Re: [cairo] Hit detect surface?

Gustavo J. A. M. Carneiro wrote:

  But there is a stronger argument against this approach: some
applications are more interested in "how far is the mouse pointer from
every object", or "what is the object nearest to the pointer" than "give
me the object under the mouse pointer".  Because most times it is not
comfortable for the user to have to place the pointer exactly over an
pixel that happens to be painted by some object.  For instance, I
remember how early nautilus used a very precise (to the pixel) hit
detection for file icons; guess what, usability studies later showed
that users were annoyed by the fact that clicking on a CD icon wouldn't
work in some cases, particularly when the pointer is exactly over the
hole :)

What I am proposing *is* to do this "nearest the pointer" type hit detection.

There is only one "pixel" in the surface, and the caller has to set the transformation so that the region of interest is mapped to the area of this pixel. You can thus hit-detect anything that draws into any parallelogram-shaped region, though typically a square around the mouse cursor will be used. If the square is 20 pixels on a side, this will pick up everything within 10 pixels of the mouse.

To find the nearest, you must draw several times, with smaller and smaller squares, until either you reach a minimum size, or only one object is detected (or it goes to zero, in which case you use the previous pass). (actually now that I think about it, it may be more efficient to start at the minimum size and go up until you hit something.)

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