Android GIMP JPG to PPM conversion

How exactly does GIMP convert a jpg into a ppm? I'm trying to convert a jpg to a ppm on Android. In Android I use the API to create an ARGB_8888 Bitmap of the jpg then drop the Alpha channel and throw the RGB values into a ppm. But I get different values for RGB from G开发者_Go百科IMP.

Have a look into the code. You can find it here: http://git.gnome.org/browse/gimp/tree/plug-ins/common/file-pnm.c