Maya Bug Watch: API2 and GetPoints()

In general I’m more or less a fan of Maya Python API 2.0. It’s more pythonic and feels faster than the old version. However, it’s not without its quirks and I just found one that really bit me in the behind.
If you want to get the vertices of an object in the api, the usual formula is:

1. Get the dagPath of the object
2. make an MFnMesh
3. call the ‘GetPoints’ method of your mesh
4. party on.

Something like this, which returns a list of MPoint objects for the verts in the mesh

import maya.api.OpenMaya as api

def get_verts(mesh):
    mobj = api.MGlobal.getSelectionListByName(mesh).getDagPath(0)
    # that's lazy, it assumes that the first child is the mesh shape.
    # in practice you need to be more careful...
    mfn_mesh =  api.MFnMesh(mobj)
    vert_array = mfn_mesh.getPoints()
    return [i for i in vert_array]

This works fine and dandy… except:
If the mesh has 256 or more verts, the first vertex comes back as garbage
Here’s an example, using the same function:

mesh, _ = cmds.polyCube(sw = 1, sh= 1, sd = 1)
print get_verts(mesh)[:4]
#> [maya.api.OpenMaya.MPoint(-0.50000000000009082, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(0.5, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.5, 0.5, 0.5, 1), maya.api.OpenMaya.MPoint(0.5, 0.5, 0.5, 1)]

# this looks good... Here's the same thing for a 226 vert cube:
mesh, _ = cmds.polyCube(sw = 8, sh= 8, sd = 3)
print get_verts(mesh)[:4]
#> [maya.api.OpenMaya.MPoint(-0.50000000000005185, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.375, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.25, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.125, -0.5, 0.5, 1)]

# but up the vert count to 258:
mesh, _ = cmds.polyCube(sw = 8, sh= 8, sd = 4)
print get_verts(mesh)[:4]
#> [maya.api.OpenMaya.MPoint(5.0277956463997711e-315, 5.0313386899592279e-315, 0.5, 1), maya.api.OpenMaya.MPoint(-0.375, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.25, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.125, -0.5, 0.5, 1)]

# that first point is gibberish: python can't go to the -315th power!

I’ll leave it to wiser heads to figure out why it works out like this. My guess is that something is borked in pointer math going on inside the wrapper around MfnMesh, but I don’t know. Luckily, there’s a workaround: if you create new MPoints out of the items coming back from the GetPoints() call, you get good data. I’m not sure why but this should be so but it appears to be reliable on my machine (Windows 7, 64 bit maya 2015). Here’s the workaround:

def safe_get_verts(mesh):
    mobj = api.MGlobal.getSelectionListByName(mesh).getDagPath(0)
    mfn_mesh =  api.MFnMesh(mobj)
    vert_array = mfn_mesh.getPoints()
    return [api.MPoint(i) for i in vert_array]  # creating new MPoints fixes the issue

mesh, _ = cmds.polyCube(sw = 10, sh= 10, sd = 10)
print safe_get_verts(mesh)[:4]
#> [maya.api.OpenMaya.MPoint(-0.5, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.40000000596046448, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.30000001192092896, -0.5, 0.5, 1), maya.api.OpenMaya.MPoint(-0.20000001788139343, -0.5, 0.5, 1)]