Re: shading in meta[post/play] & PDF

[h h extern <pragma@wxs.nl>]

Mojca Miklavec wrote:

> Problem description:
> I have a computer program, which carries the data about the grid points 
> on a sphere (possibly with different colors defined in these points) and 
> how the points are connected with each other to form a grid. I would 
> take these triangles (or quadrilaterals), draw them transformed on the 
> plane (with curves, not with straight lines) and fill them with the 
> appropriate shading pattern (colors defined in corners of the triangles 
> (darker according to normal vector) simply interpolated between the 
> three points). Yet another possibility would probably be to use 
> function-based shading and define a function which knows how the points 
> have to be shaded on the sphere, but the latter seems more complicated 
> and less universal.
> 
> Problem reduction:
> Fill a mp closed path with any more complex shading.
> 
> So I would probably need Type 6 or 7 shading (Type 4 would be OK as the 
> picture can be clipped within metapost, so that the resulting sphere 
> borders are not composed of straight lines; the articles on 
> http://www.math.ubc.ca/people/faculty/cass/graphics/text/www/ which 
> inspired me to do something so crazy actually use Type 4 shading). Type 
> one is probably the most general one, so it should be OK too, just 
> complicated to do.
> 
> However: I'm not sure if, say, a (sampled) function to be passed to PDF 
> can be specified in metapost elegantly.

i searched my machine extensively but it seems that i lost some code that could 
be used as starting point for that;

> Page 746 of PDF Reference, version 1.6 also defines an interesting new 
> feature: 3D objects (3D Artwork as they call it), but I haven't found 
> anything else about that.

there is a section about shading, the function numbers differ;

> In dvipdfm there's an example of using:
> \special{pdf: bt rotate 90 xscale 2.0 }
> 
> These lines are to be used in TeX. MetaPost also knows a "special" 
> command, I just don't know how to use it. (This is probably the 
> "communication channel" to PDF when trying to implement some more exotic 
> shadings.)

take a look at supp-mpe.tex, all we need is some variant on

\defineMPspecial{31}
   {\startMPshading{16}% type 3
    \setMPshadingcolors{4}{5}{6}{10}{11}{12}%
    \immediate\pdfobj
      {<</FunctionType 2
         /Domain [\gMPs1 \gMPs2]
         /C0 [\MPshadeA]
         /C1 [\MPshadeB]
         /N \gMPs3>>}%
    \immediate\pdfobj
      {<</ShadingType 3
         /ColorSpace /\MPshadeC\space
         /Function \the\pdflastobj\space 0 R
         /Coords [\gMPs7 \gMPs8 \gMPs9 \gMPs{13} \gMPs{14} \gMPs{15}]
         /Extend [true true]>>}%
    \stopMPshading}

(plus some more depending on what parameters need to be passed)

> I glimpsed the mp-spec.mp file, but I didn't yet understand how the 
> parameters are passed to PDF.
> 
>> [more a context mailing list issue than a metapost list issue] 
> 
> So I left the metapost group out. Actually, 90% on the people here 
> probably think I posted that to the wrong group anyway (in the sense of 
> "Are you sure you didn't want to post that question to the 3D studio max 
> mailing list?" :) The example above is probably not the most descriptive 
> one, but having the possibility to add some more advanced shading 
> patterns would be great.

if you can locate the right stuff in the pdf ref manual we can see what can be 
done (off list); i can add more 'special' handlers to the mp to pdf converted

Hans


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