C# 获取 2 点之间的所有像素坐标 - 错误

Question

So first of all, I have several X1,Y1,X2 and Y2 coordinates. X1/Y1 represents the starting points, X2/Y2 the end points so I draw a line from these. I need to store all the pixels/points between this 2 points. I have an algorithm which seems to do it but there are some errors. I have no clue that why only half of them will be stored in the dictionary.

The code (Point p1 => X1 and Y1, Point p2 => X2 and Y2):

public List<Point> GetPoints(Point p1, Point p2)
{
    List<Point> points = new List<Point>();

    // no slope (vertical line)
    if (p1.X == p2.X)
    {
        for (double y = p1.Y; y <= p2.Y; y++)
        {
            Point p = new Point(p1.X, (int)y);
            points.Add(p);
        }
    }
    else
    {
        // swap p1 and p2 if p2.X < p1.X
        if (p2.X < p1.X)
        {
            Point temp = p1;
            p1 = p2;
            p2 = temp;
        }

        double deltaX = p2.X - p1.X;
        double deltaY = p2.Y - p1.Y;
        double error = -1.0f;
        double deltaErr = Math.Abs(deltaY / deltaX);

        double y = p1.Y;
        for (double x = p1.X; x <= p2.X; x++)
        {
            Point p = new Point((int)x, (int)y);
            points.Add(p);

            error += deltaErr;

            while (error >= 0.0f)
            {
                y++;
                points.Add(new Point((int)x, (int)y));
                error -= 1.0f;
            }
        }

        if (points.Last() != p2)
        {
            int index = points.IndexOf(p2);
            points.RemoveRange(index + 1, points.Count - index - 1);
        }
    }

    return points;
}

Some explanation: at the end of this whole procedure, I'll get a Point list with all the coordinates between the points -> which is good. After that, I draw these points but as u can see on the picture, only half of them got stored (the white lines show the stored coordinates between the points). Is there anything I didn't notice in the code? Why only those got stored? Thank u in advance!

(Yellow circles are the starting points and the purple ones are the end points)

Detailed picture here

Answer 1

I have an idea. All your success stored point directed from left-top corner to bottom-right corner and all points that is not stored directed from left-bottom to top-right. So the problem is in the Y-coordinate direction.

In first scenario Y-coordinate changes from from smaller to larger value so y++; will bring us closer to the end point and last point will be equal to the endPoint and points.Last() != p2 returns false.

In second scenario deltaY is negative so y++ will move us away from the end point and int index = points.IndexOf(p2); returns -1 because there is no endPoint in the collection. So

points.RemoveRange(index + 1, points.Count - index - 1);
// points.RemoveRange(-1 + 1, points.Count - -1 - 1);
// points.RemoveRange(0, points.Count);

will delete all points.

You need to calculate deltaYdirection with var deltaYdirection = (int) Math.Sign(deltaY) and use y += deltaYdirection; instead of y++; .