[pgf-tikz] RGB colour model for plain TeX users

[Henri Menke]

>> if it does, then wonderful.  But if it does not, and if the addition
>> of \pgfutil at emu@cmyk were to suggest that it does, then that could
>> lead users to believe that they are generating CMYK graphics when
>> they are in fact generating RGB.

Actually right now the gray color model in plain TeX also converts to
RGB, which I am now realizing, is very suboptimal, because it means that
even though users may be using the gray colormodel, it is impossible to
generate "true" monochrome documents.

Kind regards,
Henri

On 11/17/19 9:05 AM, Jim Diamond wrote:
> Hi Phil,
> 
> On Sat, Nov 16, 2019 at 19:46 (+0000), Taylor, P wrote:
> 
>> Jim Diamond wrote:
>>> [A] couple of minutes of playing
>>> around gave me
> 
>>> \def\pgfutil at emu@cmyk#1#2,#3,#4,#5\@nil
>>> {
>>>      \pgfmathsetmacro{\@red@}{(1 - #2) * (1 - #5)}%
>>>      \pgfmathsetmacro{\@green@}{(1 - #3) * (1 - #5)}%
>>>      \pgfmathsetmacro{\@blue@}{(1 - #4) * (1 - #5)}%
>>>      \expandafter\edef\csname\string\color@#1\endcsname
>>>      {\noexpand\xcolor@ {}{}{rgb}{\@red@,\@green@,\@blue@}}
>>> }
> 
>>> I realize that converting cmyk to rgb is a lot more contentious than
>>> converting RGB to rgb,
> 
>> I think that it is more than "contentious"; I think that it is genuinely 
>> dangerous.  I don't know whether PGF supports CMYK —
> 
> It does for pdflatex users.  If I ever find the time, I will try to
> figure out what the difficulty for plain TeX is.  But I am not holding
> my breath.
> 
>> if it does, then wonderful.  But if it does not, and if the addition
>> of \pgfutil at emu@cmyk were to suggest that it does, then that could
>> lead users to believe that they are generating CMYK graphics when
>> they are in fact generating RGB.
> 
> This is true.  As mentioned earlier (not in your quote here) part of
> the issue is whether it is better to give plain TeX users a
> questionable approximation (so, at least, things look OK on their
> screens) or to tell plain TeX users that they can't see TikZ/pgf
> images that use cmyk at all.  Personally, I prefer the poor man's
> approximation over nothing, but others may disagree (as, apparently,
> Henri did).
> 
> On the other hand, if we added (for plain TeX users) a macro called
>         \IAmOKWithAPoorApproximationOfCmyk
> and the above \pgfutil at emu@cmyk macro was only defined after calling
>         \IAmOKWithAPoorApproximationOfCmyk
> then innocent plain TeX users wouldn't be fooled into thinking they
> had real cmyk.
> 
> Cheers.
>                                 Jim
>