Hi again Csound world! I've been thinking for ages an opcode that
limits the rate a figure can increase or decrease would be good. Kind
of like a slew effect, but more of a cap on how quickly figures can
change, rather than a general slow down of change.

It's easy to make a super simple UDO version, but it would be a great
tool to have in general:

opcode slew_linear k, ki
kIn, iIncrememt xin
  ; kIn sets where we want the slew to get to
  ; kPrev/kOut is where we are currently
  if kPrev < kIn then
    kOut = kPrev + iIncrement
  elseif kPrev > kIn then
    kOut = kPrev - iIncrement
  endif
  kPrev = kOut
  xout kOut
endop

opcode slew_exponential k, ki
kIn, iIncrememt xin
  ; kIn sets where we want the slew to get to
  ; kPrev/kOut is where we are currently
  if kPrev < kIn then
    kOut = kPrev * iIncrement
  elseif kPrev > kIn then
    kOut = kPrev / iIncrement
  endif
  kPrev = kOut
  xout kOut
endop

I'm going to make them more usable by sending a miniumum time to slide
from 0-0dbfs parameter so the code can work out the increment based on
sr/kr and what not, instead of a basic increment parameter that is
sr/kr dependant, and not so easy to conceptualise while using.

If this already exists, then fantastic :D if not I'm going to write one.

To anyone who was following, I have nearly finished the a-rate loopseg
opcode now too... one more problem to iron out and it's ready to go ;)

The "port" opcode is what people usually use for this.  It's
exponential like a capacitor, which is a little different from being
exponential like the code you posted.

If you do roll your own implementation, take care handling overshoot; you
probably don't want it to oscillate around the target value in case of not
hitting exactly.