I was happily using the dw2pdf plug-in for the DokuWiki installation at work when users began reporting broken images in the exported PDFs. One page in particular featured two images – one worked, the other did not. The only difference I found was in the palette used:
$ file deps.png deps.png: PNG image data, 361 x 256, 8-bit/color RGBA, non-interlaced $ file deps-legend.png deps-legend.png: PNG image data, 3476 x 800, 8-bit/color RGB, non-interlaced
The first one was missing in the exported PDF. Apparently dw2pdf cannot handle an alpha channel. Since my theme has a white background, I simply replaced it with white:
$ convert deps.png -background white -alpha remove deps.png && convert deps.png -alpha off deps.png
Be careful, like written here the command replaces the original file. The last file is now in RGB with a white background:
$ file deps.png deps.png: PNG image data, 361 x 256, 8-bit/color RGB, non-interlaced
Note: convert is a tool of the ImageMagick suite (sudo apt-get install imagemagick).
For files that are 8-bit gray+alpha, the following command does the trick:
$ convert file.png -background white -type Palette file.png
These tricks are mere workarounds, but time restraints did not permit further investigation into the inner workings of the plug-in.