Norman,
There is code (in 0.91, going back before my time) that rounds the
vertices of polygons (which arrows in effect are) to the center of
pixels. You can see it here inside RendererAgg::draw_polygon() in
_backend_agg.cpp:
agg::path_storage path;
for (size_t j=0; j<Npoints; j++) {
double x = xs[j];
double y = ys[j];
//snapto pixel centers
x = (int)x + 0.5;
y = (int)y + 0.5;
if (j==0) path.move_to(x,y);
else path.line_to(x,y);
}
path.close_polygon();
You could comment out these two lines:
x = (int)x + 0.5;
y = (int)y + 0.5;
and see if that corrects your wiggliness problem, just to confirm that
as the source.
The bigger question is -- there was probably a good reason that this
code was put in there in the first place, so it probably isn't a good
idea to remove it en masse. It may need to be exposed as an argument so
some things get this behavior and others don't. I know that dashed
lines that are perpendicular to the edges of the figure (e.g. grid
lines) look much worse if they aren't rounded to pixel centers. But in
general for polygons, I don't know if that's true.
Now to veer away from your question a little bit -->
On the transforms branch (which is now in SVN head, and not in any
release), this behavior has changed. The new heuristic is that if a
path (which includes everything including lines and polygons) includes
only straight and axis-aligned segments, it is rounded. If it includes
any curves or non-axis-aligned segments it is not rounded. This
heuristic seems to work quite well in practice, since it includes grid
lines, non-rotated rectangles etc., but I am interested in getting
feedback from other users if that heuristic fails in any special
circumstances.
Cheers,
Mike
Norman Davis wrote:
> Hi all,
> I' ve been using matplotlib to create some animations and it seems
> that the endpoints for arrow polygon lines are being rounded off to
> the nearest pixel. At least thats my guess. When viewing an animation
> the arrow changes shape and distorts as it moves around. The code
> below shows it on my Windows XP system where I'm using Matplotlib
> 0.91.1, SciPy 0.6.0, Numpy 1.0.4, Python 2.4. (The code is adapted
> from examples/animation_blit_tk.py) I've also seen this on my Linux
> setup when producing .png's for animation. I think I also remember
> noticing circle patches having discrete movements, so it may be more
> general than arrows.
> I'm wondering if other's find this to be a problem. Its
> distracting in my particular case. What can I do to help change this?
> I've been enjoying matplotlib along with basemap for the past two
> years and would like to help out if I can.
> Norm.
>
> - - - - - -
> import matplotlib
> matplotlib.use('TkAgg')
>
> import sys
> import pylab as p
> from matplotlib.patches import Arrow
>
> ax = p.subplot(111)
> canvas = ax.figure.canvas
>
> def run(*args):
> background = canvas.copy_from_bbox(ax.bbox)
>
> while 1:
> canvas.restore_region(background)
>
> ells = [Arrow(x = i/20.+ (run.cnt+1)/16000.,
> y = 0.4+ (run.cnt+1)/30000.,
> dx = 0.04 + 1/200.,
> dy = 0.06,
> width=0.03)
> for i in xrange(10)]
> for e in ells:
> ax.add_artist(e)
> for e in ells:
> ax.draw_artist(e)
>
> canvas.blit(ax.bbox)
>
> for e in ells:
> e.remove()
>
> if run.cnt==1000:
> sys.exit()
>
> run.cnt += 1
> run.cnt = 0
>
> p.subplots_adjust(left=0.3, bottom=0.3)
> p.grid()
> manager = p.get_current_fig_manager()
> manager.window.after(100, run)
> p.show()
> - - - - - -
>
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--
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA
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