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> Slime: His arms bend up because if they did not, his forearms would have
to
> change length, or something; try to touch your sternum with the tips of
you
> finger, with your elbow at your side --- impossible. Your elbow must be
away
> from your body to let your hands do that. This is why IK is good, it
shows
> these things without my having to specifically enumerate them.
Hmm. But for every position of the hand, there is a full circle where the
elbow can be - the set of points elbow's length from both the hand and
shoulder. What's to keep the elbow from always being at the lowest point (or
for that matter, the highest point) on this circle? (Even if you consider
the torso blocking movement, and the limitations on arm positions in real
life, there are a lot of choices for elbow position for any given hand
position.)
- Slime
[ http://www.slimeland.com/ ]
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"stm31415" <sam### [at] cs com> wrote in message
news:web.456a842f89f8419fcf1900cc0@news.povray.org...
> Slime: His arms bend up because if they did not, his forearms would have
> to
> change length, or something; try to touch your sternum with the tips of
> you
> finger, with your elbow at your side --- impossible. Your elbow must be
> away
> from your body to let your hands do that. This is why IK is good, it
> shows
> these things without my having to specifically enumerate them.
>
Actually, your elbow can remain at your side - just bend your wrist &
fingers. But if the model can't do that, yes, hand stiff means elbow must
come away from body
Sorry, as I read the post I felt like trying it out
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"Data Banks" <da_### [at] ihugcomau> wrote:
> >
> Actually, your elbow can remain at your side - just bend your wrist &
> fingers. But if the model can't do that, yes, hand stiff means elbow must
> come away from body
>
You are absolutely right. Merce Cunningham would be very upset with me.
But, as you say, he does not have wrists, so there it is.
Slime: He is at the bottom of the curve. Though that is actually what I am
currently struggling with, for that particular animation a rule was
instituted for gravity, keeping his joints in the lowest possible position
while still reaching the goal.
The IK works by feeling out the state-space --- it tryies moving each joint
in turn to see which position is closest to the goal, the best possible
position. Teh best position is defined by a function, but that function can
be undefined --- in my case it is, I use a macro that returns a number based
on the distance fo the tip of the limb to the goal, some piecewise additions
for when a joint is out of it's comfort range, and then the troublesome bit
about gravity. The iterative macro then tries to mimimize this number.
However, the trick is when parts of the limb enter negative quadrants ---
then the gravity bit, which just adds the total center of gravity of the
limb, starts *subtracting* from the final value, making the limb appear
closer to the goal than it actually is. So at the moment it is kludged by
adding the total length of my limb to the y value before finding the COG.
But that's not all that elegant. I'm posting the macros now, but it's
pretty much uncommented. I am a terrible commentor (commentator?). I forget
to. So I'll post it and then I'll re-post tomorrow (heh). I hurt my hand
today and should probably stop typing, as it hurts.
-s
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