====== Graphics Processing ==

Slack now comes with ''exiv2'' on board. This makes at least some of jheads features obsolete.

jhead is available at www.slackbuilds.org. The manual for jhead is [[http://www.sentex.net/~mwandel/jhead/usage.html|here]].

===== Set the Exif create timestamp ==

  exiv2 --Modify "set Exif.Photo.DateTimeOriginal 2019:10:07 09:51:18" image.jpg
  
Useful mainly when the timestamp is missing (e.g. in images coming from Whatsapp).

List of the exif [[https://www.exiv2.org/tags.html|tags]]
===== Rename files and set file timestamps according to the Exif create timestamp ==

  exiv2 -t mv *.jpg

===== Read all Exif Data ==

  exiv2 image.jpg
  jhead image.jpg
  php -r 'print_r(exif_read_data("image.jpg"));'

===== Adjust Exif Timestamp ==

<code bash>
# Add one hour to the timestamp:
exiv2 -a '+01' adjust *.jgp
# Sub 30 minutes from the timestamp:
exiv2 -a '-00:30' adjust *.jgp
</code>

==== old ==

  jhead -da <to_time>-<from_time> *.jpg

Example:

  jhead -da 2011:09:04/21:17:04-2009:09:04/05:55:00 *.jpg

===== Get Image Dimensions ==

  identify image.jpg [...]
  php -r 'print_r(getimagesize("image.jpg"));'
  rdjpgcom -verbose image.jpg | grep 'JPEG image'

===== Read comment ==

  rdjpgcom image.jpg

===== Write comment ==

  wrjpgcom [-replace] -comment 'comment text' image.jpg - Add/replace comment directly
  wrjpgcom [-replace] -cfile comment.txt image.jpg - Add/replace comment with text from comment.txt
  jhead -ce image.jpg - Insert comment with editor (eg. vim)
  jhead -ci comment.txt image.jpg - Replace comment with text from comment.txt
  jhead -cl 'comment text' image.jpg - Insert Comment directly

===== Delete comment ==

  exiv2 --Modify "del Exif.Photo.UserComment" *.jpg
===== Rotate an Image ==

Bild verlustfrei um 90° im Uhzeigersinn drehen (-rotate 90) wobei Kommentare, exif-header & thumbnails erhalten bleiben. Die Resolution-Info wird automatisch angepaßt: zB 1600 x 1200 -> 1200 x 1600:
  jpegtran -rotate 90 -copy all in.jpg > out.jpg

===== Resizing ==

Alle Bilder im Verz auf 2048px als größte Dimension scalieren, wobei das Seitenverhältnis erhalten bleibt, Origiale werden zersört! Nur Bilder, die größer als 2048px werden verkleinert. FIXME Kleinere Bilder werden offenbar doch angefasst
 Exif-header bleibt erhalten (ausprobiert), wobei der Resolution-Tag auf die neue Auflösung angepaßt wird:

  mogrify -verbose -resize '2048x2048>' *.jpg

Alle Bilder im Verz auf eine Fläche von 10000 Pixel skalieren, wobei das Seitenverhältnis erhalten bleibt, Origiale werden zerstört(!):

  mogrify -resize 10000@ *.jpg

''-scale'' is faster as ''-resize'', but quality is lower. [[http://imagemagick.org/script/mogrify.php|mogrify]] is part of ImageMagick.

===== Convert Image File Format ==

A script to convert from one image file format to another:
<code bash>
#!/bin/bash
oldext=pcd
newext=jpg
command=convert
for file in *.$oldext ; do
    base=$(basename $file .$oldext)
    echo $command $base.$oldext $base.$newext
    $command $base.$oldext $base.$newext
done
</code>
[[http://imagemagick.org/script/convert.php|convert]] is part of ImageMagick.

===== Get or remove embedded Thumbnails ==

  jhead -st thub.jpg bild.jpg - Save Thumbnail in thumb.jpg
  jhead -st "thumbnails/&i" *.jpg - Save thumbnails in subdir
  jhead -dt *.jpg - Delete thumbnails

===== Image Viewers ==

gthumb kuickshow <del>gqview</del> -> geeqie nview eog display seejpeg

==== display - CLI Image Viewer ==

  space     display the next image
  backspace display the former image
  Ctl-D     delete an image file
  Ctl+Z     undo last image transformation
  <         halve the image size.
  .         return to the original image size.
  >         double the image size

==== xv - CLI Image Viewer ==

  space     display the next image
  backspace display the former image
  Ctl-D     delete an image file

===== jigl.pl (obsolete?) ==

  -uec --use-exif-comment
  -it --index-title <string> - Sets the title and title-bar of the index.html page
  --nogenerate-info
  Options: xome.net/projects/jigl/