Understanding Orthogonal Projection: Formula and Definition Explained

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Can anyone tell me what Orthagonal Projection means. I know the formula is b - proj b onto a.

What does it mean exactly, I tried searching on google.

 

[LCKurtz]

Given a vector V and another vector W, the idea is to write V as the sum of a vector parallel to W and a vector perpendicular to W. Draw V and W with their tails together and drop a perpendicular from the head of W to the line of vector W. Put appropriate heads on the legs of the little right triangle that forms and you see the two vectors. The perpendicular one is the orthogonal projection.