Hi, I'm comparing the results my fractal program produces against Fractint's. In general I get the same results, but I'm getting dramatically different images for some fractals involving the acos function. Using the formula: mandelfn { z = 0: z = fn1(z*z) + pixel |z| < 4.0 } With fn1 = acos and default parameters, Fractint produces : http://www.speakeasy.org/~edyoung/fract001.gif And I get: http://www.speakeasy.org/~edyoung/acos_image.png I think this stems from different functions we use to compute acos: Fractint uses rz=Arccos(z)=-i*Log{z+sqrt(z*z-1)} Whereas my code uses: acos(z) = pi/2 - asin(z) According to MathWorld, ( see http://functions.wolfram.com/ElementaryFunctions/ArcCos/27/02/01/), the formula Fractint uses is only valid for certain domains, so I'm suspicious that Fractint's producing the wrong output. Any comments? Regards, -- Edwin