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

[Gilles Caulier]

2012/11/4 Jean-François Rabasse <jean-francois.rabasse at wanadoo.fr>

>
> 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.
>
>
Warning : quality = 100 is not lossless. Only JPEG used in medecine world
can do that. It's patented and not implemented in opensource.


>
> 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.


It use JPEG lib crop feature. Image is not decoded and re-encoded. But on
JPEG support that.

Gilles Caulier
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