From: Florent Michel <florent.m42@gmail.com>
To: Hans Hagen <j.hagen@xs4all.nl>
Cc: mailing list for ConTeXt users <ntg-context@ntg.nl>
Subject: [NTG-context] Re: Functional shading with MetaPost / MetaFun
Date: Fri, 10 Jan 2025 07:17:21 +0000 [thread overview]
Message-ID: <CALOhoeL6Z2jbMkFJmaXR+ORQ5VrGv9ecLjpEkdhyj8AD5E0W=g@mail.gmail.com> (raw)
In-Reply-To: <868b0721-4789-4361-8b77-2b0c6ba4b270@xs4all.nl>
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Thank you very much Hans for this solution! Very interesting!
One thing I'm still not understanding (but maybe I'm missing something
obvious) is how to reproduce the ‘2D gradient’ effect of the PGFplots
version, where one corner is red, one is green, and one is red with the
shading inside the triangle interpolating smoothly between the three
colours (apologies if my previous email was not clear on that!). Running
the code you sent, I get a change of colour in the vertical direction only
(it looks nice, though!). Would you know if there is a way to achieve a
similar ‘2D gradient’ effect in MetaPost?
Here are two screenshots to whos the difference: https://imgur.com/a/SlJ4BiE
Thank you also for pointing me to the test suite! It seems to have many
useful examples!
Cheers,
Florent
On Fri, Jan 10, 2025, 00:17 Hans Hagen <j.hagen@xs4all.nl> wrote:
> On 1/9/2025 10:44 PM, Florent Michel wrote:
> > Hi,
> >
> > I was trying to see if I could use MetaFun / MetaPost as my main tool
> > for generating figures. I really like its close integration with
> > ConTeXt, speed, and expressivity making it, even for a beginner like me,
> > a pleasure to work with.
> >
> > One difficulty I encountered is for generating 2D functional shadings.
> > For instance, I am not sure what is the best way to reproduce the
> > following pgfplots picture in MetaPost:
> >
> > ____________________________________________________________
> > \usemodule[pgfplots]
> > \pgfplotsset{width=10cm,compat=1.18}
> > \usepgfplotslibrary{patchplots}
> >
> > \starttext
> >
> > \startTEXpage
> > \starttikzpicture
> > \startaxis[
> > hide axis=true,
> > axis lines=none,hide axis,
> > enlargelimits=false,
> > scale only axis,
> > clip bounding box=upper bound,
> > clip=true,
> > ]
> > \addplot[
> > patch,
> > shader=interp,
> > mesh/color input=explicit,
> > data cs=polar,
> > ]
> > coordinates {
> > (90,4) [color=red]
> > (210,4) [color=green]
> > (-30,4) [color=blue]
> > };
> > \stopaxis
> > \stoptikzpicture
> > \stopTEXpage
> > \stoptext
> > ____________________________________________________________
> >
> > One idea is to draw a sequence of triangles, each with a linear gradient:
> > ____________________________________________________________
> > \startMPpage
> > def LinearIntTriangle(expr pa, pb, pc, cola, colb, colc, n_iter) =
> > for i = n_iter step -1 until 1 :
> > path p;
> > x := i / n_iter;
> > y := (i - 0.5) / n_iter;
> > p := pa -- (pa + x * (pb - pa)) -- (pa + x * (pc - pa)) -- cycle;
> > sh := define_linear_shade(pa + y * (pb - pa), pa + y * (pc - pa),
> > (1 - y) * cola + y * colb, (1 - y) *
> > cola + y * colc);
> > fill p withshade sh;
> > endfor
> > enddef;
> >
> > color cola, colb, colc;
> > cola := (0,1,0);
> > colb := (1,0,0);
> > colc := (0,0,1);
> >
> > LinearIntTriangle((0,0), (50,50*(3**0.5)), (100,0), cola, colb, colc,
> > 30);
> > \stopMPpage
> > ____________________________________________________________
> >
> > or to use a slight variation (decomposing the triangle in three so that
> > the color is ‘right’) along each edge:
> > ____________________________________________________________
> > \startMPpage
> > def LinearIntTriangleB(expr pa, pb, pc, cola, colb, colc, n_iter) =
> > color centre_color;
> > centre_color := (cola + colb + colc) / 3;
> > LinearIntTriangle((pa + pb + pc) / 3, pa, pb, centre_color, cola,
> > colb, n_iter);
> > LinearIntTriangle((pa + pb + pc) / 3, pb, pc, centre_color, colb,
> > colc, n_iter);
> > LinearIntTriangle((pa + pb + pc) / 3, pc, pa, centre_color, colc,
> > cola, n_iter);
> > enddef;
> >
> > color cola, colb, colc;
> > cola := (0,1,0);
> > colb := (1,0,0);
> > colc := (0,0,1);
> >
> > LinearIntTriangleB((0,0), (50,50*(3**0.5)), (100,0), cola, colb,
> > colc, 10);
> > \stopMPpage
> > ____________________________________________________________
> >
> > This approach gives ‘good enough’ results for large values of n_iter.
> > However, the size of the generated pdf is somewhat larger than the
> > pgfplots version: for instance, the first version above generates a 5KiB
> > pdf versus 3.8KiB for the pgfplots version, while being somewhat less
> > accurate. Increasing n_iter to 100 gives a result which is (to my not-
> > very-good eyes) very close to the pgfplots version but the pdf size
> > increases to about 20KiB, and takes significantly longer to render using
> > MuPDF.
> >
> > I'd be grateful if someone could point me to a better way to generate
> > such shadings in MetaFun / MetaPost.
>
> these things can often be found in the test suite
>
> \startMPpage
> fill unittriangle rotated 90 xyscaled (200,100)
> withshademethod "linear"
> withshadecenteronefraction (0,1)
> withshadecentertwofraction (0,0)
> withshadestep (
> withshadefraction 0
> withshadecolors (green, green)
> )
> withshadestep (
> withshadefraction 0.75
> withshadecolors (blue, green)
> )
> withshadestep (
> withshadefraction 0.25
> withshadecolors (green, blue)
> )
> withshadestep (
> withshadefraction 1
> withshadecolors (green, red)
> )
> ;
> \stopMPpage
>
>
> -----------------------------------------------------------------
> Hans Hagen | PRAGMA ADE
> Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
> tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
> -----------------------------------------------------------------
>
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prev parent reply other threads:[~2025-01-10 7:19 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-01-09 21:44 [NTG-context] " Florent Michel
2025-01-10 0:17 ` [NTG-context] " Hans Hagen
2025-01-10 7:17 ` Florent Michel [this message]
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