解释ARM霓虹灯图像采样

Question

我想写一个更好的版本的cv :: resize（）的OpenCV，我来了一个十字架的代码是在这里：https://github.com/rmaz/NEON-Image-Downscaling/blob/master/ImageResize/BDPViewController.m 该代码是下采样图像2但我无法获得算法。我想首先将该算法转换为C，然后尝试将其修改为用于学习目的。将它转换为任意大小的下采样也很容易吗？

功能是：

static void inline resizeRow(uint32_t *dst, uint32_t *src, uint32_t pixelsPerRow) 
{ 
    const uint32_t * rowB = src + pixelsPerRow; 

    // force the number of pixels per row to a multiple of 8 
    pixelsPerRow = 8 * (pixelsPerRow/8); 

    __asm__ volatile("Lresizeloop: \n" // start loop 
        "vld1.32 {d0-d3}, [%1]! \n" // load 8 pixels from the top row 
        "vld1.32 {d4-d7}, [%2]! \n" // load 8 pixels from the bottom row 
        "vhadd.u8 q0, q0, q2 \n" // average the pixels vertically 
        "vhadd.u8 q1, q1, q3 \n" 
        "vtrn.32 q0, q2 \n" // transpose to put the horizontally adjacent pixels in different registers 
        "vtrn.32 q1, q3 \n" 
        "vhadd.u8 q0, q0, q2 \n" // average the pixels horizontally 
        "vhadd.u8 q1, q1, q3 \n" 
        "vtrn.32 d0, d1 \n" // fill the registers with pixels 
        "vtrn.32 d2, d3 \n" 
        "vswp d1, d2 \n" 
        "vst1.64 {d0-d1}, [%0]! \n" // store the result 
        "subs %3, %3, #8 \n" // subtract 8 from the pixel count 
        "bne Lresizeloop \n" // repeat until the row is complete 
: "=r"(dst), "=r"(src), "=r"(rowB), "=r"(pixelsPerRow) 
: "0"(dst), "1"(src), "2"(rowB), "3"(pixelsPerRow) 
: "q0", "q1", "q2", "q3", "cc" 
); 
} 

To call it: 

// downscale the image in place 
    for (size_t rowIndex = 0; rowIndex < height; rowIndex+=2) 
    { 
     void *sourceRow = (uint8_t *)buffer + rowIndex * bytesPerRow; 
     void *destRow = (uint8_t *)buffer + (rowIndex/2) * bytesPerRow; 
     resizeRow(destRow, sourceRow, width); 
    } 

Answer 1

的算法是非常简单的。它从当前行读取8个像素，从下一行读取8个像素。然后使用vhadd（half-add）指令垂直平均8个像素。然后它转换像素的位置，以便水平相邻的像素对现在位于不同的寄存器（垂直排列）中。然后再做一组加减平均值。然后再将结果转换为原始位置并写入目的地。这个算法可以被重写，以处理不同积分大小的缩放，但是按照写法，它只能进行2x2到1的平均缩减。下面是相当于C代码的代码：

static void inline resizeRow(uint32_t *dst, uint32_t *src, uint32_t pixelsPerRow) 
{ 
    uint8_t * pSrc8 = (uint8_t *)src; 
    uint8_t * pDest8 = (uint8_t *)dst; 
    int stride = pixelsPerRow * sizeof(uint32_t); 
    int x; 
    int r, g, b, a; 

    for (x=0; x<pixelsPerRow; x++) 
    { 
     r = pSrc8[0] + pSrc8[4] + pSrc8[stride+0] + pSrc8[stride+4]; 
     g = pSrc8[1] + pSrc8[5] + pSrc8[stride+1] + pSrc8[stride+5]; 
     b = pSrc8[2] + pSrc8[6] + pSrc8[stride+2] + pSrc8[stride+6]; 
     a = pSrc8[3] + pSrc8[7] + pSrc8[stride+3] + pSrc8[stride+7]; 
     pDest8[0] = (uint8_t)((r + 2)/4); // average with rounding 
     pDest8[1] = (uint8_t)((g + 2)/4); 
     pDest8[2] = (uint8_t)((b + 2)/4); 
     pDest8[3] = (uint8_t)((a + 2)/4); 
     pSrc8 += 8; // skip forward 2 source pixels 
     pDest8 += 4; // skip forward 1 destination pixel 
    }