计算A-B线上从A到B的给定距离点

Question

我非常疯狂地试图计算出A-B，在一个给定的距离上的点，这样我就可以在两个给定的点之间“画出”线。一开始听起来很简单，但我似乎搞错了。更糟糕的是，我不知道自己哪里出了问题。几何(和一般的数学)不是我的强项。

我读过类似的问题，也有答案。事实上，我直接从Mads的答案中删除了我目前对CalculatePoint函数的实现：给定起始点和终点，以及距离，计算一条直线上的一个点。 (再加上后来的一条评论中的更正--如果我对他的理解是正确的话)，因为我试图解决这个问题的坚定尝试，除了头等舱的特快机票节约地之外，并没有给我带来任何帮助。

下面是我的更新的代码(请参阅文章底部的编辑说明)：

using System;
using System.Drawing;
using System.Windows.Forms;

namespace DrawLines
{
    public class MainForm : Form
    {
        // =====================================================================
        // Here's the part I'm having trouble with. I don't really understand
        // how this is suposed to work, so I can't seem to get it right!
        // ---------------------------------------------------------------------

        // A "local indirector" - Just so I don't have go down and edit the 
        // actual call everytime this bluddy thing changes names.
        private Point CalculatePoint(Point a, Point b, int distance) {
            return CalculatePoint_ByAgentFire(a, b, distance);
        }

        #region CalculatePoint_ByAgentFire
        //AgentFire: Better approach (you can rename the struct if you need):
        struct Vector2
        {
            public readonly double X;
            public readonly double Y;
            public Vector2(double x, double y) {
                this.X = x;
                this.Y = y;
            }
            public static Vector2 operator -(Vector2 a, Vector2 b) {
                return new Vector2(b.X - a.X, b.Y - a.Y);
            }
            public static Vector2 operator *(Vector2 a, double d) {
                return new Vector2(a.X * d, a.Y * d);
            }
            public override string ToString() {
                return string.Format("[{0}, {1}]", X, Y);
            }
        }
        // For getting the midpoint you just need to do the (a - b) * d action:
        //static void Main(string[] args)
        //{
        //    Vector2 a = new Vector2(1, 1);
        //    Vector2 b = new Vector2(3, 1);
        //    float distance = 0.5f; // From 0.0 to 1.0.
        //    Vector2 c = (a - b) * distance;
        //    Console.WriteLine(c);
        //}
        private Point CalculatePoint_ByAgentFire(Point a, Point b, int distance) {
            var vA = new Vector2(a.X, a.Y);
            var vB = new Vector2(b.X, b.Y);
            double lengthOfHypotenuse = LengthOfHypotenuseAsDouble(a,b);
            double portionOfDistanceFromAtoB = distance / lengthOfHypotenuse;
            var vC = (vA - vB) * portionOfDistanceFromAtoB;
            Console.WriteLine("vC="+vC);
            return new Point((int)(vC.X+0.5), (int)(vC.Y+0.5));
        }
        // Returns the length of the hypotenuse rounded to an integer, using
        // Pythagoras' Theorem for right angle triangles: The length of the
        // hypotenuse equals the sum of the square of the other two sides.
        // Ergo: h = Sqrt(a*a + b*b)
        private double LengthOfHypotenuseAsDouble(Point a, Point b) {
            double aSq = Math.Pow(Math.Abs(a.X - b.X), 2); // horizontal length squared
            double bSq = Math.Pow(Math.Abs(b.Y - b.Y), 2); // vertical length  squared
            return Math.Sqrt(aSq + bSq); // length of the hypotenuse
        }

        #endregion

        //dbaseman: I thought something looked strange about the formula ... the question 
        //you linked was how to get the point at a distance after B, whereas you want the
        //distance after A. This should give you the right answer, the start point plus 
        //distance in the vector direction.
        //
        // Didn't work as per: http://s1264.photobucket.com/albums/jj496/corlettk/?action=view&current=DrawLinesAB-broken_zps069161e9.jpg
        //
        private Point CalculatePoint_ByDbaseman(Point a, Point b, int distance) {
            // a. calculate the vector from a to b:
            double vectorX = b.X - a.X;
            double vectorY = b.Y - a.Y;
            // b. calculate the length:
            double magnitude = Math.Sqrt(vectorX * vectorX + vectorY * vectorY);
            // c. normalize the vector to unit length:
            vectorX /= magnitude;
            vectorY /= magnitude;
            // d. calculate and Draw the new vector, which is x1y1 + vxvy * (mag + distance).
            return new Point(
                (int)((double)a.X + vectorX * distance)     // x = col
              , (int)((double)a.Y + vectorY * distance)     // y = row
            );
        }

        // MBo: Try to remove 'magnitude' term in the parentheses both for X and for Y expressions.
        //
        // Didn't work as per: http://s1264.photobucket.com/albums/jj496/corlettk/?action=view&current=DrawLinesAB-broken_zps069161e9.jpg
        //
        //private Point CalculatePoint_ByMBo(Point a, Point b, int distance) {
        //    // a. calculate the vector from a to b:
        //    double vectorX = b.X - a.X;
        //    double vectorY = b.Y - a.Y;
        //    // b. calculate the length:
        //    double magnitude = Math.Sqrt(vectorX * vectorX + vectorY * vectorY);
        //    // c. normalize the vector to unit length:
        //    vectorX /= magnitude;
        //    vectorY /= magnitude;
        //    // d. calculate and Draw the new vector, which is x1y1 + vxvy * (mag + distance).
        //    return new Point(
        //        (int)(  ((double)a.X + vectorX * distance)  +  0.5  )
        //      , (int)(  ((double)a.X + vectorX * distance)  +  0.5  )
        //    );
        //}

        // Didn't work
        //private Point CalculatePoint_ByUser1556110(Point a, Point b, int distance) {
        //    Double magnitude = Math.Sqrt(Math.Pow(b.Y - a.Y, 2) + Math.Pow(b.X - a.X, 2));
        //    return new Point(
        //        (int)(a.X + distance * (b.X - a.X) / magnitude + 0.5)
        //      , (int)(a.Y + distance * (b.Y - a.Y) / magnitude + 0.5)
        //    );
        //}

        // didn't work
        //private static Point CalculatePoint_ByCadairIdris(Point a, Point b, int distance) {
        //    // a. calculate the vector from a to b:
        //    double vectorX = b.X - a.X;
        //    double vectorY = b.Y - a.Y;
        //    // b. calculate the proportion of hypotenuse
        //    double factor = distance / Math.Sqrt(vectorX*vectorX + vectorY*vectorY);
        //    // c. factor the lengths
        //    vectorX *= factor;
        //    vectorY *= factor;
        //    // d. calculate and Draw the new vector,
        //    return new Point((int)(a.X + vectorX), (int)(a.Y + vectorY));
        //}

        // Returns a point along the line A-B at the given distance from A
        // based on Mads Elvheim's answer to:
        // https://stackoverflow.com/questions/1800138/given-a-start-and-end-point-and-a-distance-calculate-a-point-along-a-line
        private Point MyCalculatePoint(Point a, Point b, int distance) {
            // a. calculate the vector from o to g:
            double vectorX = b.X - a.X;
            double vectorY = b.Y - a.Y;
            // b. calculate the length:
            double magnitude = Math.Sqrt(vectorX * vectorX + vectorY * vectorY);
            // c. normalize the vector to unit length:
            vectorX /= magnitude;
            vectorY /= magnitude;
            // d. calculate and Draw the new vector, which is x1y1 + vxvy * (mag + distance).
            return new Point(
                (int)(((double)a.X + vectorX * (magnitude + distance)) + 0.5) // x = col
              , (int)(((double)a.Y + vectorY * (magnitude + distance)) + 0.5) // y = row
            );
        }

        // =====================================================================

        private const int CELL_SIZE = 4; // width and height of each "cell" in the bitmap.

        private readonly Bitmap _bitmap; // to draw on (displayed in picBox1).
        private readonly Graphics _graphics; // to draw with.

        // actual points on _theLineString are painted red.
        private static readonly SolidBrush _thePointBrush = new SolidBrush(Color.Red);
        // ... and are labeled in Red, Courier New, 12 point, Bold
        private static readonly SolidBrush _theLabelBrush = new SolidBrush(Color.Red);
        private static readonly Font _theLabelFont = new Font("Courier New", 12, FontStyle.Bold);

        // the interveening calculated cells on the lines between actaul points are painted Black.
        private static readonly SolidBrush _theLineBrush = new SolidBrush(Color.Black);

        // the points in my line-string.
        private static readonly Point[] _theLineString = new Point[] {
            //          x,   y
            new Point(170,  85), // A
            new Point( 85,  70), // B
            //new Point(209,  66), // C
            //new Point( 98, 120), // D
            //new Point(158,  19), // E
            //new Point(  2,  61), // F
            //new Point( 42, 177), // G
            //new Point(191, 146), // H
            //new Point( 25, 128), // I
            //new Point( 95,  24)  // J
        };

        public MainForm() {
            InitializeComponent();
            // initialise "the graphics system".
            _bitmap = new Bitmap(picBox1.Width, picBox1.Height);
            _graphics = Graphics.FromImage(_bitmap);
            picBox1.Image = _bitmap;
        }

        #region actual drawing on the Grpahics

        private void DrawCell(int x, int y, Brush brush) {
            _graphics.FillRectangle(
                brush
              , x * CELL_SIZE, y * CELL_SIZE    // x, y
              , CELL_SIZE, CELL_SIZE        // width, heigth
            );
        }

        private void DrawLabel(int x, int y, char c) {
            string s = c.ToString();
            _graphics.DrawString(
                s, _theLabelFont, _theLabelBrush
              , x * CELL_SIZE + 5   // x
              , y * CELL_SIZE - 8   // y
            );
        }

        // ... there should be no mention of _graphics or CELL_SIZE below here ...

        #endregion

        #region draw points on form load

        private void MainForm_Load(object sender, EventArgs e) {
            DrawPoints();
        }

        // draws and labels each point in _theLineString
        private void DrawPoints() {
            char c = 'A'; // label text, as a char so we can increment it for each point.
            foreach ( Point p in _theLineString ) {
                DrawCell(p.X, p.Y, _thePointBrush);
                DrawLabel(p.X, p.Y, c++);
            }
        }

        #endregion

        #region DrawLines on button click

        private void btnDrawLines_Click(object sender, EventArgs e) {
            DrawLinesBetweenPointsInTheString();
        }

        // Draws "the lines" between the points in _theLineString.
        private void DrawLinesBetweenPointsInTheString() {
            int n = _theLineString.Length - 1; // one less line-segment than points
            for ( int i = 0; i < n; ++i )
                Draw(_theLineString[i], _theLineString[i + 1]);
            picBox1.Invalidate(); // tell the graphics system that the picture box needs to be repainted.
        }

        // Draws all the cells along the line from Point "a" to Point "b".
        private void Draw(Point a, Point b) {
            int maxDistance = LengthOfHypotenuse(a, b);
            for ( int distance = 1; distance < maxDistance; ++distance ) {
                var point = CalculatePoint(a, b, distance);
                DrawCell(point.X, point.X, _theLineBrush);
            }
        }

        // Returns the length of the hypotenuse rounded to an integer, using
        // Pythagoras' Theorem for right angle triangles: The length of the
        // hypotenuse equals the sum of the square of the other two sides.
        // Ergo: h = Sqrt(a*a + b*b)
        private int LengthOfHypotenuse(Point a, Point b) {
            double aSq = Math.Pow(Math.Abs(a.X - b.X), 2); // horizontal length squared
            double bSq = Math.Pow(Math.Abs(b.Y - b.Y), 2); // vertical length  squared
            return (int)(Math.Sqrt(aSq + bSq) + 0.5); // length of the hypotenuse
        }

        #endregion

        #region Windows Form Designer generated code
        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent() {
            this.picBox1 = new System.Windows.Forms.PictureBox();
            this.btnDrawLines = new System.Windows.Forms.Button();
            ((System.ComponentModel.ISupportInitialize)(this.picBox1)).BeginInit();
            this.SuspendLayout();
            // 
            // picBox1
            // 
            this.picBox1.Dock = System.Windows.Forms.DockStyle.Fill;
            this.picBox1.Location = new System.Drawing.Point(0, 0);
            this.picBox1.Name = "picBox1";
            this.picBox1.Size = new System.Drawing.Size(1000, 719);
            this.picBox1.TabIndex = 0;
            this.picBox1.TabStop = false;
            // 
            // btnDrawLines
            // 
            this.btnDrawLines.Location = new System.Drawing.Point(23, 24);
            this.btnDrawLines.Name = "btnDrawLines";
            this.btnDrawLines.Size = new System.Drawing.Size(77, 23);
            this.btnDrawLines.TabIndex = 1;
            this.btnDrawLines.Text = "Draw Lines";
            this.btnDrawLines.UseVisualStyleBackColor = true;
            this.btnDrawLines.Click += new System.EventHandler(this.btnDrawLines_Click);
            // 
            // MainForm
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(1000, 719);
            this.Controls.Add(this.btnDrawLines);
            this.Controls.Add(this.picBox1);
            this.Location = new System.Drawing.Point(10, 10);
            this.MinimumSize = new System.Drawing.Size(1016, 755);
            this.Name = "MainForm";
            this.SizeGripStyle = System.Windows.Forms.SizeGripStyle.Hide;
            this.StartPosition = System.Windows.Forms.FormStartPosition.Manual;
            this.Text = "Draw Lines on a Matrix.";
            this.Load += new System.EventHandler(this.MainForm_Load);
            ((System.ComponentModel.ISupportInitialize)(this.picBox1)).EndInit();
            this.ResumeLayout(false);
        }

        private System.Windows.Forms.PictureBox picBox1;
        private System.Windows.Forms.Button btnDrawLines;
        #endregion
    }

}

对不起，如果有点长，但这是从我的实际项目中挖掘出来的SSCCE，它是最短路径算法的一个实现，用于运行MazeOfBolton.也就是迷宫赛跑者。

我实际上想做的是预先计算迷宫(矩阵)中两个给定点(原点和目标)周围的“栅栏”(即缓冲的MBR)，这样“栅栏”内的所有点都在从“两点之间的直线”的给定距离内，以便快速消除数百-数千条可能的偏离目标的路径。

请注意，这个编程挑战几年前就结束了，所以这里没有“竞争剽窃”的问题。不，这不是作业，事实上我是一个专业的程序员.我只是离开了我的舒适区，即使是相对简单的几何。叹一口气。

所以..。请有人给我指点，帮助我正确地得到CalculatePoint函数到，：计算A-B线上从A到B的给定距离的一个点？

谢谢你的慷慨..。即使在阅读这么远的地方。

干杯。基思。

编辑：我刚刚更新了发布的源代码，因为：

(1)我刚刚意识到它不是自成一体的。我忘记了单独的MainForm.Designer.cs文件，我已经将其附加到发布代码的底部。

(2)最新版本包括我迄今尝试过的内容，其中有一个指向每个故障的图片的photobucket链接.它们都是一样的。惠伊？见鬼？

我想我的问题可能出在其他地方，比如以前被其他人忽略的一些古怪的windows表单设置，因为我忘记了发布设计人员生成的代码……除了(在我的实际项目中)所有的东西都画在我期望的地方，那么为什么计算的点会有什么不同呢？我不知道！？我很沮丧，而且越来越暴躁，所以我想我会把这个留到另一天;-)

这说明我们经常低估让一台计算机做ANYthing所需的努力。即使划一条简单的线..。它甚至不是一条曲线，更不用说一个大圆圈，一个横向的墨卡托或任何奇特的东西.只是一句简单的棒棒糖！？;-)

非常感谢每一个发帖的人！

再次干杯。基思。

Answer 1

计算矢量AB

首先定义从点A(1，-1)到点B(2,4)减去B的向量。向量是Vab(1,5)。

计算AB的长度

利用毕达哥拉斯定理计算矢量AB的长度。

|Vab| = SQRT(1²+5²)

长度为(四舍五入) 5.1

计算单位向量

将向量除以其长度得到单位向量(长度为1的向量)。

V1(1/5.1,5/5.1) = V1(0.2, 0.98)

计算长度为4的向量

现在用您想要的长度乘以V1，例如4，以获得Vt。

Vt(0.2*4,0.98*4) = Vt(0.8,3.92)

计算目标点

将向量Vt添加到A点以得到点T(目标)。

T = A + Vt = T(1.8,2.92)

编辑:响应您的编辑

方法LengthOfHypotenuse应该是这样的

• 修正了计算bSq的一个错误
• 并删除冗余的Math.Abs调用，因为pow为2总是正的。
• 去掉0.5的加法，不知道你为什么要那样做
• 您至少应该使用浮点数作为返回值(双小数点也可以)。 //您应该使用Vector2类而不是Point，并使用它们的长度属性私有双LengthOfHypotenuse(点a，点b) { double aSq =Math.Pow(A.X-b.X，2)；//水平长度平方双bSq =Math.Pow(A.Y-B.Y，2)；//垂直长度平方返回Math.Sqrt(aSq + bSq)；//低音长度}

绘制方法( a点，b点)应如下所示：

• 修正的DrawCell()调用 私有空图(点a，点b) {双maxDistance = LengthOfHypotenuse(a，b)；for (int距离= 0；距离< maxDistance；++distance) { var点=CalculatePoint(新的Vector2(a)，新的Vector2(b)，距离)；DrawCell(Pointt.x，point t.Y，_theLineBrush)；}}

您的CalculatePoint(点a，点b，int距离)方法：

• 将一些计算移到Vector2类中 专用点CalculatePoint(Vector2 a，Vector2 b，int距离){ Vector2 vectorAB =a；返回a+ vectorAB.UnitVector *距离；}

我为您扩展了向量类，以添加缺少的运算符(AgentFire学分)

    //AgentFire: Better approach (you can rename the struct if you need):
    struct Vector2 {
        public readonly double X;
        public readonly double Y;
        public Vector2(Point p) : this(p.X,p.Y) { 
        }

        public Vector2(double x, double y) {
            this.X = x;
            this.Y = y;
        }
        public static Vector2 operator -(Vector2 a, Vector2 b) {
            return new Vector2(b.X - a.X, b.Y - a.Y);
        }
        public static Vector2 operator +(Vector2 a, Vector2 b) {
            return new Vector2(b.X + a.X, b.Y + a.Y);
        }
        public static Vector2 operator *(Vector2 a, double d) {
            return new Vector2(a.X * d, a.Y * d);
        }
        public static Vector2 operator /(Vector2 a, double d) {
            return new Vector2(a.X / d, a.Y / d);
        }

        public static implicit operator Point(Vector2 a) {
            return new Point((int)a.X, (int)a.Y);
        }

        public Vector2 UnitVector {
            get { return this / Length; }
        }

        public double Length {
            get {
                double aSq = Math.Pow(X, 2);
                double bSq = Math.Pow(Y, 2);
                return Math.Sqrt(aSq + bSq);
            }
        }

        public override string ToString() {
            return string.Format("[{0}, {1}]", X, Y);
        }
    }

Answer 2

更好的方法(如果需要，可以重命名结构)：

struct Vector2
{
    public readonly float X;
    public readonly float Y;

    public Vector2(float x, float y)
    {
        this.X = x;
        this.Y = y;
    }

    public static Vector2 operator -(Vector2 a, Vector2 b)
    {
        return new Vector2(b.X - a.X, b.Y - a.Y);
    }
    public static Vector2 operator +(Vector2 a, Vector2 b)
    {
        return new Vector2(a.X + b.X, a.Y + b.Y);
    }
    public static Vector2 operator *(Vector2 a, float d)
    {
        return new Vector2(a.X * d, a.Y * d);
    }

    public override string ToString()
    {
        return string.Format("[{0}, {1}]", X, Y);
    }
}

要获得中点，只需执行(a - b) * d + a操作：

class Program
{
    static void Main(string[] args)
    {
        Vector2 a = new Vector2(1, 1);
        Vector2 b = new Vector2(3, 1);
        float distance = 0.5f; // From 0.0 to 1.0.
        Vector2 c = (a - b) * distance + a;
        Console.WriteLine(c);
    }
}

这将给你一个重点：

​

​

output:\> [2, 1]

在此之后，您所需要的就是从for(the distance; up toone‘从0.0到1.0并绘制像素。

Answer 3

    private static Point CalculatePoint(Point a, Point b, int distance)
    {

        // a. calculate the vector from o to g:
        double vectorX = b.X - a.X;
        double vectorY = b.Y - a.Y;

        // b. calculate the proportion of hypotenuse
        double factor = distance / Math.Sqrt(vectorX * vectorX + vectorY * vectorY);

        // c. factor the lengths
        vectorX *= factor;
        vectorY *= factor;

        // d. calculate and Draw the new vector,
        return new Point((int)(a.X + vectorX), (int)(a.Y + vectorY));
    }