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An advanced image conversion server and command line tool.




Use NPM: npm install [-g] converjon

If you want to prevent the fixed dependency versions of the npm-shrinkwrap.json file to be used on install use --no-shrinkwrap as a command line argument. See shrinkwrap docs and install docs for more information on this.

Converjon follows Semantic Versioning.


Manually on commandline, as a system service or via docker.


Start the server with converjon [--config your_config_file] or use the command line utility converjon-cli to work on local files.


docker run -t -p 8000:8000 -v $(pwd)/config/default.yml:/etc/converjon/config.yml berlinonline/converjon:latest


docker run -e USE_CONFIG_DIR=true -t -p 8000:8000 -v $(pwd)/config/:/etc/converjon/config berlinonline/converjon:latest


Let's say you have an image at To get the image through Converjon, put the original URL into the request as a URL encoded parameter:


Another alternative is to specify a file from the server's file system as the source. Instead of url, you can add the file parameter for this:


The file parameter is URL-encoded. In plain text it is foobar:some/directory/image.jpg.

Here, foobar is the name of a filesystem alias followed by a colon and a relative path. See Configuration: Aliases for more details.

More options are available as GET parameters. All parameters need to be URL encoded.

Several examples are available on the /demo page which is enabled when starting Converjon with the development config file via converjon --dev.

It's recommended to set the server port to 80 in configuration and to run Converjon on a separate subdomain of your site or behind a reverse proxy like Nginx or Varnish.

For information on how to use the command line tool, run conversion-cli --help.

Changing size

You can either supply a width, height or both. If you only specify one dimension, the other one will be derived from the original image's aspect ratio.

If you set both values, the image may be cropped to the new aspect ratio and then resized to the requested pixel dimensions.

Area of Interest

Wiki: Area of Interest

By default images are cropped from the center of the original. You can specify an "area of interest" with the aoi parameter. The AOI is a rectangle in the following format:


The AOI can also be embedded in the original image's metadata via EXIF or IPTC. The name of this metadata field can be configured and defaults to aoi. The request parameter overrides the AOI value from the image's metadata.

By default, the AOI in the URL parameters has precedence over the one from the image's metadata. To prefer the embedded AOI from the metadata, set the prefer_embedded_aoi parameter to any non-empty value.

Cropping mode

The crop parameter sets the cropping mode. Available modes are:

  • centered
  • aoi_coverage
  • aoi_emphasis
  • aoi_auto

Details about the cropping modes can be found here in the wiki. For examples, see the demo page.

If an AOI is set, cropping will ensure, that the area is always preserved.

Padding Color

The padding_color parameter sets the background color of the padding.

The color should be specified in an HTML URL Encoded format.

If none specified, the default color set in the configuration will be used. see also Converter config

Image Format

With the format parameter you can change the format of the image. Supported formats are:

  • jpg or jpeg
  • png
  • gif

If no specific format is requested, the format of the source image will be used.


The quality parameter sets the JPEG quality value. It ranges from 1 to 100.

This parameter is ignored, if the requested format is not JPEG.

Color Palette

The colors parameter sets the number of colors for GIF compression. It ranges from 2 to 256 (integer).

Interlaced Images

The interlace parameter allows the creation of interlaced images. Supported types of interlacing scheme are:

  • plane(try this for JPEGs)
  • line
  • none

A well-known example of interlaced images are progressive JPEGs. You can use this option with PNGs and GIFs as well.


The preset parameter allows the automatic usage of a preset of parameters.

New presets may be added in the configuration files as follows:

      format: "jpg"
      quality: 50
      width: 100
      hight: 100

So instead of specifying all the parameters in the URL:


you can use the preset:


Removing Metadata

The strip_metadata option removes all metadata (e.g. EXIF, IPTC) from the converted images. This option has no value, it just needs to be present in the URL query parameters.

Status Page

The URL /status leads to a summary of Converjon's current state and some statistics.

The status page is available as content type application/json via /status.json.


When launching converjon, you can specify one or more configuration files with the --config option which can be set multiple times to load multiple config files:

converjon --config conf_file1.yml --config conf_file2.json

To load a directory containing one or more config file, use the --config-dir option. Config files will be added in the order they appear in that directory.

converjon --config-dir /etc/converjon

You can use the default.yml or development.yml file as an example for writing your own.

The default configuration format is YAML but you can also use JSON files.

Every configuration file matches certain image source URLs. If a config file contains a urls setting, that configuration will only apply to URLs that match at least one of the patterns from that list. Config files without the url setting apply to all images.

Some configuration are automatically converted:

  • "true" (string) is treated as true(bool)
  • "false" (string) is treated as false(bool)
# this config will only apply to source URLs from localhost or flickr
  - "http://localhost*" #this will match URLs on localhost, this is the dev/testing default
  - "*"

Converjon uses calmcard for string pattern matching. Documentation on how these patterns work can be found there.

This way you can define different settings depending on the source of the requested images.


  port: 8000
  instance_name: null
  timeout: 20000
  send_timeout: 2000 #time that sending the repsonse data may take.
  access_log_format: "combined"
  base_url_path: "/"
  enable_load_test: false
  • server.port: port for the server to listen on
  • server.instance_name: the name of this server that will be displayed on the status page

If not set, a random name will be generated.

  • server.timout: global timeout for incoming requests
  • server.send_timeout: the time streaming a finished image file into the HTTP response may take. This usually shouldn't need to be changed, except when the servers file system is expected to be exceptionally slow.
  • server.base_url_path: Normally, Converjon's image URL paths just start with /, like in

You can set a base path to have better control over the URLs that Converjon uses. If you want the image URLs too like set the base_url_path to "photos/".

  • server.access_log_format: the formatting of access logs:
    • combined: Apache Combined Log Format (the default)
    • short: leaves out the date/time information. Use this, if you use other software for log handling that adds timestamps.


URL whitelists download.url_whitelist sets list of URL patterns that are allowed to be requested as image sources.

For example, if you host your source images on you should set the whitelist pattern to to make sure other sources are not allowed.

# this will only allow requests for images from URLs that match these patterns
    - "http://localhost*"
    - "*

Calmcard patterns are used for matching by default.

You can also prefix the pattern with ~ (like ~ ^http://(foo|bar)\\/.*) to use regular expressions. For most cases, this should not be necessary and is discouraged.

Timeout download.timeout sets a timeout after which requests are cancelled, if the source server doesn't respond in time.

Reject Invalid SSL Certificates Setting download.rejectInvalidSSL to true (default) will cause sources to be rejected, if their SSL certificates cannot be validated.


To access the server's local filesystem for source images, you can specify "aliases":

#example from the development configuration files:
alias: dev

base_file_path: "test/resources/images"

  - "test/resources/photos"
  - "test/resources/misc"

  Cache-Control: "max-age=5"

When an image is requested with a file parameter (<alias>:<path>) instead of url, the alias part of that parameter is matched against the configuration files, just like with URLs but only the configs that have that exact alias will be used for that request.

There can only be one alias per config file. If you need multiple aliases, you need to have one config file for each of them. See here on how to load multiple config files.

In a config file with an alias you can set a base_file_path. This is the directory where your source images are located. It is concatenated with the path part of the file parameter to point to the actual file. The base_file_path can be an absolute path or relative to the working directory of the server.

Additionally there can be multiple fallback_base_file_paths. If the requested file can't be found in the base_file_path, Converjon will try to find it in the first fallback path, then the second, and so on.

In addition, you can set HTTP headers that will be sent along with the converted image as if they had come from a source server.


Converjon supports HTTP basic authentication for image sources. If you include authentication credentials in a URL specific configuration file, they will be sent along with every request to URLs that match the URL pattern of that configuration file.

  username: "testuser"
  password: "testpass"


cache.basepath sets the base directory for the local file cache.

  base_path: "/tmp/converjon/cache"

cache.copy_source_file determines, if a source file should be copied, when you're using a local file as source. By default, files are copied into Converjon's cache directory. Set this to false to use the original file's location. You will have to make sure that these files are not changed while Converjon is using them. This could result in corrupted images or failing requests.

The cache directory is not automatically cleaned up and may grow over time.

Garbage Collector

Converjon can clean up the cache directory automatically. By default this is disabled. It can be enabled with the garbage_collector config directives:

  enabled: true
  source: "cache"
  target: "immediate"
  interval: 5000
  • enabled: turns the garbage collector on or off
  • source: determines when source files should be cleaned up. Possible values are:
    • cache: The file will be removed when it's cache lifetime has expired
    • immediate: The file will be removed as soon as it's no longer in use by any pending request.
    • any other value will disable the cleanup for source files
  • target: same as source but for the converted target image files
  • interval: time between garbage collector runs, in milliseconds.

Local source files that were not copied into the cache directory will not be removed by the garbage collector.


process.limit sets the maximum number of child processes that converjon will spawn for converting and analyzing images.

Increasing this will likely increase memory consumption while providing better usage of multiple CPU cores.


  • converter.padding_color sets the background color that is used, if an image needs padding to fit the requested aspect ratio.
  • converter.filter sets the filter, e.g. "Sinc"
  • converter.quality sets the quality of the resulting resized image (number, e.g. 85)


cropping.default_mode sets the default mode for cropping images. Possible options are: centered, aoi_coverage, aoi_emphasis and aoi_auto.

Wiki: Cropping Modes


Constraints can be used to limit the possible request parameters, like width and height of images. Every constraint has a min and a max value:

    min: 1
    max: 100
    min: 1
    max: 256
    min: 1
    max: 10000
    min: 1
    max: 10000


There are three logging levels: access, error and debug. Each of them can be directed to either STDOUT, STDERR or into a log file.

  error: "stderr" # will log errors to STDERR
  debug: "stdout" # will log debug logs to STDOUT
  access: "/var/log/access.log" # will log requests into a log file

To disable a log level, set it to false:

  error: "stderr"
  debug: false # "false" as a string will also work
  access: "/var/log/access.log"

Logs are not automatically rotated. Use of a tool like logrotate is recommended.


Execute tests with npm test. Please note, that you need to have all dependencies installed.


Please contribute by forking and sending a pull request. More information can be found in the file.


See for more information about changes.

Copyright Notes

Converjon is licensed under the MIT license.

The "sparrow" testing image is © Leon Weidauer, permission to use it for testing is granted.