Inspect a model as you build it

Mix.install([{:smith, "~> 0.4.0"}, {:kino, "~> 0.19.0"}])

Overview

A valid solid can still be the wrong shape. Measure evaluated geometry, state the requirements explicitly, and keep each report tied to the revision it checked. These tools work in ordinary Elixir scripts; neither vision nor Livebook is required. This lesson uses one plate to measure hole spacing, detect an overhanging feature, compare removed material, and produce a dimensioned drawing. Read A plate, step by step first. A terminal-only version is available in examples/inspection.exs.

Named inspection inputs

Names refer to snapshots you supply. They can describe complete parts, intermediate stages, or selected subshapes. They do not promise persistent face identities after an edit. Reevaluate the model and rebuild selections when dimensions change.

alias Smith.{Drawing, Inspection, Measure, Plane, Selector}

plate = Smith.box(40, 24, 6)
{:ok, plate_result} = Smith.evaluate(plate)

holes = [Smith.cylinder(2, 8, at: {10, 12, -1}), Smith.cylinder(2, 8, at: {30, 12, -1})]
{:ok, drilled} = plate_result |> Smith.from_result() |> Smith.cut(holes) |> Smith.evaluate()
Smith.Kino.render(drilled, view: :top, edges: true)

Topology queries and selectors

Topology inspection returns metadata rather than native handles. Pages include source revision, total matches, and the next offset. Native topology order is not stable across revisions. Prefer a geometric selector to a remembered face index.

{:ok, page} = Inspection.topology(drilled, :edges, selector: Selector.type(:circle), limit: 10)

left = fn edge ->
  edge.type == :circle and abs(elem(edge.center, 0) - 10) < 1.0e-6 and
    abs(elem(edge.center, 2) - 6) < 1.0e-6
end

right = fn edge ->
  edge.type == :circle and abs(elem(edge.center, 0) - 30) < 1.0e-6 and
    abs(elem(edge.center, 2) - 6) < 1.0e-6
end

{:ok, spacing} = Measure.distance(drilled, {:circle_center, left}, {:circle_center, right}, axis: :x)
{:ok, diameter} = Measure.diameter(drilled, left)
{:ok, width} = Measure.extent(drilled, :x)
Kino.DataTable.new(page.items)

A measurement selector must match exactly one item. Multiple matches return an error; refine the selector to identify one edge. Circle centers and radii come from native curves, including arcs; they are not inferred from mesh vertices.

Requirements and reports

{:ok, report} = Inspection.run(%{plate: drilled}, checks: [
  {:topology, :plate, :solids, expected: 1},
  {:bounds, :plate, {{0, 0, 0}, {40, 24, 6}}, tolerance: 1.0e-6},
  {:measurement, :plate, spacing, expected: 20, tolerance: 1.0e-6},
  {:measurement, :plate, diameter, expected: 4, tolerance: 1.0e-6}
])

:passed = report.status
{:ok, json} = Inspection.json(report)
Kino.DataTable.new(Enum.map(report.checks, &Map.take(&1, [:kind, :status, :measured, :expected, :unit])))

{:ok, report} means inspection ran. report.status tells you whether the requirements passed. A failed requirement has status: :failed, with its measured value, expectation, units, and tolerance. A kernel error, unknown name, or stale measurement is :error. An empty or failed section must not be treated as evidence that there is no material.

The JSON contains no native handles. Model summaries include revisions, native bounds, area, volume, and topology counts. A text-only agent can use these values to decide what to change and rerun the same requirements afterward.

Containment and clearance

ledge = Smith.box(4, 10, 1, at: {38, 7, 5})
{:ok, failed} = Inspection.run(%{plate: plate_result, ledge: ledge}, checks: [
  {:contained, :ledge, :plate, tolerance: 1.0e-7}
])

:failed = failed.status
[excess] = failed.checks
IO.inspect({excess.measured, excess.unit, excess.bounds}, label: "Outside material")
ledge_scene = [{plate_result, {130, 150, 170}}, {ledge, {210, 55, 45}}]
Smith.Kino.render(ledge_scene, edges: true)

This reports 20 mm³ outside the plate. Containment measures volume for solids, area for faces, and length for curves. Its tolerance uses those units. To check whether a feature is fully supported, inspect its bottom face against the supporting body. Exposed area indicates an overhang, even when the fused result is a valid solid.

For mating parts, use {:clearance, :pin, :socket_body, minimum: 0.1, tolerance: 1.0e-6}. It measures material distance and also checks common volume. A pin contained inside solid material has zero distance, but that is interference, not a successful zero-clearance fit. Linear and volume tolerances are separate.

Images and report files

{:ok, artifacts} = Inspection.write(report, "output/inspection",
  views: [:isometric, :top, :front, :right],
  sections: [Plane.xz(y: 12)],
  width: 640,
  height: 480
)
IO.puts(artifacts.report)

Each run gets its own directory and a JSON manifest. Images reference both the source and rendered geometry revisions. Failed containment checks also produce images of the excess material. These files are also usable by agents without Livebook. Read the returned manifest rather than selecting an old image by its filename. An image generation failure does not write a final report manifest; partial image files can remain in that run's directory.

Smith.Render.write/3 writes an individual PNG. It uses an orthographic software depth buffer, with no GPU, browser, display server, or additional renderer install. Top looks from +Z, front from −Y, and right from +X. Named views use world axes, not inferred product orientation. Pass a Smith.Plane for another convention. Images use tessellation. Mesh deflection, numerical measurement tolerance, manufacturing tolerance, and printer compensation are different quantities.

Compare modeling stages

{:ok, change} = Inspection.compare(plate_result, drilled)
IO.inspect({change.added_mm3, change.removed_mm3}, label: "Stage material changes")

layers = [
  {drilled, {130, 150, 170}},
  {change.removed, {210, 55, 45}},
  {change.added, {45, 170, 90}}
]
Smith.Kino.render(layers, view: :isometric, edges: true)

The removed material is red and added material green. The same layer list works with Smith.Render.png/2. These are opaque solids, so orbit or clip the scene to inspect occluded regions. Full BREP differences can be expensive for coincident curved faces. They locate changes that a total-volume comparison can miss.

Drawing dimensions

{:ok, drawing} = Drawing.new(drilled, on: :xy)
{:ok, drawing} = Drawing.dimension(drawing, width, orientation: :horizontal, offset: -6)
{:ok, drawing} = Drawing.dimension(drawing, spacing, orientation: :horizontal, offset: 18)
{:ok, drawing} = Drawing.dimension(drawing, diameter, offset: -3)
{:ok, svg} = Drawing.svg(drawing, hidden: false, title: "Measured mounting plate")
Smith.Kino.render(drawing, label: "Measured mounting plate", hidden: false)

Dimensions retain source revision, measured value, and anchors. A mismatched revision or foreshortened projection is rejected. Linear, radius, diameter, and angular dimensions are supported. SVG includes extension lines, arrows, and center marks; placement is explicit. There is no automatic label layout or sketch constraint solver. Annotated DXF currently returns :unsupported_annotations.

Working with agents

Give an agent the requirements, coordinate convention, source drawing or model, and the location of its output directory. Ask it to keep assumptions separate from measurements and to return the inspection manifest with its changes.

The agent modeling guide is available as plain Markdown through ExDoc's llms.txt. Smith also ships skills/smith-cad/SKILL.md for harnesses that support skills. The guide and skill describe which tools to use and how to check their results.