Making metafun pictures fainter

Making metafun pictures fainter

[Gavin via ntg-context]

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Hi list,

I’m trying to produce figures like this, showing motion as a series of snap-shots, with the earlier moments more faint.


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Here are three examples of metafun code using transparency and “decorated".


\startMPinclusions
picture ball;
	ball := image(
		draw fullcircle scaled 1cm
      			withshademethod "circular"
			withshadecenter (.25,.25)
       		withcolor "lightgray" shadedinto black;
		draw fullcircle scaled 1cm withpen pencircle scaled 0.8pt;);
\stopMPinclusions

\starttext

\startMPpage
draw decorated (draw ball) withtransparency (1,.2);
draw decorated (draw ball shifted (1cm, 0cm)) withtransparency (1,.4);
draw decorated (draw ball shifted (4cm, 0cm)) withtransparency (1,.6);
draw ball shifted (9cm, 0cm);
\stopMPpage

\startMPpage
draw decorated (draw ball) withtransparency (1,.2);
draw decorated (draw ball shifted (0.25cm, 0cm)) withtransparency (1,.4);
draw decorated (draw ball shifted (1cm, 0cm)) withtransparency (1,.6);
draw ball shifted (2.25cm, 0cm);
\stopMPpage

\startMPpage
draw decorated (draw ball) withtransparency (1,.2);
filldraw fullcircle scaled 1cm withcolor white withpen pencircle scaled 0.8pt shifted (0.25cm, 0cm);
draw decorated (draw ball shifted (0.25cm, 0cm)) withtransparency (1,.4);
filldraw fullcircle scaled 1cm withcolor white withpen pencircle scaled 0.8pt shifted (1cm, 0cm);
draw decorated (draw ball shifted (1cm, 0cm)) withtransparency (1,.6);
draw ball shifted (2.25cm, 0cm);
\stopMPpage

\stoptext


Transparency alone does a good job if the objects don’t overlap (first example). If they do overlap, it’s kind of a mess (second example). I get a much better result if I put a white circle behind each transparent ball so that they look lighter without showing the earlier objects through  them (third example). However, that solution is going to be difficult for more complex shapes. I tried drawing the shape using “redecorated () withcolor white", to draw the white background object, but only the outline appears white. The circular shading disappears altogether, leaving the ball fill area transparent.

I also tried to use something like “softened .8” but could not get that to produce any output.

Any ideas?

Thanks!
Gavin

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