ppmtosixel - convert a PPM image to DEC sixel format
This program is part of Netpbm(1).
ppmtosixel reads a PPM image as input and produces sixel commands (SIX) as output. The output is formatted for color printing, e.g. for a DEC LJ250 color inkjet printer.
If RGB values from the PPM file do not have maxval=100, ppmtosixel rescales them to maxval 100. A printer control header and a color assignment table begin the SIX file. Image data is in a compressed format by default. A printer control footer ends the image file.
Generally, sixel files must reach the printer unfiltered. Use the lpr -x option or cat filename > /dev/tty0?.
Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion. If the original PPM maxval was greater than 100, rescaling also reduces the image depth. While the actual RGB values from the ppm file are more or less retained, the color palette of the LJ250 may not match the colors on your screen. This seems to be a printer limitation.