How do I calculate the magnitude of the velocity of my mouse cursor, in Python?

http://dl.dropbox.com/u/779859/speedCalc_puradata.JPG

I achieved it in pure data, have a look at the schematic of what I'm thinking:

Recieving Midi Control input from ctlin 20 and 21
Pipe delays whatever signal it recieves
Pythagoras
Viola, the speed of the input. The units don't matter, as long as it is absolute.

I was thinking about doing the same but in python, for the mouse cursor.

Basically, when I move my mouse, I want to see at what speed the mouse is moving. The rate of input packets is constant at 200hz.

I might have come up with a way, though I haven't tested it yet. How about collecting, say, 51 values in a list, keeping the [0] current, and [50] the oldest. Then simply doing the math on those two values?

Solution

What you are describing will give you the magnitude of the velocity times the length of the time interval of measurement. The actual velocity will be a vector. You can get its first coordinate as (posX - delayed_posX)/t and its second coordinate as (posY-delayed_posY)/t where t is the time interval between the measurements. Note that this satisfies P_final = P_start + t V where P is our position vector. Whenever you want to know how to measure an approximation of the velocity, that's always your starting point. The smaller the time interval, the more accurate a picture of the velocity you will have.

In response to your question about time.sleep, no it will not slow down your other code: it will stop it completely unless it runs in another thread.

What exactly are you trying to do? It's hard to say if there's a better way unless we know where you need the data to be, when you need it to be there and how current you need it to be.