Detection of collisions with virtual objects usually doesn't actually involve testing for intersection of the ray with one of the scene's polygons, as a typical scene may have hundreds or thousands of polygons, which makes direct tracing of rays to polygons impractical in most cases. Instead, most applications find a way to simplify the implementation of their hit testing algorithms. It's possible—or even likely—that whatever 3D graphics engine you're using offers hit testing, especially if it's designed specifically for game development.
Collision detection is a related concept for detecting intersections of two or more different shapes, points, or lines, rather than intersection of one or more shapes with a cursor. This was first defined in SVG 1. It was initially slated to be defined in the CSS UI level 3 spec, but removed because there was some sentiment that it should not be defined until the whole hit-testing model for the open web platform is defined; the current draft was extracted and put into a wiki page for future reference.
A proposal by Paul Bakaus Zynga in April requests that the pointer-events property be extended, or a new property created, that allows variable hit-testing sensitivity on raster images based on the alpha values transparency of each pixel, so that transparent parts of a PNG could be clicked through. This would make writing sprite-based games easier and improve performance, though it may be challenging to implement, according to a reply by Alex Danilo Google.
Another simple shape for collision detection is between two circles. This algorithm works by taking the centre points of the two circles and ensuring the distance between the centre points are less than the two radii added together. Note: Here is another example without Canvas or external libraries. It's more complicated to implement than the above methods but is more powerful. The complexity of an algorithm like this means we will need to consider performance optimization, covered in the next section.
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