Bug 9450

At high zoom levels, it takes a very long time to draw a single line.
There is a thread that discusses this on the cairo mailing list,
but I wanted to follow it up with a bug report here (the title of the
thread was "newbie clipping question").  The conclusion was that the problem
was an X bug that could be worked around in the xlib backend of cairo.

Here is some code that demonstrates the problem:

#include <stdio.h>
#include <gtk/gtk.h>
#include <sys/time.h>

#define DAREA_SIZE (50)

GtkWidget *darea;
double zoom = 3200.0;
int redrawTimeout;


gint onRedrawTimeout(gpointer data)
{
GdkRectangle rect;
rect.x = 0;
rect.y = 0;
rect.width = DAREA_SIZE;
rect.height = DAREA_SIZE;
gdk_window_invalidate_rect(darea->window,
                            &rect,
                            FALSE);
redrawTimeout = 0;
return FALSE;
}

void onExpose(double *exposeRect)
{

cairo_t *cr;
cr = gdk_cairo_create (darea->window);

cairo_rectangle (cr,
                  exposeRect[0], exposeRect[1], exposeRect[2],
                  exposeRect[3]);


cairo_clip (cr);

/* draw white background */
cairo_move_to(cr, 0.0, 0.0);
cairo_line_to(cr, DAREA_SIZE, 0.0);
cairo_line_to(cr, DAREA_SIZE, DAREA_SIZE);
cairo_line_to(cr, 0.0, DAREA_SIZE);
cairo_close_path(cr);
cairo_set_source_rgb(cr, 1.0, 1.0, 1.0);
cairo_fill(cr);

/* draw line at currrent zoom level */
cairo_move_to(cr, 0.0, 0.0);
cairo_line_to(cr, 10*zoom, 10*zoom);
cairo_set_source_rgb(cr, 1.0, 0.0, 0.0);
cairo_stroke(cr);

cairo_destroy (cr);
}


gint onDrawingAreaEvent(GtkWidget *item, GdkEvent *event)
{
switch (event->type) {
case GDK_EXPOSE: {
   double exposeRect[4];
   exposeRect[0] = event->expose.area.x;
   exposeRect[1] = event->expose.area.y;
   exposeRect[2] = event->expose.area.width;
   exposeRect[3] = event->expose.area.height;
   onExpose(exposeRect);
   return TRUE;
}
}
}

int main(int argc, char *argv[])
{
gtk_init(&argc, &argv);

GtkWidget *window;
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
GtkWidget *vbox;
vbox = gtk_vbox_new(FALSE, 0);
gtk_container_add (GTK_CONTAINER(window), vbox);

darea = gtk_drawing_area_new();

gtk_box_pack_start(GTK_BOX(vbox), darea, TRUE, TRUE, 0);
gtk_widget_set_size_request (GTK_WIDGET (darea), DAREA_SIZE, DAREA_SIZE);

gtk_signal_connect(GTK_OBJECT(darea), "event",
GTK_SIGNAL_FUNC(onDrawingAreaEvent), 0);

gtk_widget_show_all(window);
gtk_main();
}