[metapost] rfc: consistent color values

[Jens-Uwe Morawski]

Hello

Since Taco told about a MP 1.0 release later this year, IMO
the following problem should be discussed before.

In the example below you can see that MP replaces
silently all negative color values with 0.

%%%
picture P ; P := nullpicture ;

addto P doublepath origin withcolor (-1,0.5,0.5) ;
addto P doublepath origin withcolor (-1,-0.5,0.5) ;
addto P doublepath origin withcolor (-1,0.5,-0.5) ;
addto P doublepath origin withcolor (-1,-0.5,-0.5) ;

for I within P:
	show (redpart I, greenpart I, bluepart I) ;
endfor;
end
%%%

IMO MP should either disallow negative color values
(err message) or should not change the values
to 0 as long as the values are accessible on macro level
(before shipout).
The latter would even allow tagging of objects, for example
to apply a transformation to objects with redpart=-1
when the transformation was still unknown when the
object has been drawn.

Is this possible to change? Makes it sense?
Ideas? Comments?

TIA

Jens