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
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