Re: multiplying matrices into tensors?

[James Cloos]

>>>>> "JGH" == Jordi Gutiérrez Hermoso <address@hidden> writes:

JGH> I'm not sure if I understand, but I imagine you want something like
JGH> this:

JGH>      c = bsxfun(@times, permute(a,[3,2,1]), permute(b,[2,3,1]))

I have to confirm each element, but, based on the above, it looks like

 c = bsxfun(@times, permute(a,[1,2,3]), permute(b,[1,3,2]))

might have been the solution, but testing reminded me that the
multiplication needs to be done a bit differently:

 c(1,u,v) = a(1,u) * b(4,v) + b(1,v) * a(4,u)
 c(2,u,v) = a(2,u) * b(4,v) + b(2,v) * a(4,u)
 c(3,u,v) = a(3,u) * b(4,v) + b(3,v) * a(4,u)
 c(4,u,v) = a(4,u) * b(4,v)

u loops over the 2nd dim of a and v over the 2nd dim of b.

I presume I can still use bsxfun() and just need to write a function
which takes a pair of column vectors representing homogeneous coordinates
and returns their product by way of the above algo, yes?

Said multiplication perhaps should be followed by something akin to
what c /= gcd(c) looks like it might do, if that syntax were legal.  

Is their an efficient way of doing that?

-JimC
-- 
James Cloos <address@hidden>         OpenPGP: 1024D/ED7DAEA6