I'm not very experienced with graphic or printer formats. My first (and only try yet) was to print a Tux.png with 265 x 314 pixel via the Chromium browser and see what Wireshark shows as USB transfer.
I turns out that the image became a 1.540.598 byte bulk transfer (noticed because it uses a designated USB endpoint). For this I only measured the payload which divides into 188 packets of 8192 byte and one left packet with 4982 byte.
Than I tried to look into the first 81920 bytes by concatenating the payloads and read the content as 32 bit integers to see if it contains some repeating patterns which maybe looked like some continuous white or black pixels - but this didn't work.
So did anybody already started an attempt to understand the data that is sent to the printer?
It would be good to have an open source variant of the driver, because the current driver is an Intel binary and won't work for example on a Raspberry PI, which could be a cheap and current-saving print server.
Update 2014-03-10: Forgot to mention that I already found the open source drivers of Brother - but they don't contain the driver for the printer, they only provide the source for the CUPS wrapper. And just to mention it (should make no difference) I own an Brother DCP 135c.