[matplotlib-devel] MEP: _zdraworder attribute for Collections

[Benjamin R. <ben...@ou...>]

Digging through mplot3d (again), I have come to realize that a lot of its
code in art3d.py could be simplified if we had a way to tell collection
objects in what order to draw their elements.

My proposal is fairly straight-forward. All collections would have an
internal _zdraworder attribute that can be anything that can index a numpy
array: slice(), array of indices, whatever. The draw() methods will then
iterate over their elements returned by indexing with _zdraworder. This
will help keep the bookkeeping to a minimum, because everything should be
sliced/indexed the same way: offsets, verts, facecolors, etc.

The default value will be slice(None), so nothing will change for regular
collections.

With this, mplot3d can greatly simplify its logic and semantics by focusing
on projecting and setting the _zdraworder by running np.argsort() on the
projected depth of the elements. Another advantage is that methods like
get_facecolors() and set_facecolors() can round-trip in mplot3d (right now,
get_facecolors() has to return a z-sorted version of the colors for
drawing).

For now, I imagine keeping this a private attribute, but I could see some
really fancy tricks in the future such as using boolean indexing to mark
some elements as visible/invisible, and doing some neat tricks for
animations.

Thoughts?
Ben Root