在C中缩放原始的24位像素图

Question

我试图缩小我捕获的X显示的大小,以便在新窗口中显示它.我尝试对各个RGB组件求平均值,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);

        }
   }

平均像素是一种不好的方法吗？我基本上试图将img缩放50%.我的假设是我在像素图上移动了一个2x2的正方形.

Answer 1

在这样做:

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)的总和不能被4整除,你将有位进入G字段(在最重要的位!).(p1G + p2G + p3G + p4G)和B场相同.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是没有必要的,它绝对没有:)

(我也仔细检查了运算符优先级,应该没问题)