def l_slope (expr P,S)(text Q) = 
 %show Q;
T:=identity;
numeric dirs[];
numeric lengths[];
for i=Q:
  dirs[redpart i]:=greenpart i;
  lengths[redpart i]:=bluepart i;
endfor;  
li:=length(P); % last
alw_perpendicular:=true;
for i=0 upto li:
  if unknown dirs[i]: dirs[i]:=-1; 
  else: 
    if dirs[i]>-1:
      dirs[i]:=((90-dirs[i]) - angle(thdir(P,i))) mod 360; 
      alw_perpendicular:=false;
    fi;
  fi;
  if unknown lengths[i]: lengths[i]:=-1; fi;
endfor;
  %for i=0 upto li: show dirs[i]; endfor;
ni:=0; % next
pi:=0; % previous
for i=0 upto li:
  d:=dirs[i];
  if d=-1:
    if (i=0) or (i=li):
      dirs[i] := angle(thdir(P,i) rotated 90) mod 360;
pi:=i;
    else:
      if ni<=i:
  for j=i upto li:
          ni:=j;
    exitif dirs[j]>-1;
  endfor;
fi;
w:=arclength(subpath(pi,i) of P) / 
   arclength(subpath(pi,ni) of P);
dirs[i]:=w[dirs[pi],dirs[ni]];
   %        if (dirs[i]-angle(thdir(P,i))) mod 360>180:
   %          dirs[i]:=w[dirs[ni],dirs[pi]];
   %	  message("*******");
   %        fi;
   fi;
  else:
    pi:=i;
  fi;
endfor;
  %for i=0 upto li: show dirs[i]; endfor;
ni:=0; % next
pi:=0; % previous
for i=0 upto li:
  l:=lengths[i];
  if l=-1:
    if (i=0) or (i=li):
      lengths[i] := 1cm; % should never happen!
thwarning("slope width at the end point not specified");
pi:=i;
    else:
      if ni<=i:
  for j=i+1 upto li:
          ni:=j;
    exitif lengths[j]>-1;
  endfor;  
fi;
w:=arclength(subpath(pi,i) of P) /   
   arclength(subpath(pi,ni) of P);
lengths[i]:=w[lengths[pi],lengths[ni]];
pi:=i;
    fi;
  else:
    pi:=i;
  fi;
endfor;
  % for i=0 upto li: show lengths[i]; endfor;
T:=identity;
boolean par;
cas := 0.3u;
krok := 0.7u;
dlzka := (arclength P);
if dlzka>3u: dlzka:=dlzka-0.6u fi;
mojkrok:=adjust_step(dlzka,1.4u) / 5;
pickup PenD;
par := false; 
forever:
  t := arctime cas of P;
  if t mod 1>0:  % not a key point
    w := (arclength(subpath(floor t,t) of P) / 
          arclength(subpath(floor t,ceiling t) of P));
    if alw_perpendicular:
      a := 90;
    else:
      a := w[dirs[floor t],dirs[ceiling t]];
    fi;
    l := w[lengths[floor t],lengths[ceiling t]];
  else:
    if alw_perpendicular:
      a := 90;
    else:
      a:= dirs[t];
    fi; 
    l:=lengths[t];
  fi;    
  a := a + angle(thdir(P,t));    
  thdraw (point t of P) -- 
    ((point t of P) + if par: 0.333 * fi l * unitvector(dir(a)));
  cas := cas + mojkrok;
  par := not par;
  exitif cas > dlzka + .3u + (krok / 3);  % for rounding errors
endfor;
if S = 1: pickup PenC; draw P fi;
   %pickup pencircle scaled 3pt;
   %for i=0 upto li: draw point i of P; endfor;
  enddef; 

Next line was modified:

  mojkrok:=adjust_step(dlzka,1.4u) / 5;

Original:

 mojkrok:=adjust_step(dlzka,1.4u) / 2;