## The basics

**origin**a point at 0,0,0**unit length**a vector that has a length of 1**normalization**scaling a vector so that it's unit length**normal**a unit vector that is used to describe a direction**magnitude**the length of a vector

## The dot product

So the dot product is an operation that looks like thisVector3 DotProduct(Vector3 a, Vector3 b) { return a.x * b.x + a.y * b.y + a.z * b.z; }Okay, sure, that's the code, but what does it

*do*?

*
The dot-product returns the distance from the origin (0,0,0)
to the projection of point B on vector A, in terms of the length of vector A.
*

*
The dot-product returns the distance from the origin (0,0,0)
to the projection of point B on vector A
*

So if the length of vector A is twice as long, then the returned dot product is half as big.

*Note that a dot product is "Commutative"*

*this which means you can swap vector A & B and you'll get the same results*

### Distance of a point along a line

So imagine having a line that is defined by a point

*lineStart*and a

**normalized**

*lineDirection*, and we want to know the distance from a point

*P*to

*lineStart*, how would we do that?

Well if we take the dot product of

*lineDirection*and point

*P*, we have the distance to the origin, which is not what we want. However, if we both move the point

*P*and

*lineStart*so that

*lineStart*is at the origin, we

**can**use the dot-product.

We can simply do this by subtracting

*lineStart*from both points

The final equation then becomeslineStart - lineStart= 0,0,0 (the origin)P - lineStart= pointPat the samerelativedistance tolineStart

distanceAlongLine = DotProduct((P-lineStart),lineDirection)

## The plane equation

Another important tool in your vector algebra arsenal is the plane equation:Ax+By+Cz = Dor

Ax+By+Cz-D = 0As you can see, a plane is stored as 4 floating point values (ABCD).

The ABC part is the normal, which signifies in what direction the plane is facing.

The D part is the (negative) distance of the plane to the origin, along that normal.

Since a plane is infinite in every direction except the direction of the normal, you can define any 2D plane in 3D space this way.

So this makes the distance of a point to a plane:

signedDistanceToPlane = DotProduct(planeNormal, point) + planeDistanceNote that the distance is signed, meaning that you can tell if a point is in front or behind the plane by checking it's sign.

If you need a point that lies on a plane, you can use:

pointOnPlane = planeNormal * -planeDistanceAnd in reverse, if you have a normal for a plane you can find the planeDistance, the D part of the plane equation, with:

planeDistance = -DotProduct(planeNormal, pointOnPlane)Note that this works with

*any*point that lies on the plane.

If you have 3 points A,B & C, you can calculate the normal by:

normal = Normalize(CrossProduct(B - A, C - A))Now imagine these 3 points all lying on the same plane, then B-A and C-A are 2 vectors that also lie on this plane. The cross product takes these 2 vectors and returns a vector that is perpendicular to both of them, which would be a vector pointing away from the plane.

*Normalize*scales it to unit length, giving us the plane normal.

Note that if these 3 points lie on the same line, it would result in a zero length normal.

If you want to calculate the normal of a polygon, don't just take the first 3 points and try to create a normal out of them, since they might be aligned even though subsequent points may not be.

For polygons it's better to use Newell's algorithm, this algorithm will work with most polygons as long as they're:

- not self-intersecting,
- planar (all points lie on the same plane)
- not degenerate (all points lie on the same line or form a singularity)

## Next time

I'll write some more posts in the future about- the relationship between the dot product and matrix algebra
- ray-plane intersections
- snapping, how you'd snap a point on an arbitrary grid in 3D, or along a line
- half-edges, fat planes, and how you'd split a polygon using both

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