I recently switched from the fltk to the qt graphics-toolkit and noticed, that the dpi related metadata is handled quite differently when printing to jpeg/png files with the resolution option "-rNUM".

Here are may findings:

• the image size is calculated correctly by both backends
• fltk backend: 'X Resolution'/'Y Resolution' (jpeg) and 'Pixels Per Unit X/Y', 'Pixel Units' (png) are set correctly
• qt backend: jpeg: 'X Resolution'/'Y Resolutions' is always set to 72 dpi. png: no dpi related metadata is written at all

Not sure what would be the right approach (setting dpi related metadata explicitly or omitting it), but imo it should be handled consistently for all raster image files (at least where applicable).

Thanks