在C中缩放原始24位像素图

Question

我正在尝试缩小我捕获的X显示的大小，以便在新窗口中显示它。我试着对各个RGB分量取平均值，但图像看起来不太好。这是我的算法：

  img = XGetImage(d_remote,RootWindow(d_remote,0),0,0,attr.width,attr.height,XAllPlanes(),ZPixmap);
   int i;
   int j;
   for(i=0;i<attr.height;i=i+2){
        for(j=0;j<attr.width;j=j+2) {
                unsigned long p1 = XGetPixel(img, j, i);
                unsigned long p1R = p1 & 0x00ff0000;
                unsigned long p1G = p1 & 0x0000ff00;
                unsigned long p1B = p1 & 0x000000ff;
                unsigned long p2 = XGetPixel(img, j+1, i);
                unsigned long p2R = p2 & 0x00ff0000;
                unsigned long p2G = p2 & 0x0000ff00;
                unsigned long p2B = p2 & 0x000000ff;
                unsigned long p3 = XGetPixel(img, j, i+1);
                unsigned long p3R = p3 & 0x00ff0000;
                unsigned long p3G = p3 & 0x0000ff00;
                unsigned long p3B = p3 & 0x000000ff;
                unsigned long p4 = XGetPixel(img, j+1, i+1);
                unsigned long p4R = p4 & 0x00ff0000;
                unsigned long p4G = p4 & 0x0000ff00;
                unsigned long p4B = p4 & 0x000000ff;
                unsigned long averageR = (p1R+p2R+p3R+p4R)/4;
                unsigned long averageG = (p1G+p2G+p3G+p4G)/4;
                unsigned long averageB = (p1B+p2B+p3B+p4B)/4;
                int average = averageR | averageG | averageB | 0x00000000;
                XPutPixel(newImg, j/2, i/2, average);
        }
   }

平均像素是一种糟糕的方法吗？我基本上是在尝试将img扩展50%。我的假设是在像素图上移动一个2x2的正方形。

Answer 1

这样做：

unsigned long averageR = (p1R+p2R+p3R+p4R)/4;
unsigned long averageG = (p1G+p2G+p3G+p4G)/4;
unsigned long averageB = (p1B+p2B+p3B+p4B)/4;
int average = averageR | averageG | averageB | 0x00000000;

你正在"污染"你的颜色：如果（p1R+p2R+p3R+p4R）的和不能被4整除，你将有位进入G字段（最高有效位！）。对于（p1G+p2G+p3G+p4G）和B字段也是如此。你应该做：

unsigned long averageR = (p1R+p2R+p3R+p4R)/4  & 0x00ff0000;
unsigned long averageG = (p1G+p2G+p3G+p4G)/4  & 0x0000ff00;
unsigned long averageB = (p1B+p2B+p3B+p4B)/4  & 0x000000ff;
int average = averageR | averageG | averageB;

最后一个`0x00000000不是必需的，它完全不起作用：）

（我还仔细检查了运算符优先级，应该可以）