Given two or three numbers, byte or int16, what is the best way to compute the "distance" function (square root of the sum of squares)?  Certainly computing the square root of the sum of squares is possible, but it seems needlessly expensive in terms of CPU time.  Can anyone offer a better algorithm for the PIC16x?

>Given two or three numbers, byte or int16, what
>is the best way to compute the "distance"
>function (square root of the sum of squares)?
>Certainly computing the square root of the sum of 
>squares is possible, but it seems needlessly 
>expensive in terms of CPU time.  Can anyone offer 
>a better algorithm for the PIC16x?

I can't offer a better algorithm, but I can
point you to a square root routine I used
when I need to take a square root of a 16-bit
integer as fast as possible.

It's in assembly, but can be "inlined" into C
code (what I did).

http://www.dattalo.com/technical/software/pic/picsqrt.html

Bill

Hi John,

i have no solution for computing it faster, but
perhaps you don't have to compute it.
In one of my projects i need to find the right distance to an object.
I only compute a^2+b^2 for every object and check it against d^2 (i compute d^2 only once).


>Given two or three numbers, byte or int16, what is the best way to compute the "distance" function (square root of the sum of squares)?  Certainly computing the square root of the sum of squares is possible, but it seems needlessly expensive in terms of CPU time.  Can anyone offer a better algorithm for the PIC16x?

>I only compute a^2+b^2 for every object and check it against d^2 (i compute d^2 only once).

In this application, for compatibility reasons, it is probably necessary to compute the actual distance (the PIC feeds numbers to another micro).  Since that other micro does a table lookup to compute the log of the numbers fed it, it may be possible to rework things to return the square rather than sqrt, but I recall reading somewhere that there was a way of computing or approximating the distance function which was as cheap or cheaper than computing the squares, let alone the square root.

Hi, 

perhaps this helps

http://www.goldenchip.com.cn/download/material/appcorpus/02/05.pdf

ciao
Jan Erik Bertram