error in docs for Sigmoidal Non-linearity Contrast
Posted: 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
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