Skia深入分析3——skia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 + width <= fDevice.width() && y + height <= fDevice.height());

uint32_t*          device = fDevice.getAddr32(x, y);
size_t             deviceRB = fDevice.rowBytes();
SkShader::Context* shaderContext = fShaderContext;
SkPMColor*         span = fBuffer;

if (fConstInY) {
if (fShadeDirectlyIntoDevice) {
// shade the first row directly into the device
shaderContext->shadeSpan(x, y, device, width);
span = device;
while (--height > 0) {
device = (uint32_t*)((char*)device + deviceRB);
memcpy(device, span, width << 2);
}
} else {
shaderContext->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 < BUF_MAX*2);
#ifdef TEST_BUFFER_OVERRITE
for (int i = 0; i < TEST_BUFFER_EXTRA; i++) {
buffer[BUF_MAX + i] = TEST_PATTERN;
}
#endif
mproc(state, buffer, n, x, y);
#ifdef TEST_BUFFER_OVERRITE
for (int j = 0; j < TEST_BUFFER_EXTRA; j++) {
SkASSERT(buffer[BUF_MAX + j] == TEST_PATTERN);
}
#endif
sproc(state, buffer, n, dstC);

if ((count -= n) == 0) {
break;
}
SkASSERT(count > 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 <= SkMatrix::kTranslate_Mask ||
!valid_for_filtering(fBitmap->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::kHigh_FilterLevel) {

int index = 0;
if (fAlphaScale < 256) {  // note: this distinction is not used for D16
index |= 1;
}
if (fInvType <= (SkMatrix::kTranslate_Mask | SkMatrix::kScale_Mask)) {
index |= 2;
}
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) < (unsigned)s.fBitmap->height() &&
(XY & 0xFFFF) < (unsigned)s.fBitmap->width());
src = ((const SRCTYPE*)(srcAddr + (XY >> 16) * rb))[XY & 0xFFFF];
*colors++ = RETURNDST(src);

XY = *xy++;
SkASSERT((XY >> 16) < (unsigned)s.fBitmap->height() &&
(XY & 0xFFFF) < (unsigned)s.fBitmap->width());
src = ((const SRCTYPE*)(srcAddr + (XY >> 16) * rb))[XY & 0xFFFF];
*colors++ = RETURNDST(src);
}
if (count & 1) {
XY = *xy++;
SkASSERT((XY >> 16) < (unsigned)s.fBitmap->height() &&
(XY & 0xFFFF) < (unsigned)s.fBitmap->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 << 4) | TILEY_LOW_BITS(f, max);
return (i << 14) | (TILEY_PROCF((f + one), max));
}

static inline uint32_t PACK_FILTER_X_NAME(SkFixed f, unsigned max,
SkFixed one PREAMBLE_PARAM_X) {
unsigned i = TILEX_PROCF(f, max);
i = (i << 4) | TILEX_LOW_BITS(f, max);
return (i << 14) | (TILEX_PROCF((f + one), max));
}

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

PREAMBLE(s);

const unsigned maxX = s.fBitmap->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，有几个感受。

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。