strange artifacts in rendering M-set

[saijanai]

http://s1174.photobucket.com/albums/r605/saijanai/?albumview=slideshow

the pictures are radically different even with the same algorithm, same iterations, etc.

taken from c code at:   http://rosettacode.org/wiki/Mandelbrot_set#PPM_non_interactive

Ideas for why this might be happening?  

[hobold]

The rounded square shape of the outermost contour looks as though your bailout test is comparing (Zx^4 + Zy^4) rather than (Zx^2 + Zy^2). Maybe you accidentally squared the Zx2 and Zy2 variables twice?

[saijanai]

The rounded square shape of the outermost contour looks as though your bailout test is comparing (Zx^4 + Zy^4) rather than (Zx^2 + Zy^2). Maybe you accidentally squared the Zx2 and Zy2 variables twice?

That was indeed it.

I was calling mpfr_mul()  twice on the same variable.

Thanks