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Thread: Define: Projection

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    Define: Projection

    What is a simple/intuitive way to explain a projection.
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  2. #2
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    Quote Originally Posted by Noxide View Post
    What is a simple/intuitive way to explain a projection.
    You mean "orthogonal projection"? If we suppose "a" is a nonzero vector in $\displaystyle R^n$ and "x" happens to be any vector in $\displaystyle R^n$, then the orthogonal projection of x onto span{a} is written as $\displaystyle proj_a x$ and is defined using this formula: $\displaystyle proj_a x = \frac{x.a}{\| a \|^2}a$
    The projection vector can be called "vector component of x along a".

    Edit: If you consider the operator $\displaystyle T: R^2 \rightarrow R^2$ that "projects" any vector $\displaystyle x \in R^2$ onto a line through the origin, by dropping a perpenicular to that line, it's called an orthogonal projection. This is the definition of projections onto lines through the origin of $\displaystyle R^2$.
    Last edited by Roam; Jan 16th 2010 at 07:28 PM.
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    Since you say "intuitive", imagine a light shining on a vector from far away. The shadow of the vector on a plane or line would be its "projection" on the plane or line. That is where the term "projection" is from.

    Also, when you separate a three-dimensional vector into x, y, and z components, those are its projections on to the x, y, and z unit vectors, respectively. Since you can always choose a basis containing any one (non-zero) vector, you can think of the projection of vector u on vector v as one "component" of u in a basis containing v.
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