Scalar Projection & Vector Projection
Refer to the note in Pre Linear algebra
about understanding Dot product.
Assume that the vector w
projects onto the vector v
.
Notation:
- Scalar projection:
Componentᵥw
, read as "Component ofw
ontov
". - Vector projection:
Projectionᵥw
, read as "Projection ofw
ontov
".
Notice that: When you read it, it’s in a reverse order! Very important!
Projection Formula
Note that, the formula concerns of these concepts as prerequisites:
- Dot product calculation
- Dot product cosine formula
- Unit vector
How to calculate the Scalar Projection
The name is just the same with the names mentioned above:
boosting
.
Componentᵥw = (dot product of v & w) / (w's length)
Refer to lecture by Imperial College London: Projection
Refer also to Khan academy: Intro to Projections
What if we know the vectors, and we want to know how much is the Scalar projection
(the shadow)?
Example:
How we’re gonna solve this is: We know the vectors, so we can get their dot product
easily by taking their linear combination; and we know the length of each vector, by using Pythagorean theorem; and then we get the projection, as in the picture.
How to calculate the Vector Projection
It’s another idea for projection, and less intuitive.
Remember that a Scalar projection
is the vector's LENGTH projected on another vector. And when we add the DIRECTION onto the LENGTH, it became a vector, which lies on another vector. Then it makes it a Vector projection
.
It can be understood as this formula:
Projectionᵥw = (Componentᵥw) * (Unit vector of v)
But usually we write it as this:
Refer also to video for formula by Kate Penner: Vector Projection Equations
Refer to video by Firefly Lectures: Vector Projections — Example 1
Example:
-