Fisheye is an effect in which a scene looks sphered. As a result, edges in the scene look curved and bowed around the center of this virtual sphere. This effect makes the scene look like wrapped around a curved surface.
The barrel distortion technique is used to achieve the present effect, which can be applied to fragments or vertices. This recipe will implement the barrel distortion on the fragment shader first and then apply it to the vertex shader. The difference between the two is this; in the former shader, the geometry does not distort. However, texture coordinates are distorted, resulting in a magnifying lens effect or a fisheye lens effect. In the latter technique, the geometry is displaced and creates different amusing distorted shapes. Note that this is not a post processing technique.