索貝爾算子（Sobeloperator）主要用作邊緣檢測(cè)，在技術(shù)上，它是一離散性差分算子，用來(lái)運(yùn)算圖像亮度函數(shù)的灰度之近似值。在圖像的任何一點(diǎn)使用此算子，將會(huì)產(chǎn)生對(duì)應(yīng)的灰度矢量或是其法矢量
Sobel卷積因子為：

http://blog.csdn.net/goodshot/article/details/10170073

http://blog.csdn.net/yanmy2012/article/details/8110316

為了加快算法速度:
先縮小圖像,步長(zhǎng)為2,然后計(jì)算梯度,
void getGrads(unsigned char* g_lpTemp, unsigned char* g_lpDivide, int IMGW, int IMGH, long r)
{
    long  x, y, i, j;
    long  vx, vy, lvx, lvy;
    unsigned char   *lpSrc = NULL;
    unsigned char   *lpDiv = NULL;
    long    num;
    int gradSum;
    int grad;
    
    r=6;
    for(y = 0; y < IMGH/2; y++)
    {
        for(x = 0; x < IMGW/2; x++)
        {            
            lpDiv = g_lpDivide + 2*y*IMGW + 2*x;
            lvx = 0;
            lvy = 0;
            num = 0;
            gradSum = 0;
            for(i = -r; i <= r; i++)    // 為提高速度，步長(zhǎng)為2
            {
                if(y+i<1 || y+i>=IMGH/2-1) continue;
                for(j = -r; j <= r; j++)    // 為提高速度，步長(zhǎng)為2
                {
                    if(x+j<1 || x+j>=IMGW/2-1) continue;
                    lpSrc = g_lpTemp + (y+i)*(IMGW/2) + x+j;
                    //求x方向偏導(dǎo)
                    vx = *(lpSrc + IMGW/2 + 1) - *(lpSrc + IMGW/2 - 1) +
                         *(lpSrc + 1)*2 - *(lpSrc - 1)*2 +
                         *(lpSrc - IMGW/2 + 1) - *(lpSrc - IMGW/2 - 1);
                    //求y方向偏導(dǎo)
                    vy = *(lpSrc + IMGW/2 - 1) - *(lpSrc - IMGW/2 - 1) +
                         *(lpSrc + IMGW/2)*2 - *(lpSrc - IMGW/2)*2 +
                         *(lpSrc + IMGW/2 + 1) - *(lpSrc - IMGW/2 + 1);

                    gradSum += (labs(vx)+labs(vy));        
                    //gradSum += vx*vx+vy*vy;

                    num++;

                }
            }

            if(num == 0)
                num = 1;
            // 求幅值，保存到g_lpDivide中，用于分割前景背景
            grad = gradSum/num;
            
            if(grad > 255)
                grad = 255;
            *lpDiv = (BYTE)grad;
            *(lpDiv + 1) = (BYTE)grad;
            *(lpDiv + IMGW) = (BYTE)grad;
            *(lpDiv + IMGW + 1) = (BYTE)grad;
        }
    }
}