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Topic: Generalizing Orthogonal Projection
Replies: 7   Last Post: Jan 24, 2013 5:17 PM

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Posts: 289
Registered: 5/23/11
Re: Generalizing Orthogonal Projection
Posted: Jan 22, 2013 3:21 AM
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22.1.2013 7:47, wrote:
> If L is a 1-D subspace of R^3, and q=(x,y,z) is a point not on L. Can we
> define the orthogonal projection of q into L, or do we need to have a plane
> P (as subspace) , to define an ortho projection of q?

Let p, d, q in R^n, where p is a point on the line L, d is the direction
vector of L, and q is the point that is to be projected onto L.
Parametrize L by

f : R --> R^n: f(t) = p + td.

We want to find a t' such that

dot(f(t') - q, d) = 0,

where dot stands for an inner product. Using bilinearity,

dot(p + t'd - q, d) = 0
t' dot(d, d) + dot(p - q, d) = 0
t' = dot(q - p, d) / dot(d, d).

Therefore the orthogonal projection of q onto L is given by

f(t') = p + t'd.


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