Fix Matrix2::look_at, add look_at_stable

[blargg]

Changes

1. Fixes Matrix2::look_at
2. Adds tests for look_at
3. Adds a new function, look_at_stable

Notes

I added a new function for 2d look at rotation. look_at is a bit weird in practice for 2d. For example, if you are making a basis matrix to orient a 2d character to look at a point, look_at will flip the character as they rotate past up or -up vectors. This is the best match for what look_at is supposed to do, I think.

look_at_stable will not flip based on orientation, you just pass in which way to flip. This is a bit easier to use to rotate 2d characters.

look_at_stable could have a better name. I think we can also consider removing the flip param, and just let the user flip the matrix with a transform later.

[kvark]

We don't need the normalization for "flip" flag computation

On Sep 3, 2019, at 16:00, blargg notifications@github.com wrote:

@blargg commented on this pull request.

In src/matrix.rs:

 pub fn look_at(dir: Vector2<S>, up: Vector2<S>) -> Matrix2<S> {

• //TODO: verify look_at 2D
• Matrix2::from_cols(up, dir).transpose()
• let basis1 = dir.normalize();
• let basis2 = if up.x basis1.y >= up.y basis1.x { Sounds good, it's way better to call into the same function for the common part. I think the only concern with that was that it would call noramlize on an already normalized vector, we would first normalize in look_at and then again when it calls look_at_stable.

Would you prefer if I Option 1: made a private function that is like look_at_stable, but assumes that the vector is normalized, that each of these call into, or Option 2: ignored the duplicate work, and call look_at_stable from look_at as you describe.

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[blargg]

Oh, thanks! I had missed the part of the initial comment. Added commit to the PR.

[blargg]

Sounds good, just updated.

[kvark]

bors r+

[bors[bot]]

Build succeeded

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