[Digikam-users] Exif information lost when resizing photo's?

[Jean-François Rabasse]

On Sun, 4 Nov 2012, sjoerd wrote:

> Thanks for the hint in the direction of Imagemagick...converting from the
> commandline (convert -resize 800x600 file.jpg smallfile.jpg) gave the same
> result, so I looked into the installed imagemagick packages and found out that
> there was only a package "graphicsmagick-imagemagick-compat" (kubuntu)
> installed and NOT the imagemagick itself. Still gave the convert tool though.
> Anyway to make a long story short: after installing imagemagick it all works
> fine now.

Hello,

Some extra comments about the use of the ImageMagick 'convert' tool.
(I use 'convert' to build my web versions of images.)

1. I confirm that 'convert' always keeps image metadata in the target
file, this is its default behaviour. And if and only if you wish *not
to keep metadata*, you need to add a specific option :
    convert file.jpg -resize '800x800>' -strip smallfile.jpg

(Btw, I'll suggest you use rather the « bounding box » syntax to resize
your images. With something like "-resize '800x800>'" you ensure
the larger dimension of your image will be 800, be it the width or
the height, and the aspect ratio will be preserved.
When using explicit dimensions, "-resize 800x600", if you happen to
process a portrait oriented image or an image with an aspect ratio
different from 4:3 (3:2 is a common value with SLR cameras) you will
get distorsions on your final image.)


2. In my opinion, 'convert' also has a useful option for web usage
and can produce progressive JPEG (more pleasant display for users
concerned with low bandwith issues as I am:-) :
    convert file.jpg -resize '800x800>' -interlace plane smallfile.jpg


3. Last, you can also control the JPEG quality factor (and compression
level) of your small images. The default value for 'convert' is 95%
but when generating small size images, 500 to 700 pixels, you can
lower to 90 or 85%, e.g. :
    convert file.jpg ... -quality 90 ... smallfile.jpg
and have an effective gain in output file size (typically half the size)
without visible losses for screen display usage.
(Smaller size means smaller disk usage on your web hosting and faster
faster loading time when browsing.)
The good quality value should be determined via test and look.


Hope these hints can help...

Regards,
Jean-François


> Oh and another thing....digikam resizing is very slow compared to
> commandline, so I'll stick to that. Also gives me an easier
> opportunity to restore the creation date of the resized file to set
> with the jhead tool :)

I confirm, command line processing is really faster, don't know why...
Probably, using a small script 'my-resizing' gives maximum flexibility.
One can process directories, tune options, quality, interlacing, and also
perform all what 'convert' can do, adding texts, copyright, watermarking,
etc.