[magick-users] Re: How to properly downsize an image

[John J Foerch]

T o n g <mlist4suntong at yahoo.com> writes:
> Hi, 
>
> What's the proper way to downsize an image? 
>
> I want to downsize my 1152x864 image to 500x, so I did:
>
>  convert -resize 500x screenshot.png screenshot-thumbnail.png
>
> I was thinking that the downsized thumbnail should be around 1/4 of the
> original size, but I am totally wrong:
>
>  $ identify screenshot*
>  screenshot-thumbnail.png PNG 500x375 500x375+0+0 DirectClass 509kb 
>  screenshot.png[1] PNG 1152x864 1152x864+0+0 DirectClass 82kb 
>
>  $ ls -l screenshot*
>  -rw-r--r-- 1 tong tong 521280 03-29 16:54 screenshot-thumbnail.png
>  -rw-r--r-- 1 tong tong  83876 2007-01-01  screenshot.png
>
> The size is not smaller but over 6 times bigger. 
>
> I remember that I need to something extra, so I tried the following, but
> that didn't help. The thumbnail is the same size as before:
>
>  convert -size 500x375 -resize 500x screenshot.png screenshot-thumbnail.png
>
> Please help. 
>
> Thanks

You are probably using an imagemagick compiled with quantum depth 16,
so images you produce have 16 bits per channel.  `-depth 8' will
produce 8 bit images, reducing file size.  The arg order in your
convert commands is also wrong.. see
http://imagemagick.org/Usage/basics/

John Foerch