matplotlib-devel Mailing List for matplotlib

On Saturday 08 March 2008 11:36:45 am Darren Dale wrote:
> On Friday 07 March 2008 4:58:03 pm Darren Dale wrote:
> > I am having some trouble with the Cursor widget with the qt4agg backend.
> > Here is a short script which works with the gtkagg backend with useblit
> > either true or false:
> >
> > ----------
> > from matplotlib import rcParams
> > rcParams['backend']='gtkagg'
> > from pylab import *
> > from matplotlib.widgets import Cursor
> >
> > t = arange(0.0, 1.0, 0.01)
> > s = sin(2*2*pi*t)
> > ax = subplot(111)
> >
> > cursor = Cursor(ax, useblit=True)
> >
> > ax.plot(t, s, 'o')
> > axis([0,1,-1,1])
> > show()
> > ------------
> >
> > If I use the qt4agg backend, with useblit False, the cursor lines do not
> > render. If useblit is True, the lines render but the pixmap inside the
> > axes is sheared. I've been looking at the backend_qt4agg code, the
> > widgets.Cursor code, and the working animation_blit_qt4 example, but I'm
> > stuck. Does anyone have any ideas?
>
> Here is an additional wrinkle, sometimes the pixmap is sheared, and
> sometimes it is not. The behavior seems to shift back and forth when I
> change the horizontal size of the figure window, bu I don't see a pattern
> emerging that would explain why: 556-559 pixels wide is sheared, 560-564
> looks ok, 565-567 is sheared, 568 is normal, 569 and 570 are sheared, etc.
> The
> animation_blit_qt4 demo also has the same problem, depending on the
> horizontal size. So confusing.

I think this problem is due to a loss of precision in _backend_agg's 
copy_from_bbox. That method takes a bbox with double precision as input, and 
constructs a rect with ints as input:

  agg::rect_i rect((int)l, height - (int)t, (int)r, height - (int)b);

A rendering buffer is created with a width based on that rect. Half of the 
time, the width of that buffer disagrees with the width reported by the 
original bbox. For example, I can kludge the bbox width so it agrees with the 
output of copy_from_bbox:

            l, b, w, h = bbox.bounds
            r = int(l+w)
            w_mod = r-int(l)

I can use w_mod instead of w as a workaround for the blitting issue I 
reported, but I wonder if there might be a better solution?

Thanks,
Darren

On Friday 07 March 2008 4:58:03 pm Darren Dale wrote:
> I am having some trouble with the Cursor widget with the qt4agg backend.
> Here is a short script which works with the gtkagg backend with useblit
> either true or false:
>
> ----------
> from matplotlib import rcParams
> rcParams['backend']='gtkagg'
> from pylab import *
> from matplotlib.widgets import Cursor
>
> t = arange(0.0, 1.0, 0.01)
> s = sin(2*2*pi*t)
> ax = subplot(111)
>
> cursor = Cursor(ax, useblit=True)
>
> ax.plot(t, s, 'o')
> axis([0,1,-1,1])
> show()
> ------------
>
> If I use the qt4agg backend, with useblit False, the cursor lines do not
> render. If useblit is True, the lines render but the pixmap inside the axes
> is sheared. I've been looking at the backend_qt4agg code, the
> widgets.Cursor code, and the working animation_blit_qt4 example, but I'm
> stuck. Does anyone have any ideas?

Here is an additional wrinkle, sometimes the pixmap is sheared, and sometimes 
it is not. The behavior seems to shift back and forth when I change the 
horizontal size of the figure window, bu I don't see a pattern emerging that 
would explain why: 556-559 pixels wide is sheared, 560-564 looks ok, 565-567 
is sheared, 568 is normal, 569 and 570 are sheared, etc. The 
animation_blit_qt4 demo also has the same problem, depending on the 
horizontal size. So confusing.