Understanding how jME3 handles character models
+(This note is largely based on analysis of Stephen Gold’s Maud).
+Dem Bones
+One potential way to vary character models is to adjust the length of their bones. In order to be able to do this, I need to get hold of them. This note documents how to get hold of them in the jMonkeyEngine 3 API (referenced as ‘jME3’).
+Model files (including, but not limited to, .j3o files) are loaded by the AssetManager as instances of Spatial. A spatial can be any sort of scene node – a building, a tree, a rock, a container. It is not inherently a character and does not inherently have a skeleton or a rig; furthermore, the abstract class Spatial has no methods to extract a skeleton or a rig – or even, actually, a geometry, material or texture.
Maud loads models as instances of maud.model.cgm.LoadedCgm, which wraps the Spatial.
Instance variable declaration:
+ /**
+ * root spatial in the MVC model's copy of the C-G model
+ */
+ protected Spatial rootSpatial = null;
+ this.rootSpatial = Heart.deepCopy(cgmRoot);
+LoadedCgm is a subclass of Cgm, where ‘CGM’ is stated in the documentation to be an acronym for ‘Computer Graphics Model’.
The Cgm class has an instance variable selectedSkeleton which is instantiated at the time the Cgm instance is constructed to a new, empty, instance of SelectedSkeleton. So how does the SelectedSkeleton instance (which is instantiated before the rootSpatial is set) get to know about the skeleton from the Spatial, which has, inherently, no skeleton? The answer is that is calls the countBones() method of the selectedSkeleton:
public int countBones() {
+ int result = 0;
+ Object selected = find();
+ if (selected instanceof Armature) {
+ result = ((Armature) selected).getJointCount();
+ } else if (selected instanceof Skeleton) {
+ result = ((Skeleton) selected).getBoneCount();
+ }
+
+ assert result >= 0 : result;
+ return result;
+ }
+Part of the complexity here is backwards compatibility. The class Armature is a newer replacement for the older (and now deprecated) class Skeleton; this appears to be part of a major re-engineering of how jME3 handles animation.
and countBones() calls find():
/**
+ * Find the selected Armature or Skeleton.
+ *
+ * @return the pre-existing instance, or null if none
+ */
+ Object find() {
+ Object result = find(null);
+ return result;
+ }
+which in turn calls find(binary[]):
/**
+ * Find the selected Armature or Skeleton.
+ *
+ * @param storeSelectedSgcFlag if not null, set the first element to true if
+ * the skeleton came from the selected S-G control or its controlled
+ * spatial, false if it came from the C-G model root
+ * @return a pre-existing Armature or Skeleton, or null if none selected
+ */
+ Object find(boolean[] storeSelectedSgcFlag) {
+ boolean selectedSgcFlag;
+ Object skeleton = null;
+ /*
+ * If the selected S-G control is an AnimControl, SkeletonControl,
+ * or SkinningControl, use its skeleton, if it has one.
+ */
+ Control selectedSgc = cgm.getSgc().get();
+ if (selectedSgc instanceof AnimControl) {
+ skeleton = ((AnimControl) selectedSgc).getSkeleton();
+ }
+ if (skeleton == null && selectedSgc instanceof SkeletonControl) {
+ skeleton = ((SkeletonControl) selectedSgc).getSkeleton();
+ }
+ if (skeleton == null && selectedSgc instanceof SkinningControl) {
+ skeleton = ((SkinningControl) selectedSgc).getArmature();
+ }
+ ...
+And so on.
+I’m going to confess here that coming back to object oriented programming after a decade of concentrating on functional programming, it’s frustrating how complicated, messy and repetitious it is. But in this instance I can’t help feeling that it would have been less messy if the abstract class CGM had a method getSkeleton() which by default returned null; and which was overridden in subclasses of CGM which represented things which did have skeletons to return their skeletons.
But this only kicks the ‘how to get the skeleton’ one step further down the road, to Control. Control also wraps a Spatial, which also isn’t instantiated at construction time, and also doesn’t have a getSkeleton() method.
To be fair I don’t know what proportion of subclasses of Control have skeletons, but on the evidence here at least four do; and an overridable instance method on AbstractControl returning null, declared on the Control interface, would have little cost and save a lot of mess.
As there are now a lot of branches to cover, I’m going to concentrate on the SkinningControl one, which seems to be the current state of the art. I haven’t at this stage investigated how AssetManager.loadModel(String) determines which classes to instantiate when loading a model, but I’m going to assume that I can coerce my models to be loaded in a non-deprecated form.
A SkinningControl has a private instance variable armature:
/**
+ * The armature of the model.
+ */
+ private Armature armature;
+which is instantiated in the constructor:
+ /**
+ * Creates an armature control. The list of targets will be acquired
+ * automatically when the control is attached to a node.
+ *
+ * @param armature the armature
+ */
+ public SkinningControl(Armature armature) {
+ if (armature == null) {
+ throw new IllegalArgumentException("armature cannot be null");
+ }
+ this.armature = armature;
+ this.numberOfJointsParam = new MatParamOverride(VarType.Int, "NumberOfBones", null);
+ this.jointMatricesParam = new MatParamOverride(VarType.Matrix4Array, "BoneMatrices", null);
+ }
+the Armature class has an instance method getJointCount(); it also has instance methods getJointList(), getJoint(int), and getJoint(String).
Note that Joint seems to be a name used in the rewrite of the animation system to avoid confusion with the Bone in the earlier animation system, and I think represents what normal animation rig nomenclature would refer to as a bone.
However, the Joint class has no length instance variable. What it has is a targetGeometry instance variable which is declared as a Geometry. Geometry, in turn, has no dimensions, but has an instance variable mesh declared as a Mesh. I suspect that it is the Mesh object – which contains lines, triangles and vertices – which provides the actual dimensioned objects.
I don’t think, however, that I either need to or should change anything in the Geometry objects themselves. Instead, the Joint object also has three instance variables bound to instances of Transform:
-
+
localTransform;
+initialTransform;
+jointModelTransform;
+
Each of these is private, and has a getter but no setter. It is my hypothesis that to alter the length of the bone, one should make its localTransform IV a scaling transform, by calling its setScale(float) method. There are alternate signatures to this method, one taking three floats, one for each coordinate, and the other taking an instance of Vector3f; it may be that one of these would be preferable because for my purposes I’m only interested in varing the length.
Skin
+In order to change the overall skin colour of a character, we have to modify the Material or the Texture of the skin;
Material has a method setColor(String, ColorRGBA) which should do the trick; and every Geometry has a Material. The Geometry class is a subclass of Spatial, so if the Spatial returned by AssetManager.loadNodel(String) is an instance of Geometry, which I believe it will be, we’re probably golden. If not, the Cgm class has a mechanism for getting the Texture, but not the Material, of the Spatial, and I’ll have to explore that route.
Broadly, I think that the material of the skin should be acted on by the genome (to set overall colour), while the texture may be acted on by some mechanism for handling acquired characteristics (to handle scars, amputations, tattoos, etc).
+Eyes, hair
+I need to be able to set eye colour, eyebrow shape and colour, hair colour, and baldness from the genome (modulated by age). The hair style should not be set from the genome but from acquired characteristics. I’m assuming that hair dyes are not a thing in the in-game culture, otherwise hair colour would also have to be in acquired characteristics.
+