Drawing Error Ellipses

Hi,

I need to draw error ellipses on a scatterplot. I'm guessing someone has
done this before.

I've found some examples, such as this one
http://matplotlib.sourceforge.net/examples/pylab_examples/ellipse_rotated.html

That led to the artist tutorial, and... ARGH! INFORMATION OVERFLOW!

Can someone explain to me, why I suddenly have to know so much about
matplotlib's internals to get an ellipse drawn?

···

--
Eero Nevalainen
System Architect
Indagon Ltd.

Hi,

I just made a function to draw uncertainty ellipses defined by a
covariance matrix P:

def plotEllipse(pos,P,edge,face):
    U, s , Vh = svd(P)
    orient = math.atan2(U[1,0],U[0,0])
    ellipsePlot = Ellipse(xy=pos, width=math.sqrt(s[0]),
height=math.sqrt(s[1]), angle=orient,facecolor=face, edgecolor=edge)
    ax = gca()
    ax.add_patch(ellipsePlot);
    show()
    return ellipsePlot

To use it: ellipsePlot=plotEllipse([x,y],P,'black','0.3')

Hope this helps,

Tinne

···

On Wed, Oct 28, 2009 at 9:06 AM, Eero Nevalainen <eero.nevalainen@...2844...> wrote:

Hi,

I need to draw error ellipses on a scatterplot. I'm guessing someone has
done this before.

I've found some examples, such as this one
http://matplotlib.sourceforge.net/examples/pylab_examples/ellipse_rotated.html

That led to the artist tutorial, and... ARGH! INFORMATION OVERFLOW!

Can someone explain to me, why I suddenly have to know so much about
matplotlib's internals to get an ellipse drawn?

I still discoverd some problems with my plotEllipse function:
1) the angle in the ellipsePlot expects and angle in DEGREES and not
in radians apparently
2) forgot a factor 2 for the width and height (it's the entire width
not the `radius`)
3) removed the show() command which sometimes behaves strange (having
to close the figure before continuing plotting)

So a new trial:

def plotEllipse(pos,P,edge,face):
    U, s , Vh = svd(P)
    orient = math.atan2(U[1,0],U[0,0])*180/pi
    ellipsePlot = Ellipse(xy=pos, width=2.0*math.sqrt(s[0]),
height=2.0*math.sqrt(s[1]), angle=orient,facecolor=face,
edgecolor=edge)
    ax = gca()
    ax.add_patch(ellipsePlot);
    return ellipsePlot;

Good luck,

Tinne

···

On Wed, Oct 28, 2009 at 9:55 AM, Tinne De Laet <tinne.delaet@...2824...> wrote:

On Wed, Oct 28, 2009 at 9:06 AM, Eero Nevalainen > <eero.nevalainen@...2844...> wrote:

Hi,

I need to draw error ellipses on a scatterplot. I'm guessing someone has
done this before.

I've found some examples, such as this one
http://matplotlib.sourceforge.net/examples/pylab_examples/ellipse_rotated.html

That led to the artist tutorial, and... ARGH! INFORMATION OVERFLOW!

Can someone explain to me, why I suddenly have to know so much about
matplotlib's internals to get an ellipse drawn?

Hi,

I just made a function to draw uncertainty ellipses defined by a
covariance matrix P:

def plotEllipse(pos,P,edge,face):
U, s , Vh = svd(P)
orient = math.atan2(U[1,0],U[0,0])
ellipsePlot = Ellipse(xy=pos, width=math.sqrt(s[0]),
height=math.sqrt(s[1]), angle=orient,facecolor=face, edgecolor=edge)
ax = gca()
ax.add_patch(ellipsePlot);
show()
return ellipsePlot

To use it: ellipsePlot=plotEllipse([x,y],P,'black','0.3')

Hope this helps,

Thanks, and yes it looks better now :slight_smile:

Tinne De Laet wrote:

I still discoverd some problems with my plotEllipse function:
1) the angle in the ellipsePlot expects and angle in DEGREES and not
in radians apparently

so it seems

2) forgot a factor 2 for the width and height (it's the entire width
not the `radius`)

I'd even say that this is a documentation bug in the Ellipse class.
Too bad that they are multiplying by 0.5 inside their code :stuck_out_tongue:

···

--
Eero Nevalainen
System Architect
Indagon Ltd.

Eero Nevalainen wrote:

2) forgot a factor 2 for the width and height (it's the entire width
not the `radius`)
    
I'd even say that this is a documentation bug in the Ellipse class.
Too bad that they are multiplying by 0.5 inside their code :stuck_out_tongue:
  

Well, it's not a good idea to change the existing behavior now, but we can improve the documentation. What would you suggest? Would you prefer to see the word "diameter" in there explicitly somehow?

It currently says:

        *width*
              length of horizontal axis

        *height*
              length of vertical axis

Mike

···

--
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA

Michael Droettboom wrote:

Eero Nevalainen wrote:

2) forgot a factor 2 for the width and height (it's the entire width
not the `radius`)
    

I'd even say that this is a documentation bug in the Ellipse class.
Too bad that they are multiplying by 0.5 inside their code :stuck_out_tongue:
  

Well, it's not a good idea to change the existing behavior now, but we can improve the documentation. What would you suggest? Would you prefer to see the word "diameter" in there explicitly somehow?

It currently says:

        *width*
              length of horizontal axis

        *height*
              length of vertical axis

OK, here are some proposals:

1.
length (2a) of horizontal axis
length (2b) of vertical axis

2.
diameter of horizontal axis
diameter of vertical axis

3.
length (diameter) of horizontal axis
length (diameter) of vertical axis

4.
length (2r) of horizontal axis
length (2r) of vertical axis

I like number one the most.

···

--
Eero Nevalainen
System Architect
Indagon Ltd.

We've had several users come to the same (incorrect) conclusion so I'd have to say it's not a rare occurrence for those comments to be misunderstood. Perhaps adding "total" in front of length would help.

width- The total width of the ellipse

···

-----Original Message-----
From: Michael Droettboom [mailto:mdroe@…86…]
Sent: Wednesday, October 28, 2009 6:59 AM
To: Eero Nevalainen
Cc: matplotlib-users@lists.sourceforge.net
Subject: Re: [Matplotlib-users] Drawing Error Ellipses

Eero Nevalainen wrote:
>> 2) forgot a factor 2 for the width and height (it's the entire width
>> not the `radius`)
>>
>
> I'd even say that this is a documentation bug in the Ellipse class.
> Too bad that they are multiplying by 0.5 inside their code :stuck_out_tongue:
>
Well, it's not a good idea to change the existing behavior now, but we
can improve the documentation. What would you suggest? Would you
prefer to see the word "diameter" in there explicitly somehow?

It currently says:

        *width*
              length of horizontal axis

        *height*
              length of vertical axis

Mike

--
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA

-----------------------------------------------------------------------
-------
Come build with us! The BlackBerry(R) Developer Conference in SF, CA
is the only developer event you need to attend this year. Jumpstart
your
developing skills, take BlackBerry mobile applications to market and
stay
ahead of the curve. Join us from November 9 - 12, 2009. Register now!
http://p.sf.net/sfu/devconference
_______________________________________________
Matplotlib-users mailing list
Matplotlib-users@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/matplotlib-users

I believe the documentation is just fine, but maybe the choices made
for the ellipses parameters can be improved.
I however I don't believe it is a very good idea to use non standard
units like degrees .... You force your users to convert their output
of mathematical calculations to non-standard units before being able
to plot.
I also think that the usage of radius in the circle patch is not
consistent with using the length of the full horizontal axis of the
ellipse patch ....

Tinne

···

On Wed, Oct 28, 2009 at 2:59 PM, Michael Droettboom <mdroe@...86...> wrote:

Eero Nevalainen wrote:

2) forgot a factor 2 for the width and height (it's the entire width
not the `radius`)

I'd even say that this is a documentation bug in the Ellipse class.
Too bad that they are multiplying by 0.5 inside their code :stuck_out_tongue:

Well, it's not a good idea to change the existing behavior now, but we
can improve the documentation. What would you suggest? Would you
prefer to see the word "diameter" in there explicitly somehow?

It currently says:

   \*width\*
         length of horizontal axis

   \*height\*
         length of vertical axis