剪辑线到屏幕坐标

Question

我有一个被定义为两点的线.start =(xs,ys)end =(xe,ye)

我正在使用的绘图功能仅接受完全在屏幕坐标中的线条.屏幕尺寸为(xSize,ySize).

左上角是(0,0).右下角是(xSize,ySize).

其他一些函数给我的行定义为例如start(-50,-15)end(5000,200).所以它的结束是在屏幕尺寸之外.

在C++中

struct Vec2
{
 int x, y
};

Vec2 start, end //This is all little bit pseudo code
Vec2 screenSize;//You can access coordinates like start.x end.y

如何计算屏幕边缘的新起点和终点,而不是屏幕外部.我知道怎么在纸上做.但我无法将其转移到c ++.在纸面上,我正在为属于边缘和线条的点进行搜索.但这是对c ++的大量计算.

你能帮我吗？

Answer 1

有许多线裁剪算法,如:

• Cohen-Sutherland 维基百科页面,包含实施
• Liang-Barsky 维基百科页面
• Nicholl-Lee-Nicholl(NLN)等等.请参阅维基百科上的线路剪辑

[编辑1] 见下图:

有3种起点:

1. sx> 0且sy <0(红线)
2. sx <0且sy> 0(黄线)
3. sx <0且sy <0(绿色和紫色线)

在情境1和2中,只需分别找到Xintersect和Yintersect,并选择它们作为新的起点.如您所见,情况3中有两种线.在这种情况下,找到Xintersect和Yintersect,并选择靠近终点的交点,即与endPoint的距离最小的点.

min(distance(Xintersect, endPoint), distance(Yintersect, endPoint))

[EDIT2]

// Liang-Barsky function by Daniel White @ http://www.skytopia.com/project/articles/compsci/clipping.html
// This function inputs 8 numbers, and outputs 4 new numbers (plus a boolean value to say whether the clipped line is drawn at all).
//
bool LiangBarsky (double edgeLeft, double edgeRight, double edgeBottom, double edgeTop,   // Define the x/y clipping values for the border.
                  double x0src, double y0src, double x1src, double y1src,                 // Define the start and end points of the line.
                  double &x0clip, double &y0clip, double &x1clip, double &y1clip)         // The output values, so declare these outside.
{

    double t0 = 0.0;    double t1 = 1.0;
    double xdelta = x1src-x0src;
    double ydelta = y1src-y0src;
    double p,q,r;

    for(int edge=0; edge<4; edge++) {   // Traverse through left, right, bottom, top edges.
        if (edge==0) {  p = -xdelta;    q = -(edgeLeft-x0src);  }
        if (edge==1) {  p = xdelta;     q =  (edgeRight-x0src); }
        if (edge==2) {  p = -ydelta;    q = -(edgeBottom-y0src);}
        if (edge==3) {  p = ydelta;     q =  (edgeTop-y0src);   }   
        r = q/p;
        if(p==0 && q<0) return false;   // Don't draw line at all. (parallel line outside)

        if(p<0) {
            if(r>t1) return false;         // Don't draw line at all.
            else if(r>t0) t0=r;            // Line is clipped!
        } else if(p>0) {
            if(r<t0) return false;      // Don't draw line at all.
            else if(r<t1) t1=r;         // Line is clipped!
        }
    }

    x0clip = x0src + t0*xdelta;
    y0clip = y0src + t0*ydelta;
    x1clip = x0src + t1*xdelta;
    y1clip = y0src + t1*ydelta;

    return true;        // (clipped) line is drawn
}