[Bf-taskforce25] PyConstraints V.2 - Proposal + Unresolved Issues

[Joshua Leung]

Hi all,

With the new style of coding py-extensions for Blender 2.5 coming in (with
the new PyAPI), it's about time that we had a look at refreshing the way
PyConstraints are coded.

Firstly, an theoretical example of how a new PyConstraint could look like...

class MyPyConstraint (bpy.types.pyconstraint):
    __name__ = "Dummy Test";
    __description__= "A silly constraint that makes use of and showcases
several features that would need to be used.";
    __num_targets__ = 1;

    # evaluate the constraint for the given matrix and target matrices
    def doConstraint (self, eval_matrix, target_matrices):
         # get location parts
         loc= eval_matrix.translationPart();
         locT= target_matrices[0].translationPart();
         # and other parts of matrix
         obrot = obmatrix.toEuler()            # Rotation
         obsca = obmatrix.scalePart()        # Scale

         # snap to grid, then add target location on top + a custom factor
         for i in xrange(3):
            loc[i]= floor(loc[i]+0.5) + locT[i] + self.factor;

         # Convert back into a matrix for loc, scale, rotation
         mtxloc = Mathutils.TranslationMatrix(loc)
         mtxrot = obrot.toMatrix().resize4x4()
         mtxsca = Mathutils.Matrix([obsca[0],0,0,0], [0,obsca[1],0,0],
[0,0,obsca[2],0], [0,0,0,1])

         # Recombine the separate elements into a transform matrix.
         outputmatrix = mtxsca * mtxrot * mtxloc;
         return outputmatrix;

    # manipulate matrix of target before giving it to the doConstraint()
callback
    doTarget(self, target_object, subtarget_bone, target_matrix):
          return target_matrix;  # for this constraint, do nothing

    # draw custom settings for constraint as part of constraints panel
    def drawSettings (self, layout):
          # add a single button for this custom factor
          layout.itemR(self, "factor");


-----------------------

As can be seen, it takes into account several of the new PyAPI style issues:
- subclassing RNA-defined base types   (bpy.types.pyconstraint)
- layout engine is used to include the buttons defined in drawSettings() at
the end of panel for instances of the constraint
- name and num-targets can be defined in the way above, or by some other
means (details to be confirmed)
- custom properties can be directly accessed from the constraint instance
like regular properties (due to RNA)

However, I think it is important to still maintain a few other
characteristics
- constraint instances still must reference text buffers/datablocks instead
of some magical global list that is maintained at startup... The main
benefit here is that in productions where multiple versions of the
constraints may be used, you don't need to register all the pyconstraints
for every machine that you work on, leading to problems with version
control...
- keeping the text buffers, we can also easily edit the scripts and test
without having to click on too many 'refresh' buttons

The example above is incomplete in some ways though. Notably, there needs to
be some way of checking for (and initialising) missing but required custom
properties - probably a initSettings() call that checks if a setting exists
already (creating a new prop if non-existant) that gets called at certain
points? Getting this feature working is currently dependent on having
bpy/RNA methods available for doing this (TODO #1)
Also, the way of specifying the number of targets required indicated above
may not work that well in practice?

Technical issues
- supposing that we compiled scripts containing such code, how would we go
about executing the constraints that are defined in this way? How are
changes such as number of targets going to cause changes?


Anybody have any comments on the proposed PyConstraints 2? (More
importantly, would be answers to the technical issues raised)

Regards,
Joshua
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