Objects with alpha (lines or shapes with an alpha fill or stroke, images
with alpha, all fonts) are displayed in OpenGL (and P3D, in fact) based on
their drawing order. This creates some annoying effects like making things
opaque if they're drawn out of order with objects above or beneath them.
This is simply how OpenGL works, but is something that we'd like to develop
a workaround for in the future. Currently, you should just try to draw
things back-to-front as best as possible.

http://processing.org/discourse/yabb_beta/YaBB.cgi?board=Programs;action=display;num=1115222332

edit]
sketch that shows the problem

*** Bug 302 has been marked as a duplicate of this bug. ***

implemented basic support via hint(ENABLE_DEPTH_SORT) for revision 114. it won't work for
many cases because it just z-sorts the triangles based on the average z of its vertices. it also
doesn't help lines, which have the same trouble.

the proper fix will be considerably more work:
- implement newell's algorithm for painter sort (have code for this from a project years ago..
just need to adapt in a way that'll be fast enough)
- needs to take into account splitting triangles as necessary
- need to merge line and triangle rendering, because they need to be sorted relative to one
another (ugh). this makes for a nasty structural change. without it, it's pretty bad.

in some cases, disabling depth testing (hint(DISABLE_DEPTH_TEST)) will also
make textures work properly in either direction, however you'll obviously
lose depth.

*** Bug 1115 has been marked as a duplicate of this bug. ***