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hm I m not gonna switch languages because of a math
function now?
thats like deciding to switch to japanese because
you feel there's some characters missing in our alphabet. (yeah its a flawed
comparison)
now I did work it out with the acos, and I ll be
needing this,
since I wanna do this on particles, and they
dont have a transform matrix.
btw it ran MUCH faster that way, but there can be
some other things that are influencing it.
right now I have one version, with the matrix
method, to place nulls on points,
and another version using the arccos method,
to place particles on points, the second being more performant.
however, it seems that the formula is flawed,
I get 'division by zero' errors.
and its quite obvious: if X=1 then (x*x + 1) gives
zero.
Arccos(X) = Atn(-X / Sqr(-X * X + 1)) + 2 *
Atn(1)
I'll have to try and find a better arccos
formula somewhere..
,
Glad
you got it sorted - microsoft's acos solution truly is elegant ;-) A google
seems to provide strange results - some places mention a math object in
VBScript though doesn't seem to work for me.
If your other solution is
working then I'd go with that - at least until you shift
languages
Cheers,
Alan.
On 9/5/05, peterb
<peter_b(at)skynet.be> wrote:
Hi Alan, seems like your answer was on right on
the spot.
but on kims suggestion I found back the thread
on xsibase:
I got it working with Rafaelle's method,
that doesnt involve dot product, nor inverse cosine.
just cross product, normalize and putting it
into a matrix that goes straight to an object's transform.
on the bottom of the page there is a method by
Andrea, that is pretty much exactly what you
suggested.
this uses the inverse cosine of the dot
product and so on:
...
var angle = Math.acos(upVec.dot(polyNormal));
...
cant find one in vbscript
though.
what I did come up with is
where inverse cosine is made up like
this:
Arccos(X) = Atn(-X / Sqr(-X * X +
1)) + 2 * Atn(1)
microsoft helps to keep things simple
:-)
if anyone knows of an inverse cosine function
in vbscript, I'd love to know.
peter
Excuse my laziness I didn't real through all your code so I've no
idea how you're tackling this, but here's a simple approach I'd recommend
anyway (I'll explain the math rather than coding it out.)
Take the
direction you want to be "up" on your object (probably the y axis I guess
so 0,1,0). Get the cross product of that with the normal you want it to
rotate to. The result is the axis you want to rotate your object around.
To get the angle you want to rotate your object get the dot product of the
up axis and the normal. Then get the inverse cosine of this and that will
be the angle in radians you want to rotate it. The only problem you'll
probably come into this way is defining how it rotates around the normal.
You may want to look at doing this with a direction vector which could be
global or using the U and V directions or
whatever.
Cheers,
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