Make the pattern from composition notebooks This challenge was xnor's idea, taken from Digital Trauma's challenge donation thread.



Your challenge is to write a program that, as xnor put it, "procedurally generate[s] the splotchy black-and-white marbled pattern on the covers of composition notebooks":



Your program can either display it on the screen or save it in a file. The image should be at least 300 pixels in each dimension, and it should be different between runs of the program. Your program should not just load an external image and use that as its output; the image should be generated by your program.

This is a popularity-contest, and it will use xnor's criteria for voting:


Similarity to the reference texture above.
Simplicity of generation method.
Mathematica

Blur[Binarize[Blur[RandomImage[1, {1000, 1000}, ImageSize -&gt; Large], 6], .52]]


Quite simple. Creates a random grayscale image, blurs it, takes the pixels above a threshold, and blurs it again. It uses a couple of hand-tweaked values. The result: