Re: Slow canvas performance

[Jody Goldberg <jody gnome org>]

On Thu, Apr 29, 2004 at 04:02:24PM +0100, Rob Clack wrote:
> I'm writing an app which has to draw 10's of thousands of squares - it's 
> human genome data, so I'm not exaggerating.  I'm using gtk2 and gnome 
> canvas running on a Compaq deskpro with 512Mb memory and Red Hat 9, but 
> finding performance is really poor.
>
> Drawing a thousand or two squares is fine, but as the numbers rise, the 
> time taken increases to levels my users are going to find unacceptable.
>
> I thought that if what I was drawing was outside the viewable area (ie 
> the canvas is bigger than the scrolled window) then it wouldn't even try 
> to draw the squares, but that doesn't seem to be the case.  eg if the 
> window can show, say, 1000 squares and I run 3 tests, drawing 1000, 2000 
> and 3000 squares respectively, I'd expect them to take the same time to 
> run, since the extra squares should simply be cropped before the display 
> was rendered on the screen.  This is not the case.  The time taken 
> increases linearly.  I thought that was the whole point of double-buffering.
>
> Have I misunderstood how this is supposed to work or is there some 
> setting I've got wrong that disables that aspect?  Or is there a better 
> way for me to set about this?
>
> To create the item I just use
>
> for (i = 0, x = 0.0; i < 100; i++, x += x_incr)
>     {
>       for (j = 0, y = 0.0; j < number; j++, y += y_incr)
>       {
>         gnome_canvas_item_new(group,
>                               type,
>                               "x1"           , x ,
>                               "y1"           , y ,
>                               "x2"           , x + 5,0,
>                               "y2"           , y + 5.0,
>                               "outline_color", "red"           ,
>                               "fill_color"   , "white"          ,
>                               "width_units"  , 1.0,
>                               NULL);
>       }
>     }
> x and y are doubles, so no casting is necessary.

I'm surprised that redraw time is effected by invisible objects, but
ignoring that question I'll still make a few suggestions 

1) You may want to use the foocanvas rather than the gnomecanvas.
   foo is a simplification of the gnomecanvas that removes option of
   antialiasing and ties more closely to the gtk redraw framework.
   It is used in nautilus and gnumeric.

2) With 10s of thousands of things you may want to consider a custom
   canvas item.  The overhead of a canvas item for each square may
   get unacceptable for large data sets.  eg in gnumeric we have one
   canvas item for the entire grid, not one per cell.

Good Luck