Force points to be on a line

I have a line identify by x1,y1,x2,y2 which are double values. Then I have several graphical objects (Let's name the class TShape) which cordinates are Left, Top, Right, Bottom: double. Only Top and Left properties are writable value. When dragging the TShape around the top and left values are updated.

I am using a function to discovery when TShape is near a Line. The function definition is:

function NearLine(const Target: TPoint; X1, Y1, X2, Y2: double; Off: integer = 5): boolean;

NearLine returns true if point specified by Target is near the line specified by Point1 and Point2. The point must be at the distance specified by Off.

I use the function with Off = 0;

In my implementation Target is the center of the TShape which I keep updated calculating it from Top and Left properties. Because Target is TPoint I do:

1-

CPoint.X := Trunc(Center.X);
CPoint.Y := Trunc(Center.Y);

2- and when the function NearLine above is true I force the mouse to release with:

3-

Mouse_Event(MOUSEEVENTF_ABSOLUTE or MOUSEEVENTF_LEFTUP, 0, 0, 0, 0);

1,2 an 3 are called within an event UpdateMove which is called while draggi开发者_JAVA技巧ng the shape.

This allow me to "stick" the TShape almost near the line however it's not exactly what I want to achive... obviously there is an error due the Trunc function.

The error is:

deltaX := Frac(Center.X);
deltaY := Frac(Center.Y);

After releasing the mouse programmatically how I can force all the center therefore all the shapes to be perfectly lined (collinear) with the line?

Any help? :(

You're asking the question wrong, that's why you cant' see the answer your self. If 3 points aren't collinear, you're not going to "force" them collinear unless you change the laws of math and/or physics.

What you probably want is to find a point on the line defined by two points that's closest to your point of reference. That's pretty simple geometry: The closest point is as at the "foot" of a perpendicular drawn from your third point to the line defined by the first two! You can solve that using the Pythagoran theory alone, you don't even need fancy analytic geometry.