Peter,
Thanks for trying to help. Because I solved the problem in another way you
might be interested or even benefit from it.
The underlying problem was drawing a border around a picture of given
dimensions. The border being either colored or left uncolored. The border color
should come from a withcolor modifier, with the uncolored case done simply by
leaving this out, for example:
borderinsideframe(u,v,2g);
and borderinsideframe(u,v,2g) withcolor red;
It turns out this can be done with an unfill. I learned that a bare unfill
erases, but add a withcolor modifier and the effect is a coloring. Thus the
trick is:
def bordercolorframe (expr w, h, off, clr) text modifier =
unfill (-off,-off)--(w+off,-off)--(w+off,h+off)--(-off,h+off)--cycle
modifier;
fill (0,0)--(w,0)--(w,h)--(0,h)--cycle withcolor clr
enddef;
The omission of a semicolon after the last statement of the def allows one to
add in a withcolor somecolor without triggering an error.
Hope this will be of use to someone.
Hans van der Meer
On 21 okt. 2011, at 17:13, Peter Rolf wrote:
> Am 21.10.2011 15:05, schrieb Hans van der Meer:
>> I cannot find the answer to the following question in Knuth's Metafont book.
>> The following definition with a trailing text argument:
>>
>> def mydef (expr a, b, c) text modifier =
>> if <modifier = empty>:
>
> Have you already tried
>
> if modifier = "" :
>
>> else:
>> fi
>> enddef;
>>
>> How can I do the switch on an empty or nonempty modifier argument? It is not
>> a string because calling its length fails with an error.
>> I must do the switch, because "fill modifier" with an empty "modifier" gives
>> horrible results.
>>
>> Usage would be calls like:
>> mydef(1,2,3) withcolor green; % with modifier text
>> mydef(1,2,3); % without modifier text
>>
>> Thanks in advance.
>> Hans van der Meer
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