faster jpeg resizing with imagemagick

I found a page with misc imagemagick tips [1], and this one on
faster jpeg resizing looks really useful:

> The -size option tells the IM jpeg decoder that the image being read
> will be resized later.  This lets it run faster  by avoiding returning
> full-resolution images, for later resizing.

    gerald@devo:/tmp; time convert -resize 256x192 a.jpg a-sm.jpg
    real    0m16.737s
    user    0m10.460s
    sys     0m0.250s

    gerald@devo:/tmp; time convert -size 512x384 -resize 256x192 a.jpg a-sm.jpg
    real    0m2.487s
    user    0m1.490s
    sys     0m0.060s

(a.jpg is a 2048x1536 photo, 1745586 bytes. The times above are
extra slow because both CPUs are also busy doing wavs->oggs :)

Does anyone know if doing this is a bad idea for some reason?

I'd like to use this when generating thumbnails and other smaller
versions of photos; I don't see any difference at e.g. 400x300. 

That part of my photo publishing process is really time consuming
(well, just for the machine, but I have to wait around for it);
would be nice to speed it up by a factor of 5+ :)

I wonder what the difference is between -resize and -geometry; I
have always used -geometry.

the whole tip:
> Faster JPEG resizing
> 
> The -size option tells the IM jpeg decoder that the image being read
> will be resized later.  This lets it run faster  by avoiding returning
> full-resolution images, for later resizing.
> 
> For example...
>    convert -size 120x120 cockatoo.jpg -resize 120x120 \
>           +profile "*" cockatoo_thumb.gif
> 
> This may not give as nice as a result as just the -resize filter itself, but
> it is a lot faster. If you like you could also use -size to scale part way and
> -resize to finalize.  For example if you are starting with a 3000x2000 jpeg
> and want a 120x80 thumbnail, you could use "-size 480x320" then "-resize
> 120x80"
> 
> NOTE: -resize uses the filter selected with -filter while -scale uses a
> simple fast scaling method.  -resize & -filter give you more control
> but might consume more CPU time.

[1] http://www.sct.gu.edu.au/~anthony/info/graphics/imagemagick.hints

-- 
Gerald Oskoboiny <gerald@impressive.net>
http://impressive.net/people/gerald/

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