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 <= fDevice.width() && y + height 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) {    co<em>本文来源gao.dai.ma.com搞@代*码(网$</em>nst 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  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：

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