Added design ramblings.

doc/design.md

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+## Graph
+
+### 1. Generalized data connections / Support of math nodes (or non-AnimNodes in general)
+
+Use case:
+
+* Enables animators to add custom math for blend inputs or to adjust other inputs (e.g. LookAt or IK
+  targets).
+
+Effects on the graph topology?
+
+* Need to generalize Output Sockets to different types instead of only "Animation Data".
+* How to evaluate? Two types of subgraphs:
+   a) Instant inputs (needed for blend node inputs) that have to be evaluated before
+   UpdateConnections. Maybe restrict to data that is not animation data dependent?
+   b) Processing nodes, e.g. for extracted bones.
+
+### 2. Support of multiple output sockets
+
+Use case:
+
+* E.g. extract Bone transform and use in pose modifying nodes.
+* Chain IK.
+
+Effects on graph topology?
+
+* Increases Node complexity:
+    * AnimOutput
+    * AnimOutput + Data
+    * Data
+
+(Data = bool, float, vec3, quat or ...)
+
+**Open Issues**
+
+1. Unclear when this is actually needed. Using more specific nodes that perform the desired logic
+   may be better (
+   c.f. https://dev.epicgames.com/documentation/en-us/unreal-engine/animation-blueprint-bone-driven-controller-in-unreal-engine).
+   Likely this is not crucial so should be avoided for now.
+
+### 3. Multi-skeleton evaluation
+
+Use case: riding on a horse, interaction between two characters.
+
+### 4. Output re-use
+
+Description:
+
+Output of a single node may be used as input of two or more other nodes.
+
+Use case:
+
+* (Related to 1.) an input that does some computation gets reused in two separate subtrees.
+
+### 5. Inputs into Subgraphs
+
+Description:
+
+An embedded blend tree can receive inputs of the surrounding blend tree. Inputs are animations or - depending on 1. - also general input values.
+
+Use case:
+
+* Reuse of some blend logic or to move logic of a part of a blend tree into its own node.
+
+Effects on graph topology:
+
+* Great flexibility and possibly reusability.
+* Improves logical block building.
+* Probably only of bigger use with 1.
+
+Open issues:
+
+* Inputs to embedded state machines?