What is DPI?

[mercanucaribe]

I'm waiting for my MX518 to arrive from Newegg and I've been trying to figure out what the DPI value means. Obviously it's not the dots per inch when you track, becacuse that would depend on speed. The faster you move the mouse, the fewer DPI there would be.

So it must be the resolution of the camera right? If an 800 dpi camera looks at a .25x.25" piece of mousepad, it's taking a 200x200 picture 100 times a second, 200 times a second, or whatever. But wait.. Why would changing the resolution of the camera change the cursor movement speed, like when you use the DPI changing buttons on the MX518?

And wouldn't it make more sense to change the cursor speed while maintaining the highest DPI possible? Of course I'm not sure what DPI is, but presumably higher is better.


What is DPI?

 

[Zepper]

No, the DPI stays the same regardless of how fast you move the mouse. The ball travels x distance and puts out y pulses in that distance. The frequency of those pulses will increase (and some may be lost if your mouse's repeat rate and resolution aren't fast enough to keep up with your movement) but the raw number of pulses will remain the same for each distance you move. If that weren't the case, it would be vary difficult to get accurate pointing with a mouse or any other pointing device.

.bh.

 

[mercanucaribe]

Are you sure? How does the mouse know it's moving unless it's updating constantly like a video camera?

 

[Zepper]

You just have to take apart an old mechanical mouse to know that my explanation is correct. The ball drives two axles and on one end of each axle is a wheel - the axles would be analogous to the X and Y axes on a standard 2D graph. The wheels have evenly spaced slots in them and they run thru the slots of optical sensors which count the slots in the wheels as they go by. The new ball-less mice operate the same way except they expect a relatively even texture in the surface they are on for their sensing - my Logitech Trackman Marble FX depends on the evenly spaced features in its Marble.
. The higher the res of your mouse (think of it as more evenly-spaced slots in each wheel), the more accurate your pointing.
. Mouse drivers have speed and "accelerator" functions which multiply the actual dpi of the mouse to move the cursor on the screen faster. Speed is a linear multiplier while "acceleration" is a geometric function. Turn both of those off to see what is actually happening. Not a lot of fun, eh? But you can get very accurate pointing at that slow speed, right down to the pixel level if you can see them.
. But the basic opto-mechanical or optical systems in the mouse still has to be a linear "X Pulses = Y Distance" or the fancy mathematical tricks of the drivers are for naught as the mouse could never yield an accurate relative position on the screen.

.bh.