High-res: 800x800
Mathematica code:
DensityPlot[
Sum[Cos[(Cos[n*2*Pi/13] + Sin[n*2*Pi/13])*x + (Cos[n*2*Pi/13] - Sin[n*2*Pi/13])*y + Pi],
{n, 0, 12, 1}], {x, -100, 100}, {y, -100, 100},
PlotPoints -> 100, Mesh -> False, Frame -> False, ColorFunction -> Hue, ImageSize -> 834]