Mouse over a dot

Question

I have created a component that has small points on it (ellipse with a radius of about 4). This is shown on top of another image and is dynamically generated at runtime.
My problem is that when I add a mouse listener to the object i need to determine if the mouse is overtop one of the dots, its not very reliable. Its almost as if it does not count all the pixels is passes... so I do not have a chance to determine if the mouse is really overtop of it. Is there a way to optimize that so it really can see every pixel going by... and determine if its moving overtop of it.
The way I am doing it is determining the distance from the center of the dot to the point in question, then squaring my radius and comparing the result... ( Pythagorean Theorem ). I have this in my mouseMoved function.
Thanks

Answer 1

Your explination shows a mathematical error which
would cause false posatives. squaring the radius
doesn't tell you much in relation to distances and if
the point is in the circle. The radius itself is
what you should be comparing to the distance rather
than its square.Actually, it's quite common, to avoid a square root computation, to compare r1^2 to r2^2. However, comparing r1 to r2^2 will lead to problems.
E.g. He said his ellipse (circle) had radius of 4.
so he should have something like:
// dotRadiusSq = 16.0;
double mx = evt.getX();
double my = evt.getY();
for each circle (c) {
         double dx = c.x - mx;
         double dy = c.y - my;
         double d = dx * dx + dy * dy;
         if (d <= dotRadiusSq) {
                  // mouse is over circle
}Another reason to use double / float is if you plan on changing the scale.
For example, if later you want your program to zoom in, then you'll need to work with double / floats. If you decide to change the shape from a circle to a more complicated shape, then you can just use the shape.contains(...) method.