"saturation" with -fx don't work?

[bio]

Hi everyone

I'm tring extracting from a jpeg image like this (in.jpg)



only the pixels that have a NOT GRAY color.

My idea is producing a black&white mask like this (mask.png)



and go to this final result (out.png)

.


So... I know that gray pixels have R=G=B or saturation=0.

The first step I tried was:

convert in.jpg -fx "saturation>0" mask.png

BUT I receive an error:

convert: unable to parse expression 'saturation'

Is this a bug or the "saturation" expression don't exist anymore?

I'm running imagemagick 6.2.9 Q8 on Windows XP.
Thanks everyone can help me.

[anthony]

saturation was the very last addition to the -fx operator. You have to be at the very latest BETA version for it to work.

[bio]

Thank you anthony!