Slow canvas performance

[Rob Clack <rnc sanger ac uk>]

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.

Hope you can help.
Rob
--
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Eat well, stay fit, die anyway.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Rob Clack                        Acedb Development,  Informatics Group
 email: rnc sanger ac uk                Wellcome Trust Sanger Institute
 Tel: +44 1223 494883                   Wellcome Trust Genome Campus
 Fax: +44 1223 494919                   Hinxton  Cambridge    CB10 1SA