error in docs for Sigmoidal Non-linearity Contrast

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jcupitt
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error in docs for Sigmoidal Non-linearity Contrast

Post by jcupitt » 2016-10-27T05:30:22-07:00

Hi, there's a small error in the equation given here:

http://www.imagemagick.org/Usage/color_mods/#sigmoidal

It reads:

( 1/(1+exp(β*(α-u))) - 1/(1+exp(β)) ) / ( 1/(1+exp(β*(α-1))) - 1/(1+exp(β*α)) )

It should be:

( 1/(1+exp(β*(α-u))) - 1/(1+exp(β*α)) ) / ( 1/(1+exp(β*(α-1))) - 1/(1+exp(β*α)) )

ie. there's a missing alpha in the second term.

You can find discussion of this error in various places, for example the second comment on:

http://php.net/manual/en/imagick.sigmoi ... timage.php

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