完成。
5、目前Skia库用的是双三次插值 mitchell 法。

SK_CONF_DECLARE(const char *, c_bitmapFilter, "bitmap.filter", "mitchell", "Which scanline bitmap filter to use [mitchell, lanczos, hamming, gaussian, triangle, box]");
详细代码见 external/skia/src/core/SkBitmapFilter.cpp，尽量这部分代码几乎无用武之地，但里面的公式很值得借鉴，随便改改就能做成 glsl shader 用。

看完这段代码，可以作不负责任的猜想：Skia设计之初，只考虑了近邻插值和双线性插值两种情况，因此采用这种模板方法，可以最小化代码量。而且MatrixProc和SampleProc可以后续分别作SIMD优化（Intel的SSE和ARM的Neon），以提高性能。
但是对于线性插值，两步法（取值——采样）在算法实现上本来就不是最优的，后面又不得不引入shader函数，应对一些场景做优化。高阶插值无法在这个设计下实现，因此又像补丁一样打上去。

四、总结
看完这一部分代码，有三个感受。
第一：绘张图片看上去一件简单的事，在渲染执行时，真心不容易，如果追求效果，还会有各种各样的花样。
第二：在性能有要求的场景下，用模板真是灾难：函数改写时，遇到模板，就不得不重新定义函数，并替换之，弄得代码看上去一下子混乱不少。
第三：从图像绘制这个角度上看，skia渲染性能虽然确实很好了，但远没有达到极限，仍然是有一定的优化空间的，如果这部分出现了性能问题，还是能做一定的优化的。关于Skia性能的讨论将放到介绍Skia系列的最后一章。
第四：OpenGL+glsl确实是轻松且高效多了，软件渲染在复杂场景上性能很有限。