GoPro Virtual Lens

Grab the image with your mouse to reframe the image.
Within the shader code above is the math to simulate GoPro lens look within 360° source.  The same math is very useful for "correcting" lens distortion.  The shader code is licensed under MIT or Apache 2.0. 
I would love someone in the comments to convert the lens radial distortion equation phi = 85.6r - 10.91r² to a OpenCV lens model. GoPro uses n-order radial distortion polynomial in the form: angle° = Ar + Br² + Cr³ where 'r' is the normalized sensor radius. OpenCV uses something more common, but I'm too lazy to learn, so I seek your help. To get OpenCV model started, here is the lens model in odd powers phi = 83.90432r - 18.26919r³ + 15.51541r⁵ - 6.51824r⁷ .

Know Your Fisheyes

When you say a GoPro is a fisheye lens, which one do you mean?  GoPro is less "fisheye" than you likely thought, somewhere between stereographic and a rectilinear (non-fisheye, pin-pole style lens -- the one people say is non-distorted.)