(2019-12-17, 17:52)Travis Cobbs Wrote: [ -> ]OK, I finally think I understand. I think what we really want to do is rotate the arbitrary p1pn vector around the sphere's axis until it lies on P2, and then measure the resulting angle between p1p2 and p1pn'. Does that sound right? And if so, do you have a suggestion on the best way to describe that in the spec?

I think this is the same as what I called "longitude / latitude mapping" in earlier posts (e.g.

https://forums.ldraw.org/thread-18219-po...l#pid18221)

It chooses the dominant axle and rotate the base vector along with it when calculating the other. This way you can go all around using a single texture meta block.

(2019-12-17, 17:52)Travis Cobbs Wrote: [ -> ]OK, I finally think I understand. I think what we really want to do is rotate the arbitrary p1pn vector around the sphere's axis until it lies on P2, and then measure the resulting angle between p1p2 and p1pn'. Does that sound right? And if so, do you have a suggestion on the best way to describe that in the spec?

I would forego the definition of P2.

Instead I would define the point q as the projection point of p onto P1. Then say that V is computed as the signed angle between the vectors p1q and p1p. The angle is divided by b to normalize it to [-.5;.5] where V=0 corresponds to a vertical line through the texture right in the middle.

I also think that illustrations are important, and examples would be valuable as well. Hopefully people will find the examples provided in the forum.