[webkit-dev] nodeAtPoint is very confusing

[Simon Fraser]

On Apr 8, 2009, at 7:06 AM, Eric Seidel wrote:

> Dave, Simon, and other rendering gurus:
>
> bool RenderObject::nodeAtPoint(const HitTestRequest&, HitTestResult&
> result, int _x, int _y, int tx, int ty, HitTestAction hitTestAction)
>
> _x, _y, tx, ty are very confusing.
>
> As far as I can tell, _x, _y are relative to the root layer (which can
> change during hit-testing!) and tx, ty are more of the HTML "offset to
> your parent" optimization which painting uses.
>
> Can you explain what the best way to convert from _x, _y to local
> coordinates?  And what I should rename _x, _y, and tx, ty to in a
> forthcoming "make hit testing readable to normal humans" cleanup
> patch?

As I understand it, _x and _y are relative to tx, ty, which in turn is  
relative
to the rootLayer.

In general tx and ty, as passed to painting and hit testing functions,  
are
the offsets that put you in the coordinate space of the parent renderer,
relative to the current rootLayer  being used for painting or hit  
testing
(which, as you note, gets reset for transforms, and I think should
also get reset for HTML/SVG boundaries).  So to get local x and y for  
hit testing,
you'll see code like:

tx += x();
ty += y()
...
updateHitTestResult(_x - tx, _y - ty)

It is all rather confusing, but much of the complexity comes from  
having to
deal with inlines.

Simon