The new feature, external tools, is actually one of the most powerful features I've seen added to DOpus in a long time. As long as you have command line tools for at least converting it you can leverage any image format and it'll be like it's one directly supported: that means it'll get thumbnail, preview, and can be opened by D8Viewer.
To do that, add the image format by its known suffixes. For example .jxr and .wdp for JPEG-XR files. When DOpus comes across a file with that suffix, it'll trigger the tool.
The Most Common Image File Conversion Tools
The default, imagemagick, will cover most file types when it comes to decoding. Using PNG for your temporary file is suggested, because it'll support most things like alpha, transparency, metadata etc. The -quality 1 flag means you won't be wasting time compressing it.
magick.exe -quality 1 %in% %out_png%
Imagemagick doesn't cover absolutely everything, however. There's also a number of other image conversion tools which can be lighter and faster, or support image types Imagemagick doesn't.
VIPS is a lighter version of Imagemagick, claiming to be faster and easier on RAM. It has a windows build.
vips.exe copy %in% %out_png%[compression=0]
Irfanview is a classic image viewer, but also has a command line and some plugins that Imagemagick doesn't. It has no quality switch however, so you'll either have to put up with compressing a PNG or use BMP.
i_view64.exe %in% /convert=%out_bmp%
Xnview MP is another popular viewer but has no command line interface for conversion. Instead use nconvert. The -c 0 again turns off png compression.
nconvert.exe -out png -c 0 -o %out_png% %in%
Those should cover the vast majority of file types. If the command isn't found, then either add the whole path to the executable or add it to your path.
Fringe Cases
Want to be able to support those retro formats not already covered by DOpus, such as 8-bit formats and obscure ones like IFF-DEEP? Install RECOIL. It comes with its own cmdline conversion tool. Then for all the suffixes you want to load, have:
recoil2png.exe -o %out_png% %in%
To add a format that isn't supported by most tools, like Better Portable Graphics, grab the encoder and decoder tools and put in your path somewhere, such as the same folder as imagemagick.
bpgdec.exe -o %out_png% %in%
Or Dropbox's Lepton?
lepton.exe %in% %out_jpg%
PGFConsole.exe -d %in% %out_png%
Or an obscure vector format?
inkscape.exe %in% --export-type=png --export-area-drawing --export-filename=%out_png%
And so on and so on. I'm not saying you should use any of these, they're just examples of the raw power Directory Opus now has.
When the Tool Isn't Friendly
Ideally, it should be blah.exe input output and output is png, gif, jpg or bmp, but the world isn't always that simple.
Here's an example - JBIG files. There's only really one tool that supports these on Windows, called jbgtopbm. Like many of the conversion tools of yesteryear, it writes a ppm to stdout, not outputs nicely to a png. No problem! Write a batch file called jbgtopng.bat:
@echo off
setlocal
set input=%~1
set output=%~2
set outdir=%~dp2
set tempname=%~n2
rem Write PBM to the same temp folder as the PNG
jbgtopbm "%input%" > "%outdir%%tempname%.pbm"
if not exist "%outdir%%tempname%.pbm" (
echo Failed to create PBM file
exit /b 1
)
magick "%outdir%%tempname%.pbm" "%output%"
del "%outdir%%tempname%.pbm"
endlocal
Then call it just like it was an executable:
jbgtopng.bat %in% %out_png%
It might not be optimal but it does the job.
JBIG is just an example. You should be able to handle practically anything with such simple bat files. Now, happy imaging!