The capture() module and the geometry-introspection function family are BCad-specific extensions to the OpenSCAD language, intended for testing and debugging SCL scripts: they let you "store" the result of a build into a variable and then inspect its geometric properties (face count, volume, bounding box, BRep validity).

capture(name)

Stores the geometry of the child node(s) into the variable name in the parent scope.

capture("cube1") cube([1, 2, 3]);
echo(str("Volume = ", volume(cube1)));

Semantics:

• name — a string; only string names are accepted (otherwise a warning is emitted).
• After capture("x") <child>, the variable x holds the SCLShape of the child.
• A single child with geometry → the variable holds its shape as-is.
• Multiple children → the variable holds a Compound with merged face colors.
• If the child has no geometry → the variable equals undef (None).
• The variable is written into the parent scope, i.e. it is available after the block where capture() was invoked.

Example:

capture("t360") linear_extrude(height=2, twist=360)
  translate([20, 0, 0])
    rotate([90, -90, 0])
      square(2);

Diagnostic functions

All functions take a geometry argument (usually a variable filled by capture()) and return a number or a vector:

Function	Returns	Notes
num_faces(geom)	number	number of faces
num_edges(geom)	number	number of edges
num_vertices(geom)	number	number of vertices
volume(geom)	number	volume (VolumeProperties().Mass())
surface_area(geom)	number	surface area
centroid(geom)	[x, y, z]	center of mass
bbox_min(geom)	[x, y, z]	bounding box min corner
bbox_max(geom)	[x, y, z]	bounding box max corner
is_valid(geom)	true/false	BRep validity (BRepCheck_Analyzer.IsValid())

Note on is_valid: BRepCheck_Analyzer validates topology strictly. Degenerate geometry (for example, extruding a profile that was rotated out of the XY plane — a "sheet" with zero thickness) is reported invalid, even though geometrically it is a legitimate 2D surface. This is expected behavior, not a bug.

Example:

capture("cyl") cylinder(h=5, r=3, $fn=12);
echo(str("faces = ", num_faces(cyl)));      // 14 (dodecagonal prism)
echo(str("valid = ", is_valid(cyl)));       // true
echo(str("bbox = ", bbox_min(cyl), " .. ", bbox_max(cyl)));

assert(condition, message)

Available both as a module and as a function. When the condition is false, it aborts the script with the given message (visible as a Traceback in test mode).

capture("sq") square([2, 2]);
assert(num_faces(sq) == 1, "a square must have 1 face");

Together with capture() and the diagnostic functions it forms the standard test template (see the existing tests in tests/).

Library tests/lib/testing.scad

Included via use <lib/testing.scad>; provides an approximate-comparison helper:

function check_approx(a, b, eps=0.01) = ...

• Compares numbers and vectors with tolerance eps (default 0.01).
• check_approx(a, b) → true when |a - b| <= eps (vectors via norm()).

Typical test: ```scl use

capture("m1") cube([1, 1, 1]); assert(check_approx(bbox_min(m1), [0, 0, 0]), "bbox_min"); assert(check_approx(volume(m1), 1), "volume"); assert(is_valid(m1), "valid");

echo("--- OK ---"); ```

Running a test:

bcad-launcher --test tests/test_xxx.scad

The test is considered failed if the output contains a Traceback (from assert()).