There is no need to discuss about the terms CPI and DPI, they both mean the same but CPI is the correct one.
Correct in the sense of "describing the function" and the sense of "used by the sensor manufacturers".
if you can, i want to know more information about what you write.
Neg. Acceleration in CS1.6:The movement is determind by the movement of the Cursor on the Desktop.
You could say a "virtual desktop" is running in the background.
Every Frame the Game checks how much the Cursor has moved on that virtual desktop.
If the cursor has for example moved 200 pixels to the left (from the center) the game will translate that to the actual ingame movement (based on the ingame sensitivity settings).
The problem is that the cursor can't move over the edge of the desktop and the maximum movement is limited by the size of the virtual desktop.
For example:
Dsiplay Resolutions: 800*600
CPI: 400
Win Sens: 6/11
FPS: 100
If you move the mouse 1 Inch (in the time of 1 Frame) it will be at the edge of the desktop, if you move it 2 Inch in the same time the cursor will still be at the edge. The Game sees at as the same movement and therefore you will move your crosshair the same distance ingame allthough you move your mouse twice as much.
That's called Neg. Acceleration. (can also be caused by hardware issues)
It's affected by CPI (more = more accel.), display resolution (higer = less accel.), FPS (more = less accel.) and the general Windows Sensitivity Settings.
With the settings of this example the max. speed you can move your mouse without getting negative acceleration would be 2,54m/s.
If you would set your mouse to 1600 CPI it would be lowered to 0,635m/s.
Why you should keep Win Sens at 6/11:Settings higher than 6/11 will multiply the movement data and you loose precision.
For example at 7/11 it will be multiplied by 1.5.
If the mouse sends 3 Counts it will get multiplied to 4.5 but because there are afaik no "half movement units" Windows can only send 4 or 5 units, both aren't linear to the mouse movement it will be inconsistent.
Same goes for settings below 6/11.
The Counts from the mouse will be multiplied with a factor below 1.
If you set it to 3/11 Windows will wait until the mouse has sent 4 counts before it sends movement data to the game.
You lose the theoretical gain of precision with higher CPI and it's also inconsistent (at least at 5/11).
That's why you should use the ingame settings, they work without this limitations.
Why SteelSeries is wrong:The idea behind setting the ingame sensitivity to 1 and adjust your sensitivity with the CPI is to get rid of the ingame interpolation.
The smallest movement you can make (smallest rotation of your avatar) is limited.
This movement/rotation has to be done in a certain unit, if the movement determined by the mouse data and the sensitivity setting isn't exactly a multiple of that unit the actual movement of the Crosshair would be either too small or to big.
But the thing is that the smallest movement/rotation is very small which makes this interpolation pretty much negligible in real life.
For example the smallest movement in the Q3 Engine is 360/65536 or ~0,0055° (it's afaik the same in CS1.6 or the Doom 3 Engine).
As a comparison at 1920*1200 and real FOV 100 the center pixel on your screen represents about 0.0597° of the gaming world (horizontal).
That alone doesn't make this way "wrong", only pretty useless or non beneficial in practice.
But there is also the factor mouse itself, which also interpolates.
The sensor of the Xai only has CPI adjustment in 90 CPI steps, all other settings are interpolated by the MCU of the mouse, and because 1 count usually means more movement than the smallest movement the game can process this interpolation of the mouse is higher.
It only works with the real CPI settings of the mouse but that limits your choice of sensitivity options. 90 CPI more or less can mean much for how much you want to move your mouse.
Imho not worth for a benefit that only exists on paper.
Another interesting way would be to sync the display resolution with the CPI of your mouse so that a certain amount of counts sent by the mouse (for example 2 or 3) would always result in a crosshair movement of exactly 1 Pixel (you can calculate that).
But again your sensitivity options would be limited by the CPI steps of your mouse.
You would also not achieve that the exact same movement of the mouse (for example 1 Inch) would allways represent the exct same movement on the screen because the CPI of the mouse aren't 100% stable.
But people shouldn't take that stuff too serious, this are high level optimations which don't have to mean much for real gaming.
But there is also one other issue and that's screen resolution. Most people base their DPI on the vertical and horizontal limits i.e. 1920x1200 or 2560x1600 etc.etc. What they forget is that in between those vertical and horizontal limits are millions of pixels.
A conclusion from what I wrote above is that dependant on display resolution it's more like 10-20 for the game engine.
Edit:
I dont understand something, I use for check these settings:
450cpi - 6windows sens.
or
1800cpi - 3windows sens.
but I dont know, 1800cpi -3windows sens feels better for me somehow.
but I worry about dont have 6 windows sens.
Just because a setting is wrong on paper or theoretically bad doesn't mean much if those settings feel best for you.
In that case I would test how 1800 CPI and Win 6/11 feel with a 4 times lower ingame sensitivity, or are you talking about CS1.6 where those settings could lead to neg. accel.?