Hi,

On Sun, May 12, 2024 at 6:24 PM Fabrice Couvreur <
fabrice1.couvreur@gmail.com> wrote:
>
> With Okular, it works !

Very good. You can still try the thing below:

\startluacode
  local exp = math.exp
  local N   = 1000
  local xstep  = 1.6/N
  local ystep  = 1.8/N
  local xmin   = 0.3
  local ymin   = 0.4

  local function f(x,y)
    local x = xstep*x - xmin
    local y = ystep*y - ymin
    local z = x + exp(-60*x) - y
    if z > 0 then
      return '1'
    else
      return '0'
    end
  end

  potrace.setbitmap("mybitmap", potrace.contourplot(N,N,f))
\stopluacode

\startMPpage[offset=1dk]
  path p ; p := lmt_potraced [
    stringname = "mybitmap",
    value      = "1",
    tolerance  = 0.1,
    threshold  = 0.2,
    optimize   = true,
  ] ;
  p := p xsized 10cm ;
  draw p withpen pencircle scaled 5 withcolor "darkblue" ;
  drawpoints p withcolor "orange" ;
  drawpointlabels p ;
  p := subpath(6,19) of p ;
  draw p withpen pencircle scaled 3 withcolor "yellow" ;
\stopMPpage

Another thing, Hans added nocycle, so you can write your loop if you want
to use it as something like

      def compute_curve(suffix f)(expr xmin, xmax, xinc) =
          for x=xmin step xinc until xmax:
               (x,f(x)) ..
          endfor nocycle ;
      enddef;

Then you do not need to think about the first and/or last point separately.