Created attachment 285204 [details] [review]
proposed implementation

Summary
-------
The following patch adds a new command for include images which are
converted using a configurable shell script.

Motivation
----------
I wanted to add support for UMLet diagrams in a similar way as it was
done for Dia diagrams.

UMLet is a free, open-source UML tool with a simple user interface:
- http://www.umlet.com
- https://code.google.com/p/umlet/

While studying the various diffs in the Git repository, I came up with a
new idea: Why not generalize the image conversion process and let a
(user supplied) script do the work? This will have the following
advantages:
1. Adding new formats will be much easier in the future
2. Let's the user add support for formats without general interest (or
   proprietary/inhouse tools)
3. The user may tweak the parameters, add special processing (e.g.
   watermarking)

The disadvantage is that a little script must be supplied by the user.

Proposed implementation
-----------------------
Add a new command \\extimagefile with a syntax mix of \\image and \\diafile:
\\extimage <file> ["caption" [<sizeindication>=<size>]]

Since this command may be used for different formats, the <file> must
contain the extension.

Processing:
1. Doxygen searches the input image file similar as other files (new
   configuration option EXTIMAGEFILE_DIRS)
2. Depending on the output format Doxygen invokes the configured script
   (configuration EXTIMAGE_CONVERTER) passing absolute paths of input
   and output file.
3. The script determines the input and output format depending on the
   file extension. Being pragmatic this would suffice in most cases. If
   one will use the same extension for different input formats, the
   script may use file magic (see file command)…
4. The script convert the file and write the output file
5. Doxygen writes the other output (e.g. <img> tag for html output)

Configuration
-------------
- EXTIMAGE_CONVERTER
  Configure executable and optional arguments. At least on windows
  platform the script mostly must be invoked by it's interpreter.
  Example:

  EXTIMAGE_CONVERTER = "bash" ".\image_converter.sh"

  Example script skeletons are provided in the manual.

- EXTIMAGEFILE_DIRS
  Similar to other *_DIRS configuration.

- EXTIMAGE_MAPPING
  Output format mapping: Define the output format depending on the input
  file extension. This mapping is applied only for HTML at the moment
  (defaults to png if not configured). Other formats will use the
  following formats:

  RTF: png
  LaTeX: eps/pdf depending on the configuration USE_PDFLATEX
  DocBook: png