http://fr.wikipedia.org/wiki/Drawing_eXchange_Format
http://www.autodesk.com/techpubs/autocad/acad2000/dxf/line_dxf_06.htm

$ perl -e 'use Math::Trig; $r0=1.5; $dr=3.7; $da=10; $maxa=1440; $w=0.2; print "0\nSECTION\n2\nENTITIES\n"; $k=1; for($a=0;;){$r=$r0+$a/360*$dr+$k*$w; $x=$r*cos($a/180*pi()); $y=$r*sin($a/180*pi()); if(defined($x0)&&defined($y0)){ printf "0\nLINE\n10\n%.6f\n20\n%.6f\n30\n0\n11\n%.6f\n21\n%.6f\n31\n0\n",$x0,$y0,$x,$y} $x0=$x; $y0=$y; if($k==1&&$a>=$maxa){$k=-1}else{last if($k==-1&&$a<=0); $a+=$k*$da}} print "0\nENDSEC\n0\nEOF\n"' > a16.dxf

$ perl -e 'use Math::Trig; $r0=1.5; $dr=3.7; $r0+=$dr/2; $da=10; $maxa=1440; $w=0.2; print "0\nSECTION\n2\nENTITIES\n"; $k=1; for($a=0;;){$r=$r0+$a/360*$dr+$k*$w; $x=$r*cos($a/180*pi()); $y=$r*sin($a/180*pi()); if(defined($x0)&&defined($y0)){ printf "0\nLINE\n10\n%.6f\n20\n%.6f\n30\n0\n11\n%.6f\n21\n%.6f\n31\n0\n",$x0,$y0,$x,$y} $x0=$x; $y0=$y; if($k==1&&$a>=$maxa){$k=-1}else{last if($k==-1&&$a<=0); $a+=$k*$da}} print "0\nENDSEC\n0\nEOF\n"' > a17.dxf