MetaPost 中是否有制作尺寸线的宏？我在 Metafun 手册和其他手册中没有找到任何有用的东西。确切地说，我想绘制类似下图中的“长度”尺寸线：
答案1
我已经看过了各种贡献的宏可在 CTAN 上使用，但我看不到任何专门用于尺寸箭头的东西，因此这是我的努力。
def drawdimarrow expr p = _apth:=p; _dimdarr enddef;
def _dimdarr text t =
save dimbar; path dimbar;
(_x, _y) = dir 1/2 ahangle;
dimbar = (down--up) scaled (ahlength / _x * _y * 1.2);
forsuffixes @=0, infinity:
draw dimbar rotated angle direction @ of _apth shifted point @ of _apth withpen currentpen t;
endfor
drawdblarrow _apth withpen currentpen t
enddef;
要使用此功能，请将上述行粘贴到您的 MP 文件中，或将它们放在本地 TEXMF 路径中某个单独的文件中，然后使用input它来包含它。然后您可以执行以下操作：
beginfig(1);
ahlength := 6; ahangle := 30;
drawdimarrow (origin -- 100 right rotated 30);
endfig;
ahlength您可以使用常用的内部参数和自定义箭头的大小和形状ahangle。这也应该可以很好地与麻雀，这样你就可以得到不同风格的箭头：
input mparrows
beginfig(1);
setarrows(open);
drawdimarrow (origin -- 100 right rotated 30);
endfig;
以下是在更复杂的绘图中使用它们的一种方法：
input mparrows
beginfig(1);
setarrows(open);
path a;
a = unitsquare xscaled 89 yscaled 55 rotated 10;
draw a;
path d[];
d1 = subpath (1, 2) of a shifted (10 unitvector(direction 3/2 of a rotated -90));
d2 = subpath (2, 3) of a shifted (10 unitvector(direction 5/2 of a rotated -90));
drawdimarrow d1;
drawdimarrow d2;
picture p[];
p1 = thelabel("55", point 1/2 of d1); unfill bbox p1; draw p1;
label.top("89", point 1/2 of d2);
endfig;
扩展箭头宏来处理标签可能很诱人，但我认为这是个错误。让每个宏只做一件事更符合普通 MP 的精神。因此，我建议使用普通工具来添加标签，以便沿路径放置标签；上面显示了两种可能性。
答案2
以下是实现此操作的方法：
leaderextension:=0.25cm;
leadergap := 0.15cm;
def _draw_dim(expr A, B, extang, leadang, obliqueang, textang, offset, Text) =
% Generic function to draw extension lines
begingroup;
pair C, D, E, F, G, H;
pair CA;
pen cpen;
cpen := currentpen;
pickup defaultpen;
C := A + offset * dir(extang);
CA := dir(angle(C-A));
% Project B-A in the direction of leadang
D := C + (
((xpart (B-A)) * (xpart (dir(leadang))))
+ ((ypart (B-A)) * (ypart (dir(leadang)))))
* dir(leadang);
% Use C-A as the angle because it's set by offset which is non-zero hopefully
E := C + leaderextension * CA;
F := D + leaderextension * CA;
G := A + leadergap * CA;
H := B + leadergap * CA;
picture lab;
picture arr;
arr := image(drawdblarrow (0,0)--(abs(D-C),0););
draw ((arr slanted (sind(obliqueang)/cosd(obliqueang))) rotated angle(D-C)) shifted C;
lab:=(thelabel(Text, (0,0)) slanted (sind(obliqueang)/cosd(obliqueang))) rotated textang shifted 0.5[C,D];
draw G--E;
draw H--F;
unfill bbox lab; draw lab;
pickup cpen;
endgroup;
enddef;
def draw_aligned_dim(expr A, B, Text, offset) =
_draw_dim(A, B, (angle(A-B) + 90), (angle(A-B)), 0, 0, offset, Text);
enddef;
def draw_vertical_dim(expr A, B, Text, offset) =
_draw_dim(A, B, (0), -90, 0, 0, offset, Text);
enddef;
def draw_horizontal_dim(expr A, B, Text, offset) =
_draw_dim(A, B, (90), 0, 0, 0, offset, Text);
enddef;
def draw_oblique_dim(expr A, B, Text, offset, measang, extang, alt) =
if alt:
_draw_dim(A, B, extang, measang, extang, measang, offset, Text);
else:
_draw_dim(A, B, extang, measang, -extang, measang, offset, Text);
fi;
enddef;
def draw_angle_dim(expr A, B, C, Text, offset) =
begingroup;
cpen := currentpen;
pickup defaultpen;
pair D, E, F, G, H, I, J;
numeric ang[];
ang1 := angle(A-C);
ang2 := angle(B-C);
ang3 := (ang1 + ang2) / 2.0;
D := A + offset * dir(ang1);
E := C + (abs (D - C)) * dir(ang2);
ang4 := angle(D-A);
ang5 := angle(E-B);
F := D + leaderextension * dir(ang4);
G := E + leaderextension * dir(ang5);
H := A + leadergap * dir(ang4);
I := B + leadergap * dir(ang5);
J := C + (abs(D - C) * dir(ang3));
draw H--F;
draw I--G;
path p;
p := D{dir(ang1 + 270)} .. J{dir(ang3 + 270)} .. E{dir(ang2 + 270)};
drawdblarrow p;
picture lab;
lab := thelabel(Text, J);
unfill bbox lab; draw lab;
pickup cpen;
endgroup;
enddef;
def draw_radial_dim(expr C, r, ang, Text, offset) =
begingroup;
pair A, B, D;
A := C + dir(ang) * r;
B := C + dir(ang) * (r + offset);
drawarrow (if offset > 0 : B else: C fi) --A;
picture lab;
lab := thelabel(Text, B);
unfill bbox lab; draw lab;
endgroup;
enddef;
以下是一些用法示例：
beginfig(0)
begingroup;
pair v[];
v[0] := (0,0);
v[1] := (2cm, 0);
v[1] := v[1] rotated 30;
v[2] := v[1] rotated 120 shifted v[1];
v[3] := v[1] rotated 120;
v[4] := v[1] rotated 240;
v[5] := v[1] shifted v[4];
v[6] := v[3] shifted v[4];
draw v[0]--v[1]--v[2]--v[3]--cycle;
draw v[0]--v[4]--v[5]--v[1];
draw v[4]--v[6]--v[3];
draw_oblique_dim(v[5], v[1], btex vert etex, 2cm, 90, 30, true);
draw_oblique_dim(v[5], v[1], btex vert etex, 2cm, 90, -30, true);
draw_oblique_dim(v[4], v[5], btex flat etex, 2cm, 30, -30, true);
draw_oblique_dim(v[6], v[4], btex flat etex, -2cm, -30, 30, true);
draw_oblique_dim(v[4], v[0], btex vert etex, 2cm, 90, -30, false);
draw_oblique_dim(v[3], v[0], btex vert etex, -2cm, -30, 30, false);
endgroup;
endfig;
beginfig(1)
begingroup;
numeric u;
u := 2in;
pair P[];
P0 = (0,0);
P1 = (1u, 0);
P2 = (0.4u, 0.7u);
draw P0--P1--P2--cycle;
draw_aligned_dim(P0, P1, btex $a$ etex, .2u);
draw_aligned_dim(P1, P2, btex $b$ etex, .2u);
draw_aligned_dim(P2, P0, btex $c$ etex, .2u);
draw_angle_dim(P0, P2, P1, btex $\theta$ etex, -.5u);
label(btex $c^2 = a^2 + b^2 - 2 a b \cos{\theta}$ etex, (0.5u, -.4u));
endgroup ;
endfig;
beginfig(2)
begingroup;
path c;
numeric radius;
radius := 0.75in;
c := fullcircle scaled (2 * radius);
draw fullcircle scaled .1in;
draw c;
draw_radial_dim((0,0), radius, 120, btex $r$ etex, -0.25in);
pair P[];
P1 := (point 5 of c);
P2 := (point 8 of c);
dotlabel.lft(btex $A$ etex, P1);
dotlabel.rt(btex $B$ etex, P2);
path beam;
beam := P1--P2;
draw beam withpen pencircle scaled 1bp;
dotlabel.ulft(btex $dog$ etex, 0.3[P1,P2]);
draw_aligned_dim(P1, P2, btex $l$ etex, 0.75in);
endgroup;
endfig;
