Re: [Matplotlib-users] Arrows using Line2D and shortening lines

Hi Jason,

I did made a similar class sometime ago and I'm attaching it just in
case. I guess it is very similar to yours but I rely on
matplolib.patches.FancyArrow class to draw the arrow head.

The circle drawn by scatter() command should be a circle with size s
(the third argument of the scatter command) in points . It seems that
it is implemented as a unit circle centered at (0,0) with a transform
corresponding to the size s (and offset). So you may try something
like below to calculate the size of the circle in data coord.

  ax = gca()
  p = scatter([0],[0], 500.)
  tr = p.get_transforms()[0] + ax.transData.inverted()
  x1, y1 = tr.transform_point([0,0])
  x2, y2 = tr.transform_point([1,0])
  r = abs(x2 - x1)

p is a collection object and p.get_transforms() is a list of transforms.
Note that a circle in the canvas coordinate(?) can be an ellipse in
data coordinates. So, I guess you'd better do things in the canvas
coordinates.

For shortening your path, if you're only concerned with a straight
line, it should be straight forward. But I guess it would a bit tricky
to do this for general bezier curves (as in the example that Alan
linked). I think (but I may be wrong) there is no universal algorithm
to find the "all" intersecting points of two bezier curves. There may
be one for between a bezier curve and a circle. And in this case where
one point is inside the circle and the other is outside, one simple
way I can think of is to recursively bisect the bezier curve (similar
to the bisect root finding).

Regards,

-JJ





On Fri, Aug 22, 2008 at 12:15 PM, Alan G Isaac <ai...@am...> wrote:
> Jason Grout wrote:
>> The other problem is a more serious problem for me: how do
>> I shorten the line so that it goes between the boundaries
>> of the circle instead of the centers, especially when the
>> circles are constructed in a scatter plot.
>
> Some years back I briefly tried to think about arrows and
> I found it trickier than expected.  Note that some famous
> software clearly does arrows wrong.  (E.g., gnuplot, at
> least last I checked.)
>
> Example: you have decided that you want to draw to the edge
> of a point, but a) is that right and b) can it be reasonably
> implemented?
>
> a) One might well argue in many applications that the arrow
> tip should go to the center of the circle.
>
> b) I'm not sure.
>
> But surely somebody out there will offer some great clues.
> Perhaps along the line of graphviz:
> http://www.graphviz.org/Gallery/directed/fsm.html
>
> Really this is not an answer to your questions ...
>
> Cheers,
> Alan Isaac
>
>
>
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