HitTesting

Hit testing, or hit testing, is the process of determining which element, region, or object lies under a given point, typically corresponding to a pointer or touch location. It is a core part of input handling in graphical user interfaces and interactive applications. A basic test checks whether a point lies within an element’s bounding box; more advanced tests account for transforms, clipping, opacity, and non-rectangular shapes.

Techniques range from simple bounding-box checks to more precise methods such as path containment tests, per-pixel hit testing, and hit-testing masks. Per-pixel hit testing uses pixel alpha or color data to decide whether a point actually hits an interactive area, enabling irregular shapes or transparency to influence interactivity. For large numbers of elements, spatial data structures and bounding volume hierarchies help optimize performance by pruning unlikely targets before precise checks are performed.

Event dispatch and targeting rely on hit testing to determine the recipient of input events. In web environments, the topmost element under the pointer is selected based on the stacking order and interaction rules; frameworks may expose APIs or properties to influence participation in hit testing, such as the CSS pointer-events property. Platform-specific examples include iOS hitTest:withEvent: and Android’s dispatchTouchEvent/onTouchEvent paths, while browsers offer document.elementFromPoint and event bubbling to resolve targets.

Common considerations include handling invisible or transparent elements, transformed coordinates, clipped regions, scrolling, and pointer capture. Performance concerns encourage using coarse preliminary checks, caching commonly used hit areas, and limiting per-pixel tests to when they are truly necessary. While related, hit testing for user input differs from collision detection in games, which often emphasizes continuous spatial overlap rather than point-based events.