[magick-users] "-channel A -fx intensity" using -evaluate?

[David Vanderson]

Anthony Thyssen wrote:
> David Vanderson on  wrote...
> | Hello,
> | 
> | A few weeks ago we were doing cloud composition, which is close to 
> | something that I've been doing - setting the alpha channel of each pixel 
> | to the intensity of each pixel.  Anthony asked why we were using -fx, 
> | which is slow, instead of -evaluate.
> | 
> | I confess that I've tried and failed a few times to figure out how to do 
> | this with -evaluate.  Can anyone enlighten me?
> | 
> | Thanks,
> | Dave
>
> Note   -channel A  -fx  intensity
> was being used to mapp a greyscale to the alpha channel.
>
>
> Try this instead.
>
>  convert  image.png  -colorspace gray \
>           -background white -channel A -combine   ouptut.png
>   
This does exactly what I want.  Thank you very much.

Dave