[Bf-committers] NZ & Game Engine Physics

[Jonathan Merritt]

Hi Kester,

I'll see if I can make it to Wellington - I'll keep in touch closer to 
the date.

I should take a good look at the game engine physics code sometime.  
Just out of interest, how easy do you think it would be to incorporate a 
specific mechanical model into the rest of the physics system?  Is there 
a good level of abstraction / modularization that might allow this?  I'm 
interested mostly because it would be interesting to be able to 
incorporate a specific mechanical sub-system into the more general 
system.  An example of a specific sub-system might be a driven kinematic 
chain representing a musculoskeletal system (or a robot, for that matter 
:-).

For my PhD, I'm working on resolving muscle forces in the equine 
forelimb.  The basic stages are as follows:
    1. Measure motion of the horse (kinematics), and ground reaction forces.
    2. Apply inverse dynamics to find the moments applied to each joint 
of a rigid-body model of the limb.
    3. Measure muscle architecture parameters and anatomical origins and 
insertions.
    4. Use static optimization to solve for muscle coordination 
according to various criteria (eg: minization of the sum of muscle 
stresses).
    5. Solve for joint reaction forces.
    6. For selected bones, predict strains induced during locomotion, 
and measure these strains using gauges implanted in-vivo for comparison 
and validation.

Step 4 is required because the horse (like virtually every mammal I can 
think of - and I've seen inside quite a few!) has more muscles in its 
forelimb (esp. in the shoulder) than are actually necessary to achieve 
any specific motion.  To find out how the force is distributed among the 
various muscles, it's necessary to introduce this extra 
physiologically-based (ie: non-mechanical) condition.

So, none of my PhD directly involves so-called "forward simulation", 
like the game engine is trying to achieve.  However, I think it would be 
extremely cool to be able to take the results of my work and drive a 
model of the horse's limb, with different muscles perhaps hilighted in 
different colours depending on the force they are applying.  However, 
there's no way that a simple engine like the one you describe could 
possibly be used drive the model accurately - I think the numerical 
errors would make the exercise quite pointless.  So, I'd require some 
way of being able to incorporate an extra basic element (the 
musculoskeletal model) that can perform its own internal calculations, 
but also interact with the remainder of the system somehow.

I've been thinking about trying to achieve most of the above using 
Python within the main part of Blender, but it would just be interesting 
to see if it could be done in the game engine.  It doesn't add anything 
to my PhD (I have the original kinematic data afterall, so why do I need 
to drive the limb to see how it moves?! :-), but I'm interested in the 
exercise itself.

Jonathan Merritt.


>I haven't got any plans to go down south, but I can meet up if you're coming 
>up to Wellington at all.
>
>The physics response is still quite simple:
>
>1. Integrate forces
>2. Integrate momentum
>3. Collision response
>4. Contact Response
>
>The integration is all Euler and the collision response is impulse based.  The 
>contact response simply moves intersecting objects that shouldn't.
>
>Kester
>
>On Friday 26 November 2004 15:32, j.merritt at pgrad.unimelb.edu.au wrote:
>  
>
>>Hello Kester,
>>
>>Just a quick email to find out if you're going to be on the South Island of
>>NZ any time during December?  I'm going to be at various places
>>down there from the 12th to the 31st, and wondered if you'd like to meet
>>up sometime so that I can congratulate you on how much work you're
>>doing in the game engine? :-)
>>
>>I'm a qualified Mechanical Engineer (BE/BSc) from Melbourne Uni, and
>>am currently doing a PhD in biomechanics.  Most of my work has very
>>different requirements to the physics simulation in the game engine,
>>but I'd gladly help out if you have any queries that you think might be
>>relevant.
>>
>>Jonathan Merritt.
>>
>>btw - Do you know if power plugs have the same geometry in NZ as
>>they do in Australia?  It only just occurred to me to wonder about that.
>>:-] _______________________________________________
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>
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