Re: GnomeCanvasLine

[Lauris Kaplinski <lauris ariman ee>]

On Wed, 3 May 2000, Gustavo Joćo Alves Marques Carneiro wrote:

>   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. 
>   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.
> 
>   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?

Depending on CPU, you probably lose about 20-200 cycles (my estimate).
Even with 100MHz CPU you can get 500000 calculations per second. As canvas
will execute ::point method only if coordinates are inside objects binding
box, it is probably sufficent even for complex drawings. If not, you
should better implement your own line item, which checks segment binding
boxes, before calculating distance.

As much, as I know, the new approach of canvas will be to use all/nothing
approach, i.e. it is no more interested in actual distance, only if point
is inside object, or not. If you still want to speed this up, save
(width * width / 4.0)
during ::update and compare this to
(dx * dx + dy * dy)

If you really need distance, you can find smallest squared value and
take square root of this once.

Calculating line inclination is probably not much faster, and
abovementioned algorithm generates visible errors with fat lines near 45
degrees angle.

Regards,
Lauris Kaplinski

> 
> -- 
> Gustavo J.A.M. Carneiro
> [reinolinux.fe.up.pt/~ee96090]
>