Animating a Guitar Strap

This tutorial describes how to use Poser's Cloth Room to handle a tricky animation issue- how to have a flexible object 'attached' to an animated one without tedious hand-adjusting of posed elements. Making a Simulated Guitar Strap
Animating a musical performance presents a number of challenges. One issue when a particular instrument has a strap- like a guitar- is to accurately simulate the behavior of a cloth item attached to two separate, independent objects. The strap must follow the movement of the instrument, conform to the figure, and look realistic in between.
This is an excellent place to use dynamic cloth, in particular its ability to use choreographed groups. A choreographed group is a piece of the cloth item which is not controlled entirely by the simulation, rather it's animated explicitly, like other props. This tutorial will describe one way of using choreographed groups on a simulated cloth strap to get a realistic, properly-behaving guitar strap.
1. Setting the Scene
We'll start by bringing in a figure and an instrument. For this example I've chosen the , and I've created a bass guitar prop from one of the instruments in the , also on CP.
I've posed the figure in the initial pose, and parented the guitar prop to his left hand. I could go ahead and animate him now, or wait until I bring in the strap, either way. The next step in any case is to set up the initial position and scaling for the strap.
2. Where's The Strap?
There are two possibilities here- if a conforming strap is available, I could bring it in from the Library and pose it properly in frame #1, such that it's touching the proper parts of the guitar and is properly positioned around the character's neck- for our purposes it's best if it's not actually touching the neck, but it should be close. The next step in this case is to export the whole posed strap as a single Wavefront OBJ file, then import it back in as a prop- which will be simulated shortly.
When exporting any conforming item with multiple 'body parts', it's a very good idea to use the Weld Body Part Seams option so that the item doesn't fall apart into its components when you simulate it.
The other option, if I don't have an existing strap, would be to build one in a modeling application like Shade. To start off, I'd save the Poser file and bring it into Shade using PoserFusion-
Now I can build the strap around the figure. I'll start by drawing an Open Line, along which I'll then sweep a rectangle. The line would look like this-
- and I'd position the rectangle at one end, then select the Open Line and click the Memorize button on the Tool palette, then select the rectangle and click the Sweep button to sweep it along the memorized Open Line.
Sweeping the rectangle along the line gives me a pretty good strap- it could use a bit of cleanup, which I can do by modifying its curves and line segments such that the strap doesn't go 'through' the figure at any point (this is important to make the simulation work properly.)
After fine-tuning the strap and converting it to a polygon mesh, I'm ready to export it as an OBJ file; I'll then save the Shade file (just in case I need to go back and adjust anything) and quit Shade, and return to Poser.
3. Time To Simulate
Here's where, whether I exported a conformed strap from Poser or a custom-built strap from Shade, I get to import the strap. I'll scale it and position it such that it's touching the guitar but is only close to the surface of the figure. I'll then parent the strap to the guitar- this is important as the choreographed cloth group I'll create should follow the guitar's movements, while the simulated part of the fabric interacts with the figure playing the guitar.
With the strap in position and parented, it's time to go to the Cloth Room. I'll set up a new simulation, clothifying the strap, and set the strap to collide with the proper parts of my Caveman figure.
The next step is to click the Choreographed Group button and use the Grouping Tool to select a small number of points at either end of the strap, which will be added to the new _choreographed_ group. The points included in the choreographed group will highlight in red; if the strap has 'hardware' on the ends (metal buckles, etc.) I'd be certain to add those so they'll hold their shape rather than being 'clothified'.
This group, since it's parented to the guitar, will 'stick' to the guitar and follow its movements; the rest of the strap will follow as best it can, given that it's also interacting with the head, neck and shoulders of the figure playing the guitar (which we selected as objects against which the strap will collide, in Figure 8 above.)
Running the simulation (by clicking the Calculate Simulation button in the Cloth Room) gives the results shown below- I might want to go back and adjust the cloth parameters, in particular raising the Shear Resistance and perhaps the Fold Resistance, so that the strap does a better job of keeping its shape when I run the simulation again- but the strap definitely does what it's supposed to here-
Note- although some experimentation may be necessary to find the proper cloth settings, in general it's best to run the simulation as the last step before rendering, to ensure that all animated elements are in place and behaving properly so that the simulation will be as accurate as possible.
4. Render Time!
Applying a texture (and in this case some strand-based hair from the Hair Room) to my caveman's strap lets me render this still image- you could also use this same technique to set up a complete animated sequence. Rock on!