Media file handling

Media classes

So-called media classes define a way to give a specific image transformation a name and re-use it amongst templates.

The image tag has a mediaclass attribute which expects the name of a mediaclass that is defined somewhere.

Zotonic uses the template lookup mechanism to look for files called mediaclass.config under the templates/ directories of your modules/sites. An example mediaclass.config file is the following:

[
 {"thumb",
  [
   {width, 120},
   {height, 120},
   crop
  ]}
].

This defines a media class called “thumb”, which can be used to display a 120x120 cropped square image. This media class can then be used in the image tag as follows:

{% image id mediaclass="thumb" %}

The image URL will have a checksum embedded in it so that when the contents of the media class is changed, all images which use that media class will be regenerated to reflect the new media class.

ImageMagick conversion options

Besides the normal image processing options, as described in image, it is possible to add literal ImageMagick convert commands to the mediaclass definition.

For example:

{magick, "-level 90%,100% +level-colors \\#FE7D18,\\#331575"}

(Note that you have to double any backslashes that were needed for the convert command line.)

This command is given as-is to the ImageMagick convert command, therefore it is best to first try it with the command-line convert command to find the correct options and command line escapes needed.

There are three variations: pre_magick, magick, and post_magick. The only difference is that the pre_magick is added before any other filter argument, magick somewhere between, and post_magick after the last filter.

In this way it is possible to pre- or post-process an image before or after resizing.

See http://www.imagemagick.org/Usage/ for examples of using ImageMagick from the command line.

User-agent specific images

Since the mediaclass.config file is resolved using the template selection mechanism, it is subject to the same selection rules that normal templates fall under.

The consequence is that you can have multiple mediaclass.config files, for instance one in desktop/, one in phone/. The media classes defined in those subdirectories can have the same names. This way you can make thumbnail sizes smaller for phones, or serve higher-quality JPEG file for desktop browsers.

See User Agent selection for the details on the user-agent selection mechanism.

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