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如何在渐近线中填充平面三维路径？

tag-icon • tag-icon 3d asymptote
如何在渐近线中填充平面三维路径？

在 Asymptote 中是否存在fill适用于平面 3D 路径的命令等效项？

我以为surface这样做了：我期望命令

fill(project(p))

和

draw(surface(p))

对于任何平面路径 3 ，始终产生相同的结果p。但是，这对于我正在处理的路径（在下面的代码中调用）不起作用tile(identity4)：基本上它是一些非凸形状，并且 3D 表面包含其凸包未被路径包围的部分。

我没能写出一个简单的 MWE。不管怎样，这是我的代码；恐怕理解起来有点复杂！我删除了与问题无关的所有定义；但当我试图进一步简化它时，它突然在合理的时间内停止编译，所以我放弃了这件事。

你可能会问为什么我不直接使用前一个命令。问题是我的图片中还有其他数据，而且当我绘制 2D 图片时，似乎所有之前绘制的 3D 对象都会被删除。

unitsize(100);
import three;
import graph3;
import solids;

real tmax = 1.15; //contraction strength
real translation_speed = 0.2; //ratio of translation part over contraction part of an infinitesimal generator
real height = 1; //height of the cutting plane
real radius = 5; //radius of the "projective sphere
real width = 2; //how far do the "infinite" surfaces actually go
int subdiv = 1; //how much we subdivide curves when projecting to the sphere

transform3 squish(real t) {
  return scale(exp(-t), 1, exp(t)) * shift(t*translation_speed*Y);
}
transform3 halfgplus = squish(tmax);
transform3 halfgminus = squish(-tmax);
transform3 gplus = squish(2*tmax);
transform3 gminus = squish(-2*tmax);
transform3 conj = rotate(90,X+Z);
transform3 halfhplus = conj * halfgplus * inverse(conj);
transform3 halfhminus = conj * halfgminus * inverse(conj);
transform3 hplus = conj * gplus * inverse(conj);
transform3 hminus = conj * gminus * inverse(conj);

triple c1 = (1,0,1);
triple c3 = (1,0,-1);
triple c5 = (-1,0,-1);
triple c7 = (-1,0,1);

path3 tennisball = scale3(width) *
(arc((1,0,0),c1,c3,-X) &
arc((0,0,-1),c3,c5,-Z) &
arc((-1,0,0),c5,c7,X) &
arc((0,0,1),c7,c1,Z));

path3[] cut(path3 p, triple center) {
//Cuts the path p by the plane x+z = height, then projects the upper part onto the plane.
//Returns {flattened upper part, lower part}.
//Assumes exactly 2 intersections.
  path p_projected = project(p, orthographic(1,0,-1));
  real [] cuttimes = times(p_projected, (0, height));
  real t1 = cuttimes[0];
  real t2 = cuttimes[1];
  pair test_point = point(p_projected, (t1+t2)/2);
  path3 upper_part = (test_point.y >= height) ? subpath(p, t1, t2) :
                                                subpath(p, t2, length(p)) & subpath(p, 0, t1);
  path3 lower_part = (test_point.y >= height) ? subpath(p, t2, length(p)) & subpath(p, 0, t1) :
                                                subpath(p, t1, t2);
  triple projection_target = planeproject(X+Z, sqrt(2)*height*Z) * center;
  path plane_upper_part = project(upper_part, perspective(center, target=projection_target));
  path3 flat_upper_part = invert(plane_upper_part, X+Z, sqrt(2)*height*Z, perspective(center, target=projection_target));
  return new path3[] {flat_upper_part, lower_part};
}

triple sph_proj(triple x, triple proj_cen) {
//Projects a point from some center (assumed to be inside the sphere) onto the sphere of center O and given radius.
  triple v = unit(x - proj_cen); //Unit vector generating the half-line.
  triple q = proj_cen - dot(v, proj_cen) * v; //Point of the line closest to O.
  return q + (radius +-+ abs(q)) * v;
}

path3 sph_proj_path(path3 p, triple proj_cen) {
//Projects a smooth, cyclic, non null path from some center (assumed to be inside the sphere) onto the sphere of center O and given radius.
  int n=length(p);
  guide3 g=sph_proj(point(p,0), proj_cen);
  for(int i=1; i < n*subdiv; ++i)
    g=g..sph_proj(point(p,i/subdiv), proj_cen);
  return g..cycle;
}

path3 side(transform3 gamma) {
  triple cen = gamma * O;
  return cut(sph_proj_path((gamma * tennisball), cen), cen)[0];
}

path3 tile(transform3 gamma) {
  path3 side_gplus = side(gamma * halfgplus);
  path3 side_gminus = side(gamma * halfgminus);
  path3 side_hplus = side(gamma * halfhplus * conj);
  path3 side_hminus = side(gamma * halfhminus * conj);
  path3 tile_contour = reverse(side_gplus) -- reverse(side_hplus) -- side_gminus -- side_hminus -- cycle;
  return planeproject(X+Z, sqrt(2)*height*Z) * tile_contour;
}

draw(surface(tile(identity4)), gray+opacity(0.8));

更新：我也试过了

draw(surface(project(p, orthographic(n)), plane=myplane))

myplane我想要绘制表面的平面在哪里n，它的法线在哪里。这会产生不同的结果，但仍然不是我期望的结果。

答案1

使用 Asymptore，3D 曲面是贝塞尔曲面片。平面路径 3 不定义这样的曲面。我知道的唯一解决方案是使用 bezulate 例程，将 2D 路径（循环路径或循环路径的并集）转换为贝塞尔三角剖分（2D）。然后，您可以使用表面（bezulate（您的 2D 路径）））绘制它。它被开发为具有 3D 标签。

我对您的代码（投影选择）有些疑问，特别是对于 halfgminus 和 halfhminus，bezulate 例程会返回错误消息。以下是您的图片的一部分

path[]  aa=  project(side(identity4*halfgplus)--cycle ^^reverse(side(identity4*halfhplus*conj)--cycle),
               orthographic(X+Z));
path[] gg=bezulate(aa);
draw(planeproject(X+Z)*surface(gg),opacity(0.4));

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