Re: GnomeCanvasLine

[Federico Mena Quintero <federico helixcode com>]

>    Because of a school project I'm working on (a 3D modeler using
>  GnomeCanvas), I need to make sure that the routine that checks if a point
>  belongs to an item. I'm using exclusively GnomeCanvasLine. 

A 3D modeller using the canvas?  Are you sure you don't want to use
OpenGL instead?

>    gnome_canvas_get_item_at  calls the function
>  gnome_canvas_item_invoke_point, which calls the item's point virtual
>  function.  This function calculates the distance between the point and
>  each of the item's edges, using formulas like 			
>  	dist = sqrt (dx * dx + dy * dy) - width / 2.0;
>  
>    This is very slow; my teacher says we should use something like
>  	dist = dx; if the line's inclination is < 1.0, or
>  	dist = dy; otherwise.

That will not calculate the correct distance to the line.

>    gnome_canvas_item_invoke_point is also invoked as the pointer moves
>  across the canvas, to synthesize enter/leave notify events to canvas
>  items. This is also very slow. Amazingly, I don't notice it being slow. I
>  must be reading the code wrong. If not, can I make a patch to GnomeCanvas?

It is not slow.  The ::point() method is only invoked for items whose
bounding box intersects the mouse position.  Moreover, you normally
don't have a million overlapping items, so you'll only need to take a
few square roots on each mouse motion.

"Floating point is slow" is a fallacy.  It depends entirely on what
you are doing.

However, having a point-distance computation method for canvas items
is overkill, and it is a leftover of the design of Tk's canvas.  I
intend to replace ::point() with a simpler ::contains() that just
returns a boolean value.  This makes hit testing easier and probably
faster as well.

  Federico