XRRay.direction should have a w of 1.
Manishearth opened this issue · comments
The direction attribute defines the ray’s 3-dimensional directional vector. The direction's w attribute MUST be 0.0 and the vector MUST be normalized to have a length of 1.0.
This doesn't make sense, if it's a vector the w should be 1, not 0, homogeneous space vectors with a w of 0 are meaningless.
The constructor test will also need to be updated
We could also return a type error if the w is not 1, or normalize to 1
Homogenoeous space vectors with w of 0 represent points at infinity, which can be viewed as representing a direction - it is exactly what we need here. Transforming those vectors by 4x4 matrices does not apply translation parts to them, which is the expected behavior for a direction.
Hmmm, I think talking about the length of this vector is incorrect, then, this cannot be "normalized" to have a length of 1 since it has a length of infinity, but I get what's being said. Perhaps we should change it to say "the first three components normalized to have a length of 1.0"?
closing, it's not super important to make that clarification but i might open a PR later.
(...) since it has a length of infinity (...)
I'd be careful with transferring concepts from 3D Euclidean space to projective space, I do not have good intuitions on what happens there (what's a length? distance from the origin? how is distance defined in projective space? what is an origin in projective space?). The way I explain the normalization algorithm to myself is that given a point (x, y, z, 0) we can import it to Euclidean space as a vector (representing direction b/c it was a point at infinity), calculate its length L in a familiar way where stuff makes sense, and then export it back to projective space as (x/L, y/L, z/L, 0).
In general, I'd prefer this spec to stay less math-focused (as long as it's clear what needs to happen) - it may be a good idea to open issues on the Geometry Interfaces Module spec if there are concepts that require clarification or more theory that backs up the math is needed.
My experiences are that computer science usage of the terminology of homogeneous spaces is highly inconsistent and it's better to be specific.