1. Explanation
A fisheye camera is similar to a perspective camera, except for the fact that instead of having a rectangular canvas, you paint on a spherical one.
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We can choose how wide the fisheye camera is both horizontally and vertically. For example, on the figure above, the fisheye camera spans an angle of 180° both horizontally and vertically. Having a 180° horizontal angle means that you can see what’s left and right of you. A horizontal angle larger than 180° allows you to see behind you. Consider the scene below:
We now move the camera to just above the white sphere and let it point towards the red sphere. The movie below shows what happens if we gradually enlarge the horizontal angle from 90° to 360°
Notice how the cyan sphere comes into view towards the end, at which point the canvas forms a complete sphere.