BMP isn't so trivial, largely because it's been extended multiple times while never having a proper spec. http://searchfox.org/mozilla-central/rev/bbc1c59e460a

BMP isn't so trivial, largely because it's been extended multiple times while never having a proper spec. http://searchfox.org/mozilla-central/rev/bbc1c59e460a27b20929b56489e2e55438de81fa/image/decoders/nsBMPDecoder.cpp#10-83 http://searchfox.org/mozilla-central/rev/bbc1c59e460a27b2092... has some of the gory details.