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Ranging the Z axis

asked 2021-01-19 15:36:01 +0200

Lars gravatar image

updated 2021-01-19 18:05:32 +0200

slelievre gravatar image

I am plotting arrangements of hyperplanes. Like this one:

H3.<x, y, z> = HyperplaneArrangements(QQ)
A = H3([(1, 2, 1), 0],
       [(-4, -3, -2), 0],
       [(3, 3, -2), 0],
       [(1, -4, 3), 0],
       [(-2, 2, 1), 0])
A.plot(ranges=[(-1, 1), (-1, 1)], aspect_ratio=(1, 1, 0.25))

Hyperplane arrangement

The problem is I need the z axis to range from -1 to 1 rather than from -4 to 4.

Is there a way to change that?

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answered 2021-01-19 21:11:58 +0200

slelievre gravatar image

Setting zmin and zmax to get a chosen z-range

Missing functionality

Inspecting the plot method of hyperplanes arrangement reveals this is not implemented in Sage yet.

Searching Ask Sage and sage-support, we find that many 3d plotting functions similarly lack the ability to limit the z-range. Providing this missing functionality is now tracked at

Workaround

Some of the questions on this theme were answered with a suggestion to use implicit_plot3d. For hyperplanes, this works very well.

We can therefore mimic the plot method of hyperplane arrangements and cook up a function which, if given only an x-range and a y-range, will simply call the plot method (which adapts the z-range to the full range of z values corresponding to the x-range and y-range), but if additionally given a z-range, will use implicit plots to limit the z-range.

Here is such a function:

def ha_plot(A, ranges, **opt):
    r"""
    Return a plot of this hyperplane arrangement.

    INPUT:

    - ``A`` -- a hyperplane arrangement

    - ``ranges`` -- a list consisting of an x-range, a y-range,
      and optionally a z-range
    """
    from colorsys import hsv_to_rgb
    if len(ranges) == 2:
        return A.plot(ranges=ranges, **opt)
    elif len(ranges) != 3:
        raise ValueError("ranges should consist in two or three tuples")
    hh = [h.coefficients() for h in A.hyperplanes()]
    N = len(hh)
    HSV_tuples = [(i*1.0/N, 0.8, 0.9) for i in range(N)]
    cols = [hsv_to_rgb(*x) for x in HSV_tuples]
    ff = lambda a, b, c, d: lambda x, y, z: d + a*x + b*y + c*z
    ip = lambda f, col: implicit_plot3d(f, *ranges, color=col, **opt)
    hp = lambda a, b, c, d, col: ip(ff(a, b, c, d), col)
    return sum(hp(a, b, c, d, col) for (d, a, b, c), col in zip(hh, cols))

Usage is as follows.

One can provide only an x-range and a y-range:

sage: ha_plot(A, [(-1, 1), (-1, 1)]).show(aspect_ratio=(1, 1, 0.25))

and get the exact same picture as in the question.

Or one can additionally provide a z-range:

sage: ha_plot(A, [(-1, 1), (-1, 1), (-1, 1)]).show(aspect_ratio=1)

Hyperplane arrangement with chosen z-range

Optional extra arguments are passed to implicit_plot3d.

This for instance allows to set the opacity:

sage: ha_plot(A, [(-1, 1), (-1, 1), (-1, 1)], opacity=0.5).show(aspect_ratio=1)

The function could be improved to accept a choice of colours. Currently it uses the same default as the plot method of hyperplane arrangements.

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Comments

That is awesome thank you!

Lars gravatar imageLars ( 2021-02-05 22:21:45 +0200 )edit

If the answer solves your problem, accept it by clicking the check mark near the top left of the answer.

This will mark the question as solved.

slelievre gravatar imageslelievre ( 2021-02-05 23:30:04 +0200 )edit

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Asked: 2021-01-19 15:36:01 +0200

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Last updated: Feb 05