[cairo] Hit detect surface?

[Gustavo J. A. M. Carneiro]

On Sex, 2006-03-03 at 15:29 -0700, chris nuernberger wrote:
> It is a lot of drawing to find a hit.  This sort of method is supposed
> to be the somewhat stupid brute force method of detecting the hit, it
> is the lowest level way to go.  I think it is entirely appropriate for
> this type of hit detection to be possible in cairo, however.
> 
> People who just want something to work will use this method, and
> people who want to build more advanced structures for their hit tests
> will do that.
> 
> There is a very similar system available through opengl.  A game or
> something like that will usually have their own set of objects and do
> hit detection themselves because they have build up BSP trees or
> octrees or whatever completely independent of the rendering engine. 
> But that doesn't make using the callback system invalid or useless by
> any means, it is still simplest way to do the hit detection so in that
> regard it needs to be stupid, robust and correct.

  I see your point.  I still think a virtual method / signal would be
appropriate.  The default implementation would do the brute force /
robust approach.  However, programmers would be allowed to override the
default implementation to implement something less computationally
intensive.

> 
> But not necessarily elegant or fast, if you want that then you roll your own.
> 
> On 3/3/06, Gustavo Carneiro <gjc at inescporto.pt> wrote:
> > <quote who="Bill Spitzak">
> > > 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.)
> >
> >   It sounds like a lot of drawing just to find if the mouse pointer is
> > near an object.  I suppose it could be useful in very complex shapes,
> > but I would imagine it would be much more efficient in most cases to
> > pretend an item is approximately a rectangle, circle, or another simple
> > shape, and then use  plain old math to determine the distance from the
> > mouse pointer to the shape.
> >
> >   I would therefore propose that a distance_to_point(x, y) virtual
> > method/signal be included in the canvas item.  Implementations can then
> > choose the best method to implement it, which can be simple math
> > calculus or more complex cairo surface manipulations (for really complex
> > shapes).
> >
> >   Regards.
> >
> > PS: sorry for entering this discussion without even looking at Sven's
> > canvas code :-P
> >
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> >
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-- 
Gustavo J. A. M. Carneiro
<gjc at inescporto.pt> <gustavo at users.sourceforge.net>
The universe is always one step beyond logic.