The goal of this effort is to define a consistent, cross-browser hit-detection behavior, and to build on that with features for practical use cases.
In computer graphics programming, hit-testing (hit detection, picking, or pick correlation) 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 shape, line, or curve drawn on the screen. This may be done for movements of the pointer or the underlying shape, or restricted to user-initiated selection, such as a mouse-click.
Hit-testing is important for user interfaces and interactivity.
There are many different algorithms that may be used to perform hit-testing, with different performance or accuracy outcomes.
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.
The SVG-turned-CSS property pointer-events (not to be confused with the Pointer Events WG's deliverable) allows authors to control whether a given element participates in hit-testing.
This was first defined in SVG 1.0, then implemented for HTML in WebKit and Firefox in 2008. 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 2011 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).
CSS Mailing List Thread
- Leif Arne Storset, August 2010 presents a strong case for standardizing hit-testing, and describes useful details and next steps.
From Hixie on www-style, February 2009:
I had some notes in the HTML5 spec about what IE does for hit testing. It was brought to my attention that the general consensus is that this should be solved by applying the 'pointer-events' property to all content. Since some interest has been expressed regarding the notes I had, I have included them below. I have also removed this issue from HTML5, under the assumption that the CSS specifications will at some future point deal with the issue. HIT TESTING TRANSPARENCY Definition: IE considers a point of an element "transparent" if any one of the following are true: 1. All of the following are true: a: The computed value of 'background-image' is 'none', and b: The computed value of 'background-color' is 'transparent', and c: The point is over a pixel of an AlphaImageLoader filter image that has an alpha value of 0 (fully transparent), or the element does not have an AlphaImageLoader filter applied; 2. The point is outside the element's CSS clip rectangle; 3. The computed value of 'visibility' is 'hidden'; 4. The element is a transparent IFRAME (in IE, an IFRAME with the custom attribute "allowtransparency"); 5. The element is an OBJECT with the custom attribute "wmode" set to "transparent" and the point in question is fully transparent. Given those definitions, when a mouse event occurs, IE finds the target element as follows: A. Take the topmost node that is under the point where the pointer was for the event. For CSS boxes, borders, padding areas and content areas are considered part of the node, margins and leading generated by the 'line-height' property are not. B. If there is no node at that point, no event is fired. STOP. C. If the node is a text node, then the event is fired at the text node's nearest ancestor element node. STOP. D. If the node is not an element, assign the node's nearest ancestor element node to a variable X. Otherwise, assign the element node itself to X. E. If the element X is the BODY element or the HTML element and its document is not the document of a transparent IFRAME, goto step H. Similarly, if the element X is a TABLE element, or is an IMG element, goto step H. F. If the point where the pointer was is, per the above definition, a point that on the element X is transparent, then ignore that element and assign the element that is below that element in the stacking order to X. If there is no element below X, or if the point on X is not transparent and so the previous condition doesn't apply, then leave X as is and go straight to step H. G. Goto step E. H. If the element X is now a BODY or TABLE element, but the element assigned to X in step D was some other element, assign the element originally assigned in step D back to X. I. The event goes to X. STOP
- Computer Graphics: Principles and Practice 2nd Edition in C, Foley et al, Addison-Wesley, 1997.