[NTG-context] transparent colors

Meer, Hans van der H.vanderMeer at uva.nl
Thu May 19 22:35:36 CEST 2016

With \definecolor[colorname][r=,g=,b=] one defines a color of name "colorname" for use in ConTeXt and in Metapost.
A transparentcolor comes with \definecolor[transparentcolorname][r=,g=,b=,a=,t=]

Problem: how to define a transparent color with \definetransparentcolor[#1][#2][#3]{...}. 
To be called for example as \definetransparentcolor[transparentcolorname][colorname][0.5] for color "colorname" with transparency 0.5.

I tried to do this by intercepting from \definecolor the vales "r=,g=,b=" in a macro \colornamergb to be called as \csname..\endcsname for the varying colorname and feeding that to a definecolor-variant. However, the parsing of  r=,g=,b= then goes wrong (\futurelet balking).

Question: is there a direct way to derive a transparent colorvariant from a parent one without transparency? The usefulness of this comes from the fact that for things like \definecolor[lighthoneydew][r=.894117,g=.95,b=.894117] the numbers need not be feed more than once.

Hans van der Meer

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