Add gallery example for structure_tensor... in 3D!

[mkcor]

Description

Addresses part of #4831.
With this PR, I'm following up on #4831 (comment).

@emmanuelle I tried telling a story but, in terms of applications, it's more didactic than scientific.
@GenevieveBuckley I used a small region of the kidney image!

I wanted to include some 3D rendering with Plotly, like:

but it would only spit out a JSON:

{'text/html': '<div>            <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-AMS-MML_SVG"></script><script type="text/javascript">if (window.MathJax) {MathJax.Hub.Config({SVG: {font: "STIX-Web"}});}</script>                <script type="text/javascript">window.PlotlyConfig = {MathJaxConfig: \'local\'};</script>\n        <script src="https://cdn.plot.ly/plotly-latest.min.js"></script>                <div id="de5ac8b7-d7bc-41f1-a094-bc28e8cdaa06" class="plotly-graph-div" style="height:525px; width:100%;"></div>            <script type="text/javascript">                                    window.PLOTLYENV=window.PLOTLYENV || {};                                    if (document.getElementById("de5ac8b7-d7bc-41f1-a094-bc28e8cdaa06")) {                    Plotly.newPlot(                        "de5ac8b7-d7bc-41f1-a094-bc28e8cdaa06",                        [{"opacity": 0.5, "slices": {"z": {"locations": [5], "show": true}}, "type": "volume", "value": [774, 566, 384, 333, ...

(plus it would be too heavy).

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[pep8speaks]

Hello @mkcor! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file doc/source/conf.py:

Line 199:1: E402 module level import not at top of file

Comment last updated at 2021-01-27 10:25:51 UTC

[mkcor]

@jni iirc you would recommend these changes: 1a5887b
What's the name for this way of formatting strings again? Thanks!

[jni]

Yes I would and they are f-strings

[mkcor]

@scikit-image/core please review when you get a chance!

[rfezzani]

Thank you @mkcor! I just left some minor suggestions 😉

[mkcor]

@rfezzani thank you for the thorough review!

[grlee77]

Thanks for working on this @mkcor! I have some comments, but think the general approach here is fine and this dataset should be okay to demonstrate the basic principles

[grlee77]

I think it would be nice to also plot all 3 eigenvalues for a single slice (e.g. 1 subplot per eigenvalue, using uniform brightness scale across subplots).

[grlee77]

I was suggesting a figure something like this, which shows the spatial distribution of the eigenvalues. Do you think it is helpful?

fig, axes = plt.subplots(1, 3)
vmax = eigen[0][coords[1]].max()
axes[0].imshow(eigen[0][coords[1]], vmin=0, vmax=vmax, cmap=plt.cm.viridis)
axes[0].set_title('eigenvalue 0')
axes[1].imshow(eigen[1][coords[1]], vmin=0, vmax=vmax, cmap=plt.cm.viridis)
axes[1].set_title('eigenvalue 1')
axes[2].imshow(eigen[2][coords[1]], vmin=0, vmax=vmax, cmap=plt.cm.viridis)
axes[2].set_title('eigenvalue 2')
for ax in axes:
    ax.set_axis_off()

[mkcor]

I'm aware I hadn't addressed all of your review comments -- looking now! Thanks.

[mkcor]

Yes, I think it is helpful: It makes it easier to follow the rest of the story. I've added the plot in d82d754. I couldn't resist but use facet_col with plotly's imshow... ❤️ @emmanuelle 😉

[grlee77]

Thanks @mkcor. I am approving this now, but please see one other suggestion regarding potentially showing the eigenvalue images themselves.

[rfezzani]

Thank you @mkcor 🎉.