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.

    • Mar 06 2006 | 4:48 am
      Couple of parentheses should clear things up:
      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
      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.
      -------------- --------------
      Peter Castine | ^
      | Litter Power & Litter Bundle for Jitter | | iCE: Sequencing, Recording, and Interface Building | for Max/MSP
      | Extremely cool
    • Mar 06 2006 | 6:52 pm
      That works!