Fractal noise is, channel by channel i.e. before mixing with alpha, centered around 128, Gaussian (or pink) noise using 128 as zero point.

Turbulence comes by taking numerical value of this (again 128 as zero point) and interpreting the result with zero point out in one end,  so it cannot be centered around 128. Fractal noise comes closer to, what you expected from turbulence.

The fractal noise is also known as “Perlin Noise” after Carl Perlin, who invented it in the 1980es. He has made a better version – simplex noise – later, but that was after the old Perlin noise was adopted by everybody. Simplex noise does not give the artifacts with the systematic white pattern, you can see in turbulence.

There is a description of simplex noise here: http://webstaff.itn.liu.se/~stegu/simplexnoise/simplexnoise.pdf

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From: jf barraud [mailto:jf.barraud@...400...]
Sent: 25 June, 2009 07:53
To: inkscape-devel
Subject: [Inkscape-devel] turbulence filters

Hi,

Isn't our turbulence filter producing darker output than expected? The attached svg file is borrowed from the w3c test suite, and also contains the expected png output: our render looks draker.

I bumped into this trying to use turbulence as a displacement map: the displacement always globaly shifts the image downward and to the right. This is because the avarage value in each turbulence output channel is not 128 but much darker.

I think this is a bug... (maybe premuliplied alpha versus normal alpha color mode?)

Am I wrong?
jfb.