mesh-tool: mesht - create/inspect/edit OBJ+STL with ASCII multi-view rendering

- OBJ read/write (multi-object, ngon fan-triangulation, negative indices),
  STL binary+ascii with auto-detect and vertex welding
- info: bbox/size/area/volume + watertightness via edge manifold stats
- view: z-buffered orthographic ASCII renders (front/side/top/iso/back)
- transform: center/mirror/scale/rotate/translate/fit, per-object filter,
  winding auto-flip on negative determinant
- create: box/sphere/cylinder/cone/plane/torus; merge; convert
- go tests: primitive volumes vs analytic values, roundtrips, topology

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmdG9GqfSWCzts7AkDwRDh

mesh-tool/mesh/vec.go

@@ -0,0 +1,122 @@
+package mesh
+
+import "math"
+
+// Vec3 is a point or direction in 3D space.
+type Vec3 struct{ X, Y, Z float64 }
+
+func (a Vec3) Add(b Vec3) Vec3    { return Vec3{a.X + b.X, a.Y + b.Y, a.Z + b.Z} }
+func (a Vec3) Sub(b Vec3) Vec3    { return Vec3{a.X - b.X, a.Y - b.Y, a.Z - b.Z} }
+func (a Vec3) Mul(s float64) Vec3 { return Vec3{a.X * s, a.Y * s, a.Z * s} }
+func (a Vec3) Dot(b Vec3) float64 { return a.X*b.X + a.Y*b.Y + a.Z*b.Z }
+func (a Vec3) Len() float64       { return math.Sqrt(a.Dot(a)) }
+func (a Vec3) Cross(b Vec3) Vec3 {
+	return Vec3{
+		a.Y*b.Z - a.Z*b.Y,
+		a.Z*b.X - a.X*b.Z,
+		a.X*b.Y - a.Y*b.X,
+	}
+}
+
+// Norm returns the unit vector, or the zero vector for zero-length input.
+func (a Vec3) Norm() Vec3 {
+	l := a.Len()
+	if l == 0 {
+		return Vec3{}
+	}
+	return a.Mul(1 / l)
+}
+
+// Min/Max return the component-wise minimum/maximum.
+func (a Vec3) Min(b Vec3) Vec3 {
+	return Vec3{math.Min(a.X, b.X), math.Min(a.Y, b.Y), math.Min(a.Z, b.Z)}
+}
+func (a Vec3) Max(b Vec3) Vec3 {
+	return Vec3{math.Max(a.X, b.X), math.Max(a.Y, b.Y), math.Max(a.Z, b.Z)}
+}
+
+// Mat4 is a row-major 4x4 transform matrix.
+type Mat4 [16]float64
+
+func Identity() Mat4 {
+	return Mat4{
+		1, 0, 0, 0,
+		0, 1, 0, 0,
+		0, 0, 1, 0,
+		0, 0, 0, 1,
+	}
+}
+
+// Mul returns m * n (n is applied first when transforming points).
+func (m Mat4) Mul(n Mat4) Mat4 {
+	var r Mat4
+	for row := 0; row < 4; row++ {
+		for col := 0; col < 4; col++ {
+			sum := 0.0
+			for k := 0; k < 4; k++ {
+				sum += m[row*4+k] * n[k*4+col]
+			}
+			r[row*4+col] = sum
+		}
+	}
+	return r
+}
+
+// Apply transforms a point (w = 1).
+func (m Mat4) Apply(v Vec3) Vec3 {
+	return Vec3{
+		m[0]*v.X + m[1]*v.Y + m[2]*v.Z + m[3],
+		m[4]*v.X + m[5]*v.Y + m[6]*v.Z + m[7],
+		m[8]*v.X + m[9]*v.Y + m[10]*v.Z + m[11],
+	}
+}
+
+// Det3 is the determinant of the upper-left 3x3. Negative means the
+// transform mirrors space, which flips triangle winding.
+func (m Mat4) Det3() float64 {
+	return m[0]*(m[5]*m[10]-m[6]*m[9]) -
+		m[1]*(m[4]*m[10]-m[6]*m[8]) +
+		m[2]*(m[4]*m[9]-m[5]*m[8])
+}
+
+func Translate(t Vec3) Mat4 {
+	m := Identity()
+	m[3], m[7], m[11] = t.X, t.Y, t.Z
+	return m
+}
+
+func ScaleXYZ(s Vec3) Mat4 {
+	m := Identity()
+	m[0], m[5], m[10] = s.X, s.Y, s.Z
+	return m
+}
+
+func RotateX(deg float64) Mat4 {
+	s, c := math.Sincos(deg * math.Pi / 180)
+	return Mat4{
+		1, 0, 0, 0,
+		0, c, -s, 0,
+		0, s, c, 0,
+		0, 0, 0, 1,
+	}
+}
+
+func RotateY(deg float64) Mat4 {
+	s, c := math.Sincos(deg * math.Pi / 180)
+	return Mat4{
+		c, 0, s, 0,
+		0, 1, 0, 0,
+		-s, 0, c, 0,
+		0, 0, 0, 1,
+	}
+}
+
+func RotateZ(deg float64) Mat4 {
+	s, c := math.Sincos(deg * math.Pi / 180)
+	return Mat4{
+		c, -s, 0, 0,
+		s, c, 0, 0,
+		0, 0, 1, 0,
+		0, 0, 0, 1,
+	}
+}