Reference: Converting bitmaps to EPS efficiently

2012 March 1

Yet another entry in a surprisingly long series of reference posts about converting image formats. Sometimes you have a bitmap plot that needs to be converted to EPS for submission to a journal. My go-to command is convert foo.png foo.eps but this can give bad results.

For a large PNG file, the best approach using standard tools that I’ve discovered is to use the NetPBM programs:

pngtopnm figure.png |pnmtops -equalpixels -scale 0.24 -psfilter \
  -flate -noturn -ascii85 -nocenter - >figure.eps

Annoyingly all of these options seem to be necessary to give useful output. The scale factor is the number of inches per 72 pixels; 0.24 gives you about 300 DPI.

The pnmtops documentation says that the -flate option (which turns on compression and can be a huge deal for large images) sometimes produces busted output. There’s also an -rle option that will be less efficient (especially for bitmaps that don’t contain long stretches of single colors). The documentation also references a program called bmeps that is claimed to do a better job of this kind of conversion. The same seems to be said of sam2p, but one of the benefits of the above command is that the NetPBM tools are installed on most Linux machines.

When all of these options are working, I can generate a good EPS that’s only 15% larger (on-disk) than the original PNG, which is pretty cool.