Kreisbogen — Georg Nees, 1969

Dissertation 1969 · p. 135

ALGOL 60

1 'BEGIN'
2 'COMMENT' DEMONSTRATION KREISBOGEN.,
3 'INTEGER' I., 'REAL' U, V, R.,
4 OPEN(X,Y), JI1.=JS1., JI2.=JS2.,
5 JA1.=0., JE1.=130., JA2.=-80., JE2.=80.,
6 'FOR' I.=1 'STEP' 1 'UNTIL' 200 'DO'
7 'BEGIN' ANF ..
8   U.=J1., V.=J2.,
9   'IF' ABS(V) 'LESS' 1.0 'THEN' 'GOTO' ANF .,
10  R.=.5*(U*U+V*V)/V .,
11  LEER(U,V), ZIRK(0.0,R,R,-U,V)
12 'END',,
13 CLOSE
14 'END' DEMONSTRATION KREISBOGEN.,

This page · drawing kernel

JavaScript

for (let I = 1; I <= 200; I++) {
  let U, V;
  do { U = J1(0, 130); V = J2(-80, 80); }
  while (Math.abs(V) < 1);
  const R = 0.5 * (U * U + V * V) / V;
  LEER(U, V);
  ZIRK(0, R, R, -U, V);
}
// Circle through (U,V) and the origin,
// centre (0,R). Sheet 90° like the scan.

Approximate rendering. The plate’s seeds are not recorded; JS1 and JS2 are those of p. 122. Section 2.3 introduces language G3: ZIRK(M,N,R,P,Q) draws a circular arc from the current stylus to P,Q. Centre M,N, radius the absolute value of R; non-negative R runs clockwise. Fig. 13: 200 random points U,V (seeds JS1, JS2 as on p. 122), circle through the origin, arc to −U,V. |V| < 1 is rejected, or the radius would explode. Sheet 90° clockwise like the scan: the long arcs stand upright, the two eyes left and right of the origin.