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Specular Lenses and the z-axis bug

post #1 of 5
Thread Starter 
There are two correct ways of using the phrase "z-axis bug".

One is the strong definition, which describes mice that move when you pick them up, but move a different amount when you put them back down. As a consequence, they end up in a different position, even if you pick up and put back down the mouse perfectly.

The second is the weak definition, which describes mice that move when you pick them up, and move when you put them back down, but it moves around the same amount both times, and in opposite directions. As a consequence, the general position of the mouse doesn't change over time just because of picking the mouse up and putting it back down.

The strong definition is usually caused by bad liftoff detection, where stopping tracking happens at a higher height than starting tracking. This causes putting the mouse back down to move the cursor less, period. However, the strong definition always requires a z-axis bug to be present in general. Really, what's happening here is funky liftoff behavior in addition to a z-axis bug.

The weak definition isn't normally a problem. For low sensitivities, the weak version of the z-axis bug basically doesn't matter at all. The weak version of the z-axis bug basically only matters to mid and high sensitivities, think nine or fewer inches per 360, or high DPIs on the desktop.

In optical mice, the z-axis bug is caused by the use of a "specular" lense.

http://imgur.com/a/Qrg8j

Specular lenses have the actual magifying part of the lense off-center relative to the pinhole into the sensor's IC. (This magnifying part is normally in an inlet that ensures the lense is in the desired position relative to the sensor.)

This makes the sensor to literally see the tracking surface at an angle, which is done because it improves surface compatibility. However, because it sees the tracking surface at an angle, moving the mouse in a vertical straight line changes the horizontal displacement of the part of the tracking surface that it can see.

In reality, most mice have a small amount of z-axis drift because lenses are never perfectly centered and can even rattle a little (lol 3310), but it's always at least a minor, weak z-axis bug if the lense is shaped in a way that prevents the magnifier from being vertically centered under the IC's pinhole.

Z-axis bugs are one of the biggest reasons for the obsession with pathologically low liftoff distances, which ironically hurt surface compatibility a little. Surface compatibility is something that specular lenses, which always cause at least a minor weak z-axis bug, are used in order to improve, not make worse.

The default lenses of the 3320 and 336X are specular. The G102/G203's lense is also specular. The low liftoff distance of most mice with the 336X means that people with low sensitivities don't even notice the weak version of the z-axis bug is there, but that doesn't mean it doesn't affect people that have high sensitivities in-game, or a high DPI on the desktop.

Source of this use of the term "specular": https://www.logitech.com/en-us/articles/11539
Edited by wareya - 4/25/17 at 8:11pm
post #2 of 5
I have to say I started noticing that after you talked about it on ino's stream...

very informative! thankfully I'm 50-60cm / 360 so it doesn't bother my gaming at all. smile.gif
post #3 of 5
Quote:
Originally Posted by wareya View Post

The strong definition is usually caused by bad liftoff detection, where stopping tracking happens at a higher height than starting tracking.
Would this be due to badly done firmware and implementations on the mouse, or is it inherent to certain sensors themselves?
post #4 of 5
Thread Starter 
It's a concept called hysteresis and it actually has some merit. You only notice it when the thresholds are in significantly different places. Crappy firmware might have dumb thresholds for no good reason, but good firmware with a crappy sensor might need dumb thresholds just to make the mouse usable, or it would momentarily stop tracking when you move it too fast over funky illustrations (because you can't really tell z distance apart from general image quality).
post #5 of 5
Good read. I was wondering why all the new sensors did that. +rep
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