Size of ellipses in EllipseCollection

Yannick_Copin1 · February 1, 2010, 3:15pm

Hi,

I would like to plot lots of error ellipses on my plot. I thought I could use an EllipseCollection to do so, but I didn't manage to get the ellipse sizes, expressed in data units, right (see test script attached).

When using a plain list of Ellipse's, everything looks fine:

ells = [ P.matplotlib.patches.Ellipse(xy=xyi,
                                       width=wi, height=hi,
                                       angle=ai,
                                       edgecolor='b',
                                       facecolor='b',
                                       alpha=0.3)
          for xyi,wi,hi,ai in zip(xy,w,h,a)]
for ell in ells:
     ax.add_artist(ell)

But when using EllipseCollection (as explained in http://matplotlib.sourceforge.net/examples/pylab_examples/ellipse_collection.html),

coll = P.matplotlib.collections.EllipseCollection(widths=w,
                                                   heights=h,
                                                   angles=a,
                                                   units='x',
                                                   offsets=xy,
                                                   transOffset=ax.transData,
                                                   edgecolor='r',
                                                   facecolor='r',
                                                   alpha=0.3)
ax.add_collection(coll)

the size of the ellipses (expressed in data units) appears incorrect, and depends furthermore of the aspect ratio of the figure (try pan/zoom or resize the interactive window).

I naively thought the two approaches should give the same result... Did I miss something in the way to use EllipseCollections?

Cheers,

ellipseColl.py (1.47 KB)

···

--
    .~. Yannick COPIN (o:>* Doctus cum libro
    /V\ Institut de physique nucleaire de Lyon (IN2P3 - France)
   // \\ Tel: (33/0) 472 431 968 AIM: YnCopin ICQ: 236931013
  /( )\ http://snovae.in2p3.fr/ycopin/
   ^`~'^

Eric_Firing1 · February 1, 2010, 6:15pm

Yannick Copin wrote:

Hi,

I would like to plot lots of error ellipses on my plot. I thought I could use an EllipseCollection to do so, but I didn't manage to get the ellipse sizes, expressed in data units, right (see test script attached).

When using a plain list of Ellipse's, everything looks fine:

ells = [ P.matplotlib.patches.Ellipse(xy=xyi,
                                      width=wi, height=hi,
                                      angle=ai,
                                      edgecolor='b',
                                      facecolor='b',
                                      alpha=0.3)
         for xyi,wi,hi,ai in zip(xy,w,h,a)]
for ell in ells:
    ax.add_artist(ell)

But when using EllipseCollection (as explained in http://matplotlib.sourceforge.net/examples/pylab_examples/ellipse_collection.html),

coll = P.matplotlib.collections.EllipseCollection(widths=w,
                                                  heights=h,
                                                  angles=a,
                                                  units='x',
                                                  offsets=xy,
                                                  transOffset=ax.transData,
                                                  edgecolor='r',
                                                  facecolor='r',
                                                  alpha=0.3)
ax.add_collection(coll)

the size of the ellipses (expressed in data units) appears incorrect, and depends furthermore of the aspect ratio of the figure (try pan/zoom or resize the interactive window).

Maybe I am misunderstanding, but the EllipseCollection in the example is panning and zooming as I expect. The shapes and orientations are staying the same while the scale expands and contracts with the x-axis.

The problem is that the option you are looking for does not exist yet. I am not sure yet whether what you need is identical to an option I added to quiver (from which I partially derived EllipseCollection). I suspect it is.

Do you want width to scale with x and height to scale with y? And the angle to scale such that a 45-degree angle always corresponds to equal increments in x and in y, all in data units?

I naively thought the two approaches should give the same result... Did I miss something in the way to use EllipseCollections?

EllipseCollection is derived more from quiver than from Ellipse.

Eric

Yannick_Copin1 · February 2, 2010, 3:02pm

Eric Firing wrote:

Maybe I am misunderstanding, but the EllipseCollection in the example is panning and zooming as I expect. The shapes and orientations are staying the same while the scale expands and contracts with the x-axis.

The problem is that the option you are looking for does not exist yet. I am not sure yet whether what you need is identical to an option I added to quiver (from which I partially derived EllipseCollection). I suspect it is.

Thanks, I now understand better what is the main goal of EllipseCollection: in a quiver-like use, you don't want indeed the plotted ellipses to change size and/or orientation with scaling and zooming.

Do you want width to scale with x and height to scale with y? And the angle to scale such that a 45-degree angle always corresponds to equal increments in x and in y, all in data units?

Yes, in the context of error-ellipses, I would like to see my ellipses scale with x and y, as they do when I plot them individually as Ellipse patches (see example scripts, the blue ellipses are what I would like to obtain using an EllipseCollection), something like "units='data'" in EllipseCollection...

Cheers,

···

--
    .~. Yannick COPIN (o:>* Doctus cum libro
    /V\ Institut de physique nucleaire de Lyon (IN2P3 - France)
   // \\ Tel: (33/0) 472 431 968 AIM: YnCopin ICQ: 236931013
  /( )\ http://snovae.in2p3.fr/ycopin/
   ^`~'^

Eric_Firing1 · February 2, 2010, 4:43pm

Yannick Copin wrote:

Eric Firing wrote:

Maybe I am misunderstanding, but the EllipseCollection in the example is panning and zooming as I expect. The shapes and orientations are staying the same while the scale expands and contracts with the x-axis.

The problem is that the option you are looking for does not exist yet. I am not sure yet whether what you need is identical to an option I added to quiver (from which I partially derived EllipseCollection). I suspect it is.

Thanks, I now understand better what is the main goal of EllipseCollection: in a quiver-like use, you don't want indeed the plotted ellipses to change size and/or orientation with scaling and zooming.

Do you want width to scale with x and height to scale with y? And the angle to scale such that a 45-degree angle always corresponds to equal increments in x and in y, all in data units?

Yes, in the context of error-ellipses, I would like to see my ellipses scale with x and y, as they do when I plot them individually as Ellipse patches (see example scripts, the blue ellipses are what I would like to obtain using an EllipseCollection), something like "units='data'" in EllipseCollection...

I agree, this really is needed, and I will put it in ASAP. It may take a little while, though, so in the meantime you are stuck with the individual patches.

Thanks for bringing it to my attention.

Eric