On Fri, 1 Feb 2002, Edson Brusque wrote:

> Hello Harold,
>
> > See http://www.piclist.org/techref/microchip/math/scale/index.htm
> > Good stuff there!
>
>     as you also work with lighting, you got what I need. :)
>
>     But it seens to me that those routines aren't exactly what I need.
>
>     I have two values let's say that one is the CHANNEL and other is the
> MASTER as in the lighting consoles. Both values can be anywhere from zero to
> 255 and the result is 8 bits also, from 0-255.
>
>     The expression "RESULT = (CHANNEL * MASTER) / 255" is perfect. The
> problem is that the execution time is very long.
>
>     Some examples :
>         X    Y    X*Y   /255   /256
>         0  255      0      0      0
>        48   47   2256      8      8
>       200  199  39800    156    155
>       210  210  44100    172    172
>       220  221  48620    190    189
>       230  230  52900    207    206
>       255  255  65025    255    254
>
>     If someone tell me a very fast way to make this I'll be eternally
> grateful.


note 65025 = 0xfe01

Call that res_hi:res_lo

follow your 8X8 multiplication routine with:

  btfsc  res_hi_,7
   incf  res_hi,F

and you'll be very close. There may be a few instances where this fails...

This is the hint I mentioned in the other post:

1/255 = 1/256 + 1/256^2 + 1/256^3 + ...

If you want to have more accuracy than what the two instructions provide
you could do this:

   movf   res_lo,w
   addwf  res_hi,w
   andlw  0x80

   skpc
    skpz
     incf res_hi,f

That should be pretty accurate.

Scott

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