crossproduct

Sep 27, 2007 at 9:45am

crossproduct

Just a quick question to all the mathbuffs out there…

I want to get the crossproduct of 2 vectors, and for this I set up 2
matrices going into jit.expr with the following expression :

expr “(in[1].p[2]*in[2].p[3])-(in[1].p[3]*in[2].p[2])” “(in[1].p[3]*in
[2].p[1])-(in[1].p[1]-in[2].p[3])” “(in[1].p[1]*in[2].p[2])-(in[1].p
[2]*in[1].p[3])”

I was wondering if this is correct, and maybe if there’s a much more
simple expression or object for this…?

grtz.

Brecht Debackere

http://www.autofasurer.net
M+32(0)498319302
F+32(0)70429559
Skype : Bdebackere
brecht@visualantics.net

#33856
Sep 27, 2007 at 10:56am

Hi Brecht,

I’m not sure but maybe a shader can help. Parts of the shaders derivatives
are don with matrix calculations.
It’s only an idea. Maybe a bit more complicated. But matrices calculations
are much faster on gpu than on cpu.

grr.
Ranja

2007/9/27, Brecht Debackere :
>
> Just a quick question to all the mathbuffs out there…
> I want to get the crossproduct of 2 vectors, and for this I set up 2
> matrices going into jit.expr with the following expression :
>
> expr “(in[1].p[2]*in[2].p[3])-(in[1].p[3]*in[2].p[2])”
> “(in[1].p[3]*in[2].p[1])-(in[1].p[1]-in[2].p[3])”
> “(in[1].p[1]*in[2].p[2])-(in[1].p[2]*in[1].p[3])”
>
> I was wondering if this is correct, and maybe if there’s a much more
> simple expression or object for this…?
>
> grtz.
> Brecht Debackere
>
> http://www.autofasurer.net
> M+32(0)498319302
> F+32(0)70429559
> Skype : Bdebackere
> brecht@visualantics.net
>
>
>
>
>
>

#113313
Sep 27, 2007 at 11:50am

The idea is to create quad_strip ribbon my filling up a matrix, with
the width of the ribbon being calculated using the crossproduct using
previous positions :

vector 1 being :
current position – previous position

vector 2 :
previous position – the position before that

and then calculating the crossproduct of those 2 vectors, normalizing
it, and then some stuff with adding the current position to the
crossproduct and using that value for the even vertices of the
quad_strip. (I think it sounds clearer in my head, but it is for
doing something like this : http://www.youtube.com/watch?v=fM1qj_OFY4w )

On 27 Sep 2007, at 12:56, Ranja Achbar wrote:

> Hi Brecht,
>
> I’m not sure but maybe a shader can help. Parts of the shaders
> derivatives are don with matrix calculations.
> It’s only an idea. Maybe a bit more complicated. But matrices
> calculations are much faster on gpu than on cpu.
>
> grr.
> Ranja
>
> 2007/9/27, Brecht Debackere :
> Just a quick question to all the mathbuffs out there…
>
> I want to get the crossproduct of 2 vectors, and for this I set up
> 2 matrices going into jit.expr with the following expression :
>
> expr “(in[1].p[2]*in[2].p[3])-(in[1].p[3]*in[2].p[2])” “(in[1].p[3]
> *in[2].p[1])-(in[1].p[1]-in[2].p[3])” “(in[1].p[1]*in[2].p[2])-(in
> [1].p[2]*in[1].p[3])”
>
> I was wondering if this is correct, and maybe if there’s a much
> more simple expression or object for this…?
>
> grtz.
>
> Brecht Debackere
>
> http://www.autofasurer.net
> M+32(0)498319302
> F+32(0)70429559
> Skype : Bdebackere
> brecht@visualantics.net
>
>
>
>
>
>

Brecht Debackere

http://www.autofasurer.net
M+32(0)498319302
F+32(0)70429559
Skype : Bdebackere
brecht@visualantics.net

#113314
Sep 27, 2007 at 5:50pm

Xray.jit.crossproduct maybe can help you.
http://www.mat.ucsb.edu/~whsmith/

#113315
Sep 28, 2007 at 10:04am

wonderfull. this does exatly what I want, and normalizes as well :)
thx.

On 27 Sep 2007, at 19:50, jasmin wrote:

>
> Xray.jit.crossproduct maybe can help you.
> http://www.mat.ucsb.edu/~whsmith/
> –
> jaz

Brecht Debackere

http://www.autofasurer.net
M+32(0)498319302
F+32(0)70429559
Skype : Bdebackere
brecht@visualantics.net

#113316

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