With node curve, the mouse can drag the node to adjust the curve, similar to Photoshop

One of the more commonly used functions in Photoshop is curve adjustment, as shown in the figure

You can add, delete, and drag curve nodes with the mouse to adjust image parameters. In terms of its idea (only the curve itself is considered here, data storage is not included in this list), this function is relatively simple:

1. The curve is represented by a set of Points

2. Mousing the node with the mouse actually modifies a single Point

3. Insert and delete Point

4. A node is a handle. It’s just a small square

5. Draw a curve passing through all nodes in PaintDrawCurve

6. Draw a crosshair to represent the current node

7. When the mouse is pressed, determine whether it is in an existing node. If so, mark it, otherwise add a new node

8. The mouse is pressed and Move, if there is already a node, the node coordinates are the mouse coordinates

9. Refresh the drawing

Completed program operation demonstration (animation):

The following is part of the sample code:

Node:

 List<point> points;</point>

Draw node handle:

Rectangle getHandle(Point p)
{
    Rectangle rect = new Rectangle(
        p.X - 3,
        p.Y - 3,
        6,
        6);
    return rect;
}

判断某点是否位于手柄区域：

bool isHandle(Point p)
{
    foreach (Point pt in points)
    {
        if (isInside(p, getHandle(pt)))
        {
            downIndex = points.IndexOf(pt);
            downPoint = pt;
            current = pt;
            return true;
        }
    }
    return false;
}

注意这个部分可以适当放大一下判断区域，这样便于鼠标操作（手柄太小，不易点击）。

Draw handle:

void drawHandle(Graphics g, Point p)
{
    if (points.IndexOf(p) == downIndex)
        g.FillRectangle(
            Brushes.Black,
            getHandle(p));
    else
        g.DrawRectangle(
            Pens.Black,
            getHandle(p));
}

Draw curve:

 void drawCurve(Graphics g)
 {
     g.DrawCurve(Pens.Black, points.ToArray());
 }

Curve drawing uses the cardinal spline drawing method of the Graphics class. Default tension is 0.5.

Drawing cross positioning line (auxiliary function):

void drawCrosshair(Graphics g, Point p)
{
    g.DrawLine(
        Pens.Gray,
        0, p.Y,
        clientRect.Width,
        p.Y);
    g.DrawLine(
        Pens.Gray,
        p.X,
        0,
        p.X,
        clientRect.Height);
}

Mouse dragging:

protected override void OnMouseMove(MouseEventArgs e)
{
    mousePoint = e.Location;
    if (mouseDown)
    {
        if (Current != null)
        {
            Current = mousePoint;
        }
        Refresh();
    }
}

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