Currently, euclid and servo use points to describe vectors. It works out but it is a tad dangerous in some cases because we use the same type to express different things and also because transforming a point and transforming a vector does not involve the same computation (translations apply to points but don't apply to vectors, for instance). Euclid currently can't express transforming a vector, you have to transform two points and compute the difference, which is easy to forget when you also use the point types to store vectors).

I Propose adding TypedVector2D, TypedVector3D, types which would implement the usual arithmetic operations. Points would be changed so that:

• Points + Point is not implemented
• Point + Vector = Point
• Point - Vector = Point
• Point - Point = Vector

Places where we currently use points to express vectors (like Rect::translate) would take vectors instead. points and vectors would implement conversion helpers to_vector/to_point to avoid the burden of writing my_point - Point2D::origin() and Point2D::origin() + my_vector

Matrix transformation would be implemented separately for points (homogeneous coordinate w = 1) and vectors (homogeneous coordinate w = 0).

This would let us remove Point4D which is very awkward because it exposes the homogeneous coordinate, but we always expect it to be 1.
At a glance, Servo only uses Point4D in one place (rust-layers) where Point3D could (should) be used instead, and gecko does not use Point4D at all, so the 4d type is mostly dead weight and a source of confusion since its member w is mostly ignored.

As opposed to my other proposals to euclid, this is actually not implemented in gecko. However it has been a source of confusion for me and having distinct types for points and vectors would have avoided a few mistakes and helped make better self-documented APIs.

I have a prototype of this in my vectors branch if you would like to have a look at what the API would be like.

Thoughts?