Hit-testing

In computer graphics programming, hit-testing (hit detection, picking, or pick correlation [1]) is the process of determining whether a user-controlled cursor (such as a mouse cursor or touch-point on a touch-screen interface) intersects a given graphical object (such as a shape, line, or curve) drawn on the screen. Hit-testing may be performed on the movement or activation of a mouse or other pointing device.

Hit-testing is used by GUI environments to respond to user actions, such as selecting a menu item or a target in a game based on its visual location. In Web programming languages such as HTML, SVG, and CSS, this is associated with the concept of pointer-events (e.g. user-initiated cursor movement or object selection).

Collision detection is a related concept for detecting intersections of two or more different graphical objects, rather than intersection of a cursor with one or more graphical objects.

Algorithm

There are many different algorithms that may be used to perform hit-testing, with different performance or accuracy outcomes. One common hit-test algorithm is presented in the pseudo-code below:

function HitTest(Rectangle r1, Rectangle r2) returns boolean
{
    return ((r1.X + r1.Width >= r2.X) and (r1.X <= r2.X + r2.Width) and (r1.Y + r1.Height >= r2.Y) and (r1.Y <= r2.Y + r2.Height));
}

See also

References
  1. Computer Graphics: Principles and Practice 2nd Edition in C, Foley et al, Addison-Wesley, 1997.
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