As I only use one transparency method anyway I could also fix this by passing a numeric instead of a pair and use the standard withtransparency (I don’t like overriding a standard MetaFun method, one never knows what might break in future updates).

vardef Foo( expr w, h, fillColor, fillTransparency) =
  show "FOO:", w, h, fillColor, fillTransparency;
  save pic; picture pic;
  save circ; path circ;
  circ := (fullcircle xyscaled (h,h));
  pic := image (
    fill (circ shifted (w-h/2,-h/2)) withcolor fillColor withtransparency (1,fillTransparency);
  );
  pic
enddef;

picture pic;
pic := Foo( 150, 50, (0.686,1.000,1.000), 0.5);
draw pic;
\stopMPpage
\stoptext


> On 9 Apr 2020, at 18:06, Hans Hagen <j.hagen@xs4all.nl> wrote:
> 
> On 4/9/2020 4:58 PM, Gerben Wierda wrote:
>> \starttext
>> \startMPpage[instance=doublefun]
>> vardef Foo( expr w, h, fillColor, fillTransparency) =
>>   show "FOO:", w, h, fillColor, fillTransparency;
>>   save pic; picture pic;
>>   save circ; path circ;
>>   circ := (fullcircle xyscaled (h,h));
>>   pic := image (
>>     fill (circ shifted (w-h/2,-h/2)) withcolor fillColor withtransparency fillTransparency;
>>   );
>>   pic
>> enddef;
>> pic := Foo( 150, 50, (0.686,1.000,1.000), (1,1.000));
>> draw pic;
>> \stopMPpage
>> \stoptext
> 
> Maybe this definition is better then but I need to test it more
> 
> \starttext
> \startMPpage[instance=doublefun]
> 
> def withtransparency expr t =
>    if pair t :
>        withprescript "tr_alternative="  & decimal transparency_alternative_to_number(xpart t)
>        withprescript "tr_transparency=" & decimal ypart t
>    else :
>        mfun_with_transparency (transparency_alternative_to_number(t))
>    fi
> enddef ;
> 
> def mfun_with_transparency (expr a) expr t =
>    withprescript "tr_alternative="  & decimal a
>    withprescript "tr_transparency=" & decimal t
> enddef ;
> 
> vardef Foo ( expr w, h, fillColor, fillTransparency) =
>  image (
>    fill fullcircle xyscaled (h,h) shifted (w-h/2,-h/2)
>        withcolor fillColor
>        withtransparency fillTransparency
>    ;
>  )
> enddef;
> 
> vardef FooX ( expr w, h, fillColor) =
>  image (
>    fill fullcircle xyscaled (h,h) shifted (w-h/2,-h/2)
>        withcolor fillColor
>        withtransparency (normaltransparent, .5)
>    ;
>  )
> enddef;
> 
> vardef FooY ( expr w, h, fillColor) =
>  image (
>    fill fullcircle xyscaled (h,h) shifted (w-h/2,-h/2)
>        withcolor fillColor
>        withtransparency normaltransparent .5
>    ;
>  )
> enddef;
> 
> draw Foo  ( 350, 50, (0.5,1,0), (1,.5));
> draw FooX ( 250, 50, (1,0.5,1));
> draw FooY ( 150, 50, (1,1,0.5));
> 
> \stopMPpage
> \stoptext
> 
> 
> -- 
> 
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>                                          Hans Hagen | PRAGMA ADE
>              Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
>       tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
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