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/ascii.go

@@ -0,0 +1,169 @@
+package mesh
+
+import (
+	"fmt"
+	"math"
+	"strings"
+)
+
+// View is an orthographic camera basis: Right/Up span the screen plane,
+// Toward points from the scene toward the viewer (bigger depth = closer).
+type View struct {
+	Name              string
+	Axes              string // human-readable axis legend
+	Right, Up, Toward Vec3
+}
+
+var Views = map[string]View{
+	"front": {"front", "X→right Y↑up (seen from +Z)", Vec3{1, 0, 0}, Vec3{0, 1, 0}, Vec3{0, 0, 1}},
+	"back":  {"back", "-X→right Y↑up (seen from -Z)", Vec3{-1, 0, 0}, Vec3{0, 1, 0}, Vec3{0, 0, -1}},
+	"side":  {"side", "-Z→right Y↑up (seen from +X)", Vec3{0, 0, -1}, Vec3{0, 1, 0}, Vec3{1, 0, 0}},
+	"top":   {"top", "X→right Z↓down-screen (seen from above, +Y)", Vec3{1, 0, 0}, Vec3{0, 0, -1}, Vec3{0, 1, 0}},
+	"iso": {"iso", "isometric from (+X +Y +Z)",
+		Vec3{1, 0, -1}.Norm(), Vec3{-1, 2, -1}.Norm(), Vec3{1, 1, 1}.Norm()},
+}
+
+// ViewOrder is the canonical ordering for multi-view output.
+var ViewOrder = []string{"front", "side", "top", "iso", "back"}
+
+const shadeRamp = " .:-=+*#%@"
+
+// charAspect compensates terminal cells being ~2x taller than wide.
+const charAspect = 0.5
+
+// RenderASCII draws the mesh from the given view into a text block of
+// the given character width. Triangles are z-buffer rasterized and
+// shaded by how much each face points toward the light (over the
+// viewer's shoulder), so curvature and depth read as brightness.
+func RenderASCII(m *Mesh, v View, width int) string {
+	if width < 8 {
+		width = 8
+	}
+	if len(m.Tris) == 0 {
+		return "(empty mesh)\n"
+	}
+
+	// project all vertices into view space
+	type pv struct{ x, y, z float64 }
+	pts := make([]pv, len(m.Verts))
+	minX, minY := math.Inf(1), math.Inf(1)
+	maxX, maxY := math.Inf(-1), math.Inf(-1)
+	for i, w := range m.Verts {
+		p := pv{w.Dot(v.Right), w.Dot(v.Up), w.Dot(v.Toward)}
+		pts[i] = p
+		minX, maxX = math.Min(minX, p.x), math.Max(maxX, p.x)
+		minY, maxY = math.Min(minY, p.y), math.Max(maxY, p.y)
+	}
+	spanX, spanY := maxX-minX, maxY-minY
+	if spanX == 0 {
+		spanX = 1e-9
+	}
+	if spanY == 0 {
+		spanY = 1e-9
+	}
+	height := int(float64(width) * (spanY / spanX) * charAspect)
+	if height < 1 {
+		height = 1
+	}
+	if height > 4*width {
+		height = 4 * width
+	}
+	sx := float64(width-1) / spanX
+	sy := float64(height-1) / spanY
+
+	depth := make([]float64, width*height)
+	for i := range depth {
+		depth[i] = math.Inf(-1)
+	}
+	shade := make([]float64, width*height)
+	for i := range shade {
+		shade[i] = -1
+	}
+
+	light := v.Toward.Mul(0.8).Add(v.Up.Mul(0.5)).Add(v.Right.Mul(0.3)).Norm()
+
+	for ti, t := range m.Tris {
+		n := m.FaceNormal(ti)
+		// abs: downloaded models often have mixed winding; treat both
+		// sides as lit so the silhouette never goes black
+		lum := 0.15 + 0.85*math.Abs(n.Dot(light))
+
+		a, b, c := pts[t[0]], pts[t[1]], pts[t[2]]
+		ax, ay := (a.x-minX)*sx, (maxY-a.y)*sy
+		bx, by := (b.x-minX)*sx, (maxY-b.y)*sy
+		cx, cy := (c.x-minX)*sx, (maxY-c.y)*sy
+
+		x0 := int(math.Floor(math.Min(ax, math.Min(bx, cx))))
+		x1 := int(math.Ceil(math.Max(ax, math.Max(bx, cx))))
+		y0 := int(math.Floor(math.Min(ay, math.Min(by, cy))))
+		y1 := int(math.Ceil(math.Max(ay, math.Max(by, cy))))
+		if x0 < 0 {
+			x0 = 0
+		}
+		if y0 < 0 {
+			y0 = 0
+		}
+		if x1 >= width {
+			x1 = width - 1
+		}
+		if y1 >= height {
+			y1 = height - 1
+		}
+
+		area := (bx-ax)*(cy-ay) - (by-ay)*(cx-ax)
+		if area == 0 {
+			continue
+		}
+		for py := y0; py <= y1; py++ {
+			for px := x0; px <= x1; px++ {
+				fx, fy := float64(px), float64(py)
+				w0 := (bx-ax)*(fy-ay) - (by-ay)*(fx-ax)
+				w1 := (cx-bx)*(fy-by) - (cy-by)*(fx-bx)
+				w2 := (ax-cx)*(fy-cy) - (ay-cy)*(fx-cx)
+				if !sameSide(w0, w1, w2, area) {
+					continue
+				}
+				// barycentric depth: w2 tracks b, w0 tracks c
+				l1 := w2 / area
+				l2 := w0 / area
+				l0 := 1 - l1 - l2
+				z := l0*a.z + l1*b.z + l2*c.z
+				idx := py*width + px
+				if z > depth[idx] {
+					depth[idx] = z
+					shade[idx] = lum
+				}
+			}
+		}
+	}
+
+	var sb strings.Builder
+	mn, mx := m.BBox()
+	size := mx.Sub(mn)
+	fmt.Fprintf(&sb, "%s view — %s\nmodel %.3g x %.3g x %.3g (XYZ)\n",
+		v.Name, v.Axes, size.X, size.Y, size.Z)
+	ramp := []rune(shadeRamp)
+	for py := 0; py < height; py++ {
+		for px := 0; px < width; px++ {
+			s := shade[py*width+px]
+			if s < 0 {
+				sb.WriteByte(' ')
+				continue
+			}
+			i := int(s * float64(len(ramp)-1))
+			if i >= len(ramp) {
+				i = len(ramp) - 1
+			}
+			sb.WriteRune(ramp[i])
+		}
+		sb.WriteByte('\n')
+	}
+	return sb.String()
+}
+
+func sameSide(w0, w1, w2, area float64) bool {
+	if area > 0 {
+		return w0 >= 0 && w1 >= 0 && w2 >= 0
+	}
+	return w0 <= 0 && w1 <= 0 && w2 <= 0
+}