As you know, simplest linear gradient will make things look like cylinders. Why this happen?
Matte surface reflects the light in all directions, but the brightness depend on the angle between light direction and surface.
For point A the angle is 90° – maximum brightness.
For point B the angle is 0° – minimum brightness.
For point C the angle is 45° – half.
For the circle, angle between light and surface is proportional to the angle between DA and DS lines (D is the centre of the cylinder and S is the point on the surface). As for this angle is changing linear, brightness will change linear too, and that’s why we have an illusion of the cylinder.