I understood what you guys say, when using 2 leds it makes sense, but in this mouse there is only one led/ receiver per wheel... so... is it all about the pulsed signal and the skipped pulses? how to define direction with only one "optical barrier"?
Hmm, may be they got two-pixel sensor in the same package? I don't see how else could some get a direction info, except if the reticle have very specific coding (even then though the light source have to move relative to the sensor, what is not the case of a LED/Photodetector pair), yet so far there is nothing specific into the mice weel. It is anisotropic about the both rotational directions.