Monday, August 8, 2022

Changes to the MHX rig

Recently there were some changes to how the MHX rig is controlled. 

  • Previously it was controlled by a mix of RNA and id properties (the difference is explained in more detail below), now only (API defined) RNA properties are used. Whereas you could argue which is the best choice, mixing the two types of properties was very confusing, e.g. because they are animated separately.
  • Previously MHX used armature properties, now it uses object properties. In order to use armature properties in a linked file, they had to be made both library overridable and library editable, whereas object properties only need to be library overridable. And armature RNA properties could not be keyframed at all.
  • Stretchiness is now controlled by the floating point influence rather than the boolean enable, to make animation smoother. Other control may be converted into float properties in the future for the same reason.

This may sound technical, but the changes should not alter the panels and the behaviour very much, at least after engetudouiti pointed out all the bugs that I introduced in the process. It means that MHX rigs generated with previous versions do not work. However, they can be updated to the latest version with the Update MHX button that appears in all MHX panels. Note that the update is irreversible, so if you have a scene that works with a previous version of the MHX RTS it might be better to stick to that. Another possibility, which I favour myself, is to keep a file with the original genesis character around, and regenerate the MHX rig from scratch.

RNA and id properties

An RNA property is defined by the python script when is loaded, and it is available for all data of a given type. An id property is defined dynamically at runtime, and is only defined for a single datum. An example of id properties are the morphs, which are loaded at runtime. Different characters can have different sets of morphs, although they are all armature objects. Hence morphs must be id properties.

Let us compare how the two types are accessed in Python. First RNA properties. They are defined by a statement like this (the last line makes the property accessible in a linked file).

bpy.types.Object.RNA_property = bpy.props.FloatProperty(
    name = "RNA Property",
    description = "RNA Property",
    default = 0.0,

To access an RNA property, we use getattr, setattr, or the dot notation, e.g.

    setattr(ob, "RNA_property", 0.5)
    ob.RNA_property = 0.5

Id properties don't have to be defined at all, they are just assigned and then they exist.

    ob["id_property"] = 1.0
A limitation with id properties is that they can only be floats, integers, or strings, but not booleans, enums, vectors, or collections. So instead of a boolean, we could use an integer property limited to the range [0,1]. This works in principle, but an integer is displayed as a slider rather than a checkbox. Since that is ugly, the old MHX RTS displayed the RNA property (a true boolean) rather than the id property (an integer).

But then came the next problem. Armature RNA properties don't seem to be animatable. So if you right-clicked on a float property like Gaze Follows Head (an id property since a float is natually displayed as a slider), you could set a keyframe, but if you right-clicked on a boolean property like Left Arm Stretch (an RNA property), keyframing was not available.

OK, so this was confusing? The bottom line is that by consistently using object RNA properties, all problems went away. Properties are animatable, booleans display as checkboxes, and everything works with file linking. At least after all bugs have been ironed out.