Incorrect color mapping using categories=True for non-linear categories

[user27182]

Describe the bug, what's wrong, and what you expected.

When plotting categorical data with a non-linear or non-sequential integer mapping, the categories are not colored correctly.

Edit: For context, this bug means that the use of categories=True should probably be avoided until this is fixed, unless it's certain that the categories in the data being plotted are sequential, contiguous, 0-indexed integers. E.g any examples in the docs which currently use the download_frog_tissue or download_whole_body_ct and plot categorical=True are likely showing the segmentation labels incorrectly.

Steps to reproduce the bug.

E.g. Plot mesh with three categories for cell data: [0, 2, 8]

import pyvista as pv
import numpy as np
sphere = pv.Sphere()
sphere['labels'] = np.zeros(sphere.n_cells)      # category: '0'
sphere['labels'][range(sphere.n_cells//2)] = 2   # category: '2'
sphere['labels'][range(sphere.n_cells//4)] = 8   # category: '8'
sphere.plot(cmap='glasbey', categories=True)

Will produce this, which shows 3 colors in the scalar bar for 3 categories, but visually only colors the mesh with two.

Setting categories=False, we can see the "correct" coloring for the sphere (with non-categorical scalar bar). I would have expected this to as the coloring with categories=True, however.

I suspect the issue has something to do with this line:

pyvista/pyvista/plotting/lookup_table.py

Line 762 in 3accbf0

values = cmap(np.linspace(0, 1, n_values)) * 255

which applies a linear mapping for the colors. In this case, the apply_cmap function has limited information about the data, as it only receives an n_values parameter. To resolve this issue, I think apply_cmap needs to know the actual unique values in the data so that it can create a non-linear mapping based on the actual data values, rather than just the number of values in the data.

For reference, the value of n_values which is passed to apply_cmap, is set here when categories=True:

pyvista/pyvista/plotting/plotter.py

Lines 4269 to 4270 in 3accbf0

	if categories is True:
	n_colors = len(np.unique(scalars))

EDIT: A non-linear mapping applied on its own may result in abnormally sized color widths in the scalar bar, e.g. the scalar bar might look like this for the example above [0 0 0 0 0 0 0 0 7 8] with the 0 color being disproportionally wide. In this case, perhaps it's better to keep the linear mapping, but only set each category to a single color value, and set all other values in the lookup table to NaN and use SetNanColor to format the colors for categories not present in range. E.g., format the color bar as [0 nan nan nan nan nan nan 7 8]

System Information

pyvista 0.44.dev0

Screenshots

No response