[NTG-context] enhancing MetaPost presentation progress graphics

[Henning Hraban Ramm <texml@fiee.net>]

Hi,
I’d like a presentation progress indicator that looks like a boat on 
waves – and if it works, it can also become a bike on hills or a mars 
rover on dunes. But my math skills are lacking.

Below’s a mostly working draft.

* I find the waves not yet very convincing. Would it make sense to use a 
randomized sinus? (How?)

* The boat should sit on the middle wave and be rotated by the current 
slope. I know I need the “time” of the upper curve and get the 1st 
derivation, but how?

"""
\setuppapersize[SW]

\startuseMPgraphic{Waves}
numeric height,stops,yoffset;
stops := 10;
height := OverlayHeight/10;
path wave;

draw (0,0)--(OverlayWidth,0)--(OverlayWidth,OverlayHeight) withcolor 
white withpen pencircle scaled 0.01;

for j=1 upto 3:
   definecolor [name="Sea", y=(j/20), c=2*(j/10), m=(j/20)];
   yoffset := height/(j*2);
   wave := (0,0)--(0,yoffset)...
   for i=1 upto stops:
     (OverlayWidth*i/(stops+1), (yoffset) randomized (height/2)) ...
   endfor
   (OverlayWidth,yoffset)--(OverlayWidth,0)--cycle;
   fill wave withcolor "Sea";
endfor;

pair pos;
pos := (OverlayWidth * RealPageNumber/NOfPages, height/2 randomized 2);
path ship;
ship := (0,10)---(70,10)...(60,0)---(10,0)...cycle;

fill ship xysized (10,5) shifted pos rotatedaround (pos, 15) withcolor red;
\stopuseMPgraphic

\defineoverlay[Waves][\useMPgraphic{Waves}]

\setupbackgrounds[state=repeat]
\setupbackgrounds[page][background={Waves}]

\starttext

\dorecurse{10}{\recurselevel\page}

\stoptext
"""

Hraban
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