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x/mobile/exp/f32: Mat4.Rotate incorrectly rotates around axes with non-zero X values #14989

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bmatsuo opened this issue Mar 27, 2016 · 0 comments
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@bmatsuo bmatsuo commented Mar 27, 2016

I have put together the following example to demonstrate a problem with Rotate method on f32.Mat4. I attempt to rotate a vector v (edit: vector u) around the X axis. The resulting vector does not have the same length (in this extreme case the "rotation" has collapsed a unit vector to zero-length).

http://play.golang.org/p/INyRQCldPm

When I actually use the resulting transformation in OpenGL what I have observed is that my models gets flattened onto a plane (presumably the Y-Z plane, perpendicular to the X axis, at X=0).

I believe the problem lies in how the first row of the rotation matrix is constructed.

https://github.com/golang/mobile/blob/467d8559f22b81884dfbc37d1cef95d0fab78049/exp/f32/mat4.go#L157-L160

I kind of suck at this stuff. But you can see the subsequent rows in the matrix have a pattern which is not observed in the first row. So I tried changing the first row of the rotation matrix to the following.

    {
        c + d*a[0]*a[0],
        0 + d*a[0]*a[1] + s*a[2],
        0 + d*a[0]*a[2] - s*a[1],
        0,
    }

And that looks like it works.

The following demonstrates the altered Rotate function: http://play.golang.org/p/8ZrOpdv_AY

This second example confirms rotation of [1, 0, 0] around the X axis yields [1, 0, 0]. Obviously length was preserved.

@bradfitz bradfitz added this to the Unreleased milestone Apr 9, 2016
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