Tuesday, January 29, 2013

Obscure Blender Features: Skeleton Sketching

While looking into a bug report this afternoon, I came across a feature which I've rarely used or heard of being used which some of you may find interesting.

The Skeleton Sketching (or "Etch-A-Ton" tool, as it used to be called) allows you to draw your armatures using a combination of freehand (click-drag) and straight-line-segment ("click, move, click" - a la 'Poly' mode in Grease Pencil) strokes.

These strokes can then be converted to actual bones, with various options to control how the strokes are interpreted (e.g. how many segments should be created, or where segments are placed along the strokes).

There are also gesture-based (use Shift-LMB) tools for editing the strokes (in addition to traditional select+edit operators). For example, these include ways to reverse the direction of the strokes and to edit the strokes that are present.

Also, apparently these tools are supposed to work with the volume snapping options to ensure that the joints end up getting placed in the middle of the meshes/volumes they're being built to deform. In my tests so far, this does not appear to be working, though your mileage may vary on this (NOTE: I've never actually gotten this volume snapping stuff to actually work correctly, so that may be a part of the problem here...)


For more information, check out the wiki pages on this, which for once are actually surprisingly complete (more or less - a few of the retargetting options were still undocumented when I checked). The same though can't be said for many of the other animation related pages, which are not only generally incomplete (as in, mostly anything useful/interesting/valid is not there) or have some hideous amalgam of 2.4x and early 2.5 screenshots. Anyways, here are the links:
Disclaimer: the retargetting page was actually blank - the content there now I've just copied verbatim over from the 2.4 docs. By and large though, it should actually still be relevant today, though from what I've heard, Bassam's system sounds like it might actually be more powerful and flexible than this. In any case, this may be useful info to someone...


These tools were created by Martin Poirier (aka theeth, aka Transform System guy) as part of (or was it an outgrowth of) his university thesis work on auto-generating armatures using Reeb graphs or so.


Quick Start Guide
Here's a brief guide to getting started with this tool, based on a few minutes of playing around with it today:

1) Enable NKEY region, and scroll right down to the bottom to find the "Skeleton Sketching" panel.
2) Enable the checkbox in the header here to enable Skeleton Sketching.
WARNING: disable this checkbox as soon as you're done. Otherwise you may be left wondering why various tools won't work correctly.
3) Start drawing strokes using LMB:
  • Click-drag to draw freehand (i.e. smooth strokes)
  • Click-move-click to place stroke endpoints to get straight segments (i.e. "Poly" mode in Grease Pencil terms)
4) RMB to end a stroke
5) Shift-LMB and drag to initiate gestures
  • Single-cut across stroke to manually place segment/joint positions
  • V-cut across stroke to delete stroke
  • ... Continue with a random squiggle-scrub out to cancel gesture if you make a mistake
6) "Convert to Bones" to convert sketches.
  • Play around with the stroke conversion settings before doing so though. In particular, pay attention to where the green dots end up, as those are going to be your new joints.
  • Enable the "Quick" option to skip this step. Bones should be created once you end the stroke (RMB)
7) Disable the checkbox in the Skeleton Sketching panel header to get back to normal.

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