Am 25.11.2025 um 13:05 schrieb Mason, Roger via ntg-context:
Hello,
This code:
\startMPcode
u = 2cm; % unit length;
xmax = 1.5 ; ymax = 1.5 ; % axes parameters
path circle; circle = fullcircle scaled 2u; draw circle withcolor blue;
x = -1.0 ; % any value between -1 and 1
len = 3u; % tangent length
t = (acos x)/45 ; % point-node of fullcircle corresponding to x
pair I; I = point t of circle ;
if (x <> 0.0) and (abs(x) <> 1.0): draw origin -- I ; fi ;
% tangent on the upper part of circle
pair v ; v = unitvector I rotated 90 ;
draw I - 0.5len*v -- I + 0.5len*v withcolor red ;
draw I withpen pencircle scaled 3bp;
drawarrow (-xmax*u, 0) -- (xmax*u, 0) ; % horizontal axis
drawarrow (0, -ymax*u) -- (0, ymax*u) ; % vertical axis
label.bot(btex $x$ etex, (xmax*u, 0)) ;
\stopMPcode
Should draw a circle with a tangent at x = -1 parallel to y. However,
the point of tangency is shown at approximately x = 1, y = 0.01 or so.
I got the code from a metapost example:
u = 2cm; % unit length;
xmax = 1.5 ; ymax = 1.5 ; % axes parameters
beginfig(1);
path circle; circle = fullcircle scaled 2u; draw circle withcolor blue ;
x = -1.0 ; % any value between -1 and 1
len = 3u; % tangent length
t = (acos x)/45 ; % point-node of fullcircle corresponding to x
pair I; I = point t of circle ;
if (x <> 0) and (abs(x) <> 1): draw origin -- I ; fi ;
% tangent on the upper part of circle
pair v ; v = unitvector I rotated 90 ;
draw I - 0.5len*v -- I + 0.5len*v withcolor red ;
draw I withpen pencircle scaled 3bp;
drawarrow (-xmax*u, 0) -- (xmax*u, 0) ; % horizontal axis
drawarrow (0, -ymax*u) -- (0, ymax*u) ; % vertical axis
label.bot(btex $x$ etex, (xmax*u, 0)) ;
endfig;
end;
The original code generates the expected output when run using:
mpost --mem=metafun --numbersystem="double" tangent.mp
The result of acos is different in LMTX and MkIV.
\starttext
\startMPcode
label(acos(-1),origin);
\stopMPcode
\stoptext
LMTX: 3.1415926535897931
MkIV: 180
Wolfgang
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