Get list of coordinates from plot object

asked 2012-08-22 07:35:15 +0200

Anakonda
23 ●1 ●2 ●7

I'm looking for a way to get a plain list of coordinates from a plot object. For instance, after creating an implicit plot of a circle

p = implicit_plot(x^2+y^2-1,(x,-2,2),(y,-2,2))

I am able to save the image in .pdf or even .svg format (where coordinates are present), which is a very good start. What I really want, though, is a plain list of coordinates of, say, 2000 points lying on that circle. It should be possible to extract some from the .svg file, but is there a more convenient method? Maybe even one where I can set the sample size?

Thank you in advance for any suggestions.

Comments

Oh, definitely don't do this from the svg file! But I'm having a lot of trouble figuring out how to access this too. Basically, the implicit plot is a matplotlib contour subplot object - but acquiring it from p.matplotlib() seems challenging. Maybe I'm missing something obvious.

kcrisman ( 2012-08-24 17:08:20 +0200 )edit

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answered 2012-08-24 17:08:57 +0200

kcrisman
12222 ●41 ●135 ●254

updated 2012-08-24 17:13:00 +0200

I think maybe this works. I'm not an mpl expert, but at least the points correspond to the output of p.

sage: var('y')
y
sage: p = implicit_plot(x^2+y^2-1,(x,-2,2),(y,-2,2),plot_points=4)
sage: P = p.matplotlib()
sage: R = P.get_children()[1]
sage: S = R.collections[0]
sage: S.get_paths()
[Path([[-0.66666667 -0.70833333]
 [-0.70833333 -0.66666667]
 [-0.70833333  0.66666667]
 [-0.66666667  0.70833333]
 [ 0.66666667  0.70833333]
 [ 0.70833333  0.66666667]
 [ 0.70833333 -0.66666667]
 [ 0.66666667 -0.70833333]
 [-0.66666667 -0.70833333]], None)]
sage: S = R.collections[1]
sage: S.get_paths()
[Path([[-0.66666667 -0.70833333]
 [-0.70833333 -0.66666667]
 [-0.70833333  0.66666667]
 [-0.66666667  0.70833333]
 [ 0.66666667  0.70833333]
 [ 0.70833333  0.66666667]
 [ 0.70833333 -0.66666667]
 [ 0.66666667 -0.70833333]
 [-0.66666667 -0.70833333]], None)]

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Comments

Yes, this does exactly what I wanted. This is just great. I never would have guessed that descent into the depths of the matplotlib arrays. Thank you very much!

Anakonda ( 2012-08-24 21:33:25 +0200 )edit

Yeah, I don't really understand it either, but the graphic itself is NOT generated by Sage, but by mpl, so you have to just try stuff. Tab-completion rules!

kcrisman ( 2012-08-27 10:38:26 +0200 )edit

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Get list of coordinates from plot object

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