math is over my head


    Mar 06 2006 | 3:04 am
    I'm trying to put an s-curve into a buffer~ to use as a lookup table, but I can't figure out the damn math. I gather the formula to use would be: y= 1./ 1 + exp((b*x) -a)
    b being a curve constant, & a shifting it. I was trying to peek into a buffer using [expr 1. / 1. + exp(($f1*$f2) - $f3)] my curve rockets up, but never levels out at the top.
    Can someone who understands math tell me what I'm doing wrong?
    Thank you o wise math people.
    M

    • Mar 06 2006 | 4:48 am
      Couple of parentheses should clear things up:
      try:
      1./(1.+exp(($f1*$f2)-$f3)))
      Right now you're adding 1/1 to exp(($f1...etc., so it will never level out. (because the exp. is going to keep growing in value.) Once you add the parentheses, you should see the expected asymptotic behavior.
      Peter McCulloch
    • Mar 06 2006 | 10:31 am
      On around Mar 6, 2006, at 4:04, mattyo said something like: > Can someone who understands math tell me what I'm doing wrong?
      Peter McC. answered your top-level question. But if you're really having a hard time with the math, you might want to take a look at lp.scampf from Litter Power, which is there particularly for creating S-shaped and other shaped-curves, doing all the hard math for you. Just play with the number boxes in the help file until you get the shape you want.
      Here's a snapshot showing what it looks like:
      I've removed the alpha channel to see if that helps the folks who have been having problems viewing Max-generated TIFFs and colors are trimmed to 4-bit resolution to minimize bandwidth. Forum readers will probably have to download the Litter Starter Pack (URI below) to see it; sorry, that's beyond my power to control.
      Best, Peter C.
      > -------------- http://www.bek.no/~pcastine/Litter/ -------------- Peter Castine | ^ | Litter Power & Litter Bundle for Jitter pcastine@gmx.net | pcastine@bek.no | iCE: Sequencing, Recording, and Interface Building 4-15@kagi.com | for Max/MSP | Extremely cool | http://www.dspaudio.com | http://www.dspaudio.com/software/software.html
    • Mar 06 2006 | 6:52 pm
      That works! Cheers Roger