Poggendorff Illusion

Manipulate[
α=Pi/3;
a=6;
b=1.3;
vO={0,0};
vA={rem +5,-b};
vB={rem+ 5,b};
vC={-5,b};
vD={-5,-b};
vF={-a Sin[α], -a Cos[α]};
vE={a Sin[α], a Cos[α]};
rect={RGBColor[0.3,0.5,0.8],Polygon[{vA,vB,vC,vD}]};
FO={Red,Thickness[0.02],Line[{vF,vO}]};
OE={Red,Thickness[0.02],Translate[Line[{vO,vE}],{adjust,0}]};
Graphics[{FO,OE,rect},ImageSize->{380,300},PlotRange->{{-7,7},{-5,5}}],
{{adjust,-4,"adjust upper red line"},-4,2},{{rem,0,"check by removing rectangle"},-9,0},TrackedSymbols->Manipulate]

THIS NOTEBOOK IS THE SOURCE CODE FROM

"Poggendorff Illusion" from The Wolfram Demonstrations Project
 http://demonstrations.wolfram.com/PoggendorffIllusion/

Contributed by: Sándor Kabai

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