It generates the image calculating a math expression for every pixel and every component of the color (red, green or blue), the math expression uses this variables:
$X:horizontal position from -100 to 100.
$Y:vertical position from -100 to 100
$R:Radius of the position from the center of the image:is sqrt($X^2+$Y^2)
$A:Angle of the position from the center of the image in radians
The math expression is an expression that can use any math function: sin(Angle), cos, tan, asin, acos, atan(in radians), pow(base,exponent), log(value),log10, abs(value), sinh(value), cosh, tanh, floor, round,rand() (returning a value from 0 to 1), fmod, ceil.