[Bf-committers] Armatures Bones and Constraints Oh My!

[bf-committers@blender.org]

In whatever system that eventually replaces the current Armature and 
Constraint setup in blender I see several key things to keep in mind

Bones: 
	Bones need to store several key pieces of data. 
		1. Rest Position (currently in head tail and roll)
		2. Pose Position (currently stored in quat and duplicated in pose-
>quat)
		3. IK flag
		4. Parent Bone
		5. Child Bone List
		6. RootBone (could be calculated by following the parents up)
		7. Constraint list

	anything else is really unneccessary information. Pose position 
should be stored in the same format as the rest position. This is 
where we get our data for drawing the bone and deforming the mesh. 
The rest position is only used for going to the bones rest position. 
it should never be used for drawing the bone or deforming the mesh. 
When we return to rest position we just copy the rest position values 
to the pose position. This is the basis of a working efficient Bone 
system. Some of you may see the need for additional pieces of data. 
Please respond if you do. I'd like some feedback.
 
Constraints:
	Constraints need to be evaluated per every transform operation. I 
think they would be better served as a test function not performing 
actual operations on the data. returning a boolean value for pass or 
fail perhaps. the only hitch there is many transformations operate on 
more than one axis. an indication of which axis operation failed 
might be in order. Or perhaps transformations could be calculated on 
a per axis basis. Such a method might tie in with working transform 
handles also. Constraint's would be easier to use also in the code if 
they were linked to the object's struct. Like in the proposed bone 
structure above each object should have a list of constraints on that 
object. No more looking up the constraint in the global list.

Jeremy Wall

PS. Sorry about the title there I have something of a sense of humor 
as you'll find out.