Again, if you’re just tuning in to this thread on XROMM, please refer to my previous posts on what XROMM is, why I am excited about using it, and what I have done so far, including 3-D animating a chewing pig.
The past few days have seen us busy and social. We’ve now learned to do scientific rotoscoping, or the art of properly aligning three-dimensional, anatomically-accurate models of vertebrates with two X-ray movies shot from different angles. I have previously described the basic steps involved in making these XROMM movies and coordinating and calibrating the X-ray movies, so I refer you to my previous post on this.
Unlike the method of XROMM I described previously that uses bone markers to synch the moving images and the bones, in scientific rotoscoping you manually align bones frame by frame with x-ray movies. It can be tedious at times, but essentially it is, to quote Steve Gatesy, quoting Ken Dial, fitting a digital key (the bones) into a visual lock (the X-ray movies). In other words, you are posing the skeleton in three-dimensional coordinates based on the constraints imposed by X-ray movies. Put simply, if your digital skeleton deviates from the reality of the X-ray movies, you are doing something wrong.
What is perhaps most important to emphasize in all of this is that because you are fitting bones into a virtual reality space of what happened when the animal was filmed, you can now recover data about skeletal and joint movements. That’s right: you can actually retrieve reliable, repeatably scientific data on gait and movement from these 3-D animations. For example, you assign joint markers and then measure how much the mouth of a pig opens, or how much rotation is happening in a bird knee, or even how parts of a fish skull move in relation to the skeleton and muscles.
So, you are not just making a nice skeleton movie — you are recovering what would normally be unrecoverable data. The sort of data that allows you to more objectively describe what happens in living vertebrates, and hopefully, data that can be used as a baseline constraint for limiting what fossil vertebrates may or may not have been capable of.
All in all, this has been a great experience. I wish to thank Beth Brainerd, Steve Gatesy, David Baier, Ariel Camp, Sabine Moritz, and Erika Giblin for all their help, information, and assistance this week. It has been a pleasure.