[NTG-context] metapost: remapping colors of an SVG picture

[Hans Hagen]

On 2/18/2020 12:18 PM, mf wrote:
> Hello list,
> I tried this:
> 
> \starttext
> \startMPcode
> picture p ; p := lmt_svg [ filename = "sample.svg" ] ;
> draw p ;
> color black_rgb ; black_rgb := ( 0, 0, 0 ) ;
> color red_rgb   ; red_rgb   := ( 1, 0, 0 ) ;
> remapcolor( black_rgb, red_rgb ) ;
> recolor p ;
> draw p shifted ( bbwidth(p) + 5mm, 0) ;
> \stopMPcode
> \stoptext

\setupcolors[rgb=no]
\startMPcode
     draw lmt_svg [ filename = "sample.svg" ] ;
\stopMPcode


> but recolor gives an error:
> metapost log    > >> picture
> metapost log    > ! Wrong picture color model: redpart of grey object.
> metapost log    > <for(picture)> _f_:=(redpart(EXPR0),
> metapost log    > greenpart(EXPR0),bluepart(EXPR0));if.bou...
> metapost log    >
> metapost log    > repathed->...fi;fi;else:addto._p_.also.i;fi;endfor
> metapost log    > ;setbounds._p_.to._b_;_p_....
> metapost log    > <to be read again>
> metapost log    >                    ;
> metapost log    > <*> ...apcolor( black_rgb, red_rgb ) ; recolor p ;
> metapost log    > draw grayed p shifted ( b...
> metapost log    >
> 
> My goal is to remap colors of an SVG image, like this:
> 
> \starttext
> \startMPcode
> picture p ; p := lmt_svg [ filename = "sample.svg" ] ;
> draw p ;
> color black_rgb      ; black_rgb  := ( 0, 0, 0 ) ;
> cmykcolor black_cmyk ; black_cmyk := ( 0, 0, 0, 1 ) ;
> remapcolor( black_rgb, black_cmyk ) ;
> recolor p ;
> draw p shifted ( bbwidth(p) + 5mm, 0) ;
> \stopMPcode
> \stoptext
> 
> It relates to a recent thread in the mailing list about color management.
> It's an attempt to use inkscape-made SVG graphics inside a CMYK 
> workflow, without using color management.
> 
> Naturally it would work under some constraints, but there's a lot of 
> graphics you can do with Inkscape that falls inside these:
> - the number of colors in the input SVG should be limited, so that every 
> color gets converted
> - no embedded bitmaps, only vector graphics
> - perhaps some other constraints on gradients, transparency, etc.
> 
> A good alternative would be a remapper based on a custom lua function 
> like this:
> 
> function rgb2cmyk( r, g, b )
>    ...
>    return c, m, y, k
> end
> 
> Massi
> 
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> 
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