The image, produced with Ultra fractal, is a two-dimensional slice
of the four-dimensional parameter space of cubic polynomials written
in the form p(z) = z^3 - 3a^2 z + b. More information about this subject
can be found here.

The image is a detail of the slice when:

a_real = 0.4
a_imag = pixel (horizontal)
b_real = 0
b_imag = pixel (vertical, the axis divides the image in two halves)

Two layers are used, in one z is initialised to the first critical
point +a and in the other z is initialised to the second critical point
-a. No filter or post-processing except anti-aliasing is used.