2000.11.20 02:54 "Re: Reading other formats beside RGBA", by Joris Van Damme
Is there a TIFFGetR/G/B/A equivalant for a single byte
The RGBA level is an extremely high, and, as you noticed, extremely wastefull level. In fact, in my opinion it shouldn't even exist. If you want to deal with the data in a more appropriate way, you'll have to drop down the lower level. This means handling scanlines/strips and tiles (and, if you also want to deal with downsampled ycbcr, encoded strips). The help that comes with LibTiff includes info on that.
My advice is to forget about RGBA.