Computer Mouse Technical Question - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: December 05, 2022, 01:18:41 am
content: I have a regular computer mouse and a clear layer of plexiglass on my computer desk (normally I use a mouse pad). If I place the mouse in certain spots, I've noticed the pointer may move on its on a bit.Being interested in why this happens due to thinking the mice shouldn't work on clear tops, I've tried different mice and find that the sensitivity varies. I understand how they work, an LED shines onto a surface and a camera on the bottom of the IC detects objects such as lines changing direction.I believe the plastic over the camera lens is to magnify, but not certain. The LED is reflected at a specific angle that is directed to the camera.As for the sensitivity where it moves on its own, but not consistent at different locations, why would this happen? The surface isn't changing, and, if it's a matter of light reflection, it should should always the same reflection angles since the mouse isn't moving.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 05, 2022, 01:54:03 am
content: The sensor in an optical mouse is in fact a very low resolution webcam focused on the pad surface. It films the surface of the pad. Detection is based on optical flow in the filmed image. If the pad surface is very smooth and uniform in color, the camera will see no change when the mouse is moved, so no optical flow detected, no move in the cursor.There are so called LASER mice, where the lighting LED is replaced with a LASER diode, and because of this some interference/diffraction patterns can be observed by the sensor even on very smooth mirror like plastic or glass.

author: bostonman
author_rank: Super Contributor
timestamp: December 05, 2022, 03:19:55 am
content: Then why does the pointer move when it's sitting still on the plexiglass surface, but only on certain sections of it?

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 05, 2022, 03:33:38 am
content: Quote from: bostonman on December 05, 2022, 03:19:55 amI suppose that some of the light is reflected in deeper layers of the plexiglass and plexiglass is never completely homogeneous, so it may create interference patterns that are erroneously interpreted as motion by the algorithm inside the mouse? Since this kind of surface is not what they develop and validate mice against, it's pretty likely that algorithms go fully bonkers in that kind of situation.

author: tooki
author_rank: Super Contributor
timestamp: December 05, 2022, 11:57:46 am
content: There are also “dark field sensor” mice from Logitech (like the MX Anywhere) that use very shallow side illumination to highlight dust and surface imperfections to allow them to function on clear glass.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 05, 2022, 02:45:26 pm
content: Quote from: bostonman on December 05, 2022, 03:19:55 amSeen a few times the same thing you describe. I suspect this might be because of some auto-sensitivity adjustment in the sensor, a control loop that is trying to adjust the sensor's ISO up and down, in seek of a decent number of distinct pixels. At different sensitivity, different details of the surface may become visible, and thus the optical flow detection algorithmhttps://en.wikipedia.org/wiki/Optical_flowmight take that variation as movement.This is the first cause I would investigate, a wild guess, I don't know for sure why it is happening.

author: magic
author_rank: Super Contributor
timestamp: December 05, 2022, 02:53:55 pm
content: No idea. I assumed it's noise, because what else.Maybe shot noise of the illuminator, somehow accentuated by some particular structures of the surface

author: tooki
author_rank: Super Contributor
timestamp: December 06, 2022, 01:18:39 am
content: Or maybe in the affected locations, there's slightly changing reflected light from other sources than the mouse's own illuminator.

author: bostonman
author_rank: Super Contributor
timestamp: December 07, 2022, 04:03:35 am
content: It's not really a problem as I normally use a mouse pad anyway. It was more being curious since nothing moves.I'm thinking maybe certain surfaces cause a kaleidoscope affect with the mouse causing it to constantly see different objects.