ara3d-authoring

name	ara3d-authoring
description	Author Ara 3D Studio tools — generators (make geometry) and modifiers (transform geometry) — as standalone C# scripts. Use when the user says "write a modifier", "create a generator", "make an Ara 3D tool", "new Studio script", asks how modifiers/generators/Eval/parameters/EvalContext work, or wants to add a mesh operation, deformer, analyzer, or heat-map to Ara 3D Studio.

Authoring Ara 3D Studio tools (generators & modifiers)

Ara 3D Studio users extend the app with standalone C# scripts. Drop a .cs file into the Scripts folder; Studio compiles it on save and surfaces every tool it finds in the UI with an auto-generated parameter panel. No project, no build step, no restart.

Two roles cover almost everything:

• Generator — makes new geometry from parameters. Implements IGenerator, has an Eval() that takes no geometry input.
• Modifier — transforms geometry flowing through the graph. Implements IModifier, has an Eval(input) that takes an upstream geometry object.

Everything below is the contract the host actually enforces (by reflection), followed by the attributes that shape the UI, the valid input/output types, deployment, and two worked examples.

Paths below are relative to the ara3d-sdk repo root (where this skill lives). A few cited files live in the Ara 3D Studio host app repo, not the SDK — those are marked (host repo).

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1. The runtime contract (what the host requires)

The host wraps your object in EvaluatorWrapper and calls it by reflection. The rules:

1. Marker interface. Implement IGenerator or IModifier (namespace Ara3D.Studio.API). Both are empty marker interfaces — they only tag the class for discovery.
2. A public method named exactly Eval. The wrapper does type.GetMethod("Eval"). The name, casing, and public visibility all matter. Exactly one Eval per class.
3. Eval arguments define the input:
   • Generator: Eval() — zero geometry args. May take a single EvalContext.
   • Modifier: Eval(TInput) — the first parameter is the geometry it consumes. The host converts the upstream graph object to TInput (see §4).
   • Optional EvalContext may be added as an extra parameter (generator: the only arg; modifier: the second arg). Use it for RefreshUI, animation time, and host access.
4. Return type = output. Whatever Eval returns becomes the object flowing downstream and gets rendered. Must be a renderable/convertible geometry type (see §4).
5. Parameters = public fields or properties. Every public read/write field or property becomes an editable control in the panel. Read-only properties ({ get; private set; }) become non-editable read-outs.

Minimal shapes:

public class MyGenerator : IGenerator
{
    [Range(1, 10)] public int Count = 3;
    public TriangleMesh3D Eval() => /* build geometry from Count */;
}

public class MyModifier : IModifier
{
    [Range(0f, 1f)] public float Amount = 0.5f;
    public TriangleMesh3D Eval(TriangleMesh3D mesh) => /* transform mesh using Amount */;
}

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2. Parameters and UI attributes

Parameters are plain public fields/properties. Fields and properties both work — pick the house style of the surrounding file. Attributes refine how they render:

Attribute	Applies to	Effect	Source namespace
[Range(min, max)]	numeric field/prop	slider with bounds	System.ComponentModel.DataAnnotations
[Options(nameof(Factory))]	int field/prop	dropdown; Factory is a method/prop returning IEnumerable<string>, selected value is the index	Ara3D.PropKit
[Category(nameof(Categories.X))]	the class	groups the tool under a named category in the UI	System.ComponentModel (+ your Categories enum)
[OnDemand]	the class	do not re-evaluate on every parameter change; run only when explicitly requested	Ara3D.Studio.API
[Animated]	the class	tick every frame; read ctx.AnimationTime (or implement IAnimated)	Ara3D.Studio.API
[PointerTracking]	the class	re-evaluate while the pointer or camera moves; read ctx.Services.ViewportInput (null on headless hosts)	Ara3D.Studio.API

Notes:

• Default (no [OnDemand]) = live: changing any parameter re-runs Eval immediately.
• Categories is an ordinary enum you define/extend per script set. The examples project's is examples/Ara3D.Studio.Examples/Categories.cs (Meshes, Curves, Sdf, Topology, …).
• Supported parameter types include the numeric primitives, bool, string, and SDK vector types like Vector3 / Vector2 (see Transform, Box in the examples). A bool renders as a checkbox; a Vector3 renders as three fields.

Read-out values (computing stats back to the panel)

Expose computed results as read-only properties and push them to the panel from inside Eval:

public int VertexCountBefore { get; private set; }
public int VertexCountAfter  { get; private set; }

public TriangleMesh3D Eval(TriangleMesh3D mesh, EvalContext ctx)
{
    var r = mesh.WeldVertices();
    VertexCountBefore = mesh.Points.Count;
    VertexCountAfter  = r.Points.Count;
    ctx.Services.RefreshUI(this);   // repaint the panel with the new read-outs
    return r;
}

This is the whole WeldVertices example (examples/Ara3D.Studio.Examples/Modifiers/WeldVertices.cs).

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3. Optional EvalContext

EvalContext (src/Ara3D.Studio.API/EvalContext.cs) carries:

• Services — the IEvalServices host surface (RefreshUI(this), ViewportInput, DerivedDataCache).
• AnimationTime — seconds, for [Animated] tools.
• Input — the raw upstream FlowObject (attachments, presentation) if you need more than the converted geometry — e.g. BIM metadata via Input.GetAttachment<T>().
• Services.ViewportInput — pointer state for in-canvas widgets (IViewportInput: normalized CursorUV, world-space CursorRay, HoverObjectId, PrimaryDown, ConsumePrimaryClick()). Null on hosts without a viewport, so always keep a fallback. Pair with [PointerTracking] so the host re-evaluates the node as the pointer moves. A "click" is press+release without dragging — drags orbit the camera. Worked example: examples/.../Demos/GridSnapPickDemo.cs.

Add it only when you need it. Signatures the host accepts:

QuadMesh3D    Eval();                                  // generator
QuadMesh3D    Eval(EvalContext ctx);                   // generator + context
TriangleMesh3D Eval(TriangleMesh3D mesh);              // modifier
TriangleMesh3D Eval(TriangleMesh3D mesh, EvalContext ctx); // modifier + context

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4. Valid input and output types

Input (modifier first parameter). The host converts the upstream object to your parameter type via FlowObjectConverters (src/Ara3D.Studio.API/FlowObjectConverters.cs). The two most flexible choices:

• IModel3D — accepts a TriangleMesh3D, ColoredTriangleMesh3D, QuadMesh3D, QuadGrid3D, Model3D, or RenderModelData upstream. Use this for model-level work (instances, materials, whole assemblies).
• TriangleMesh3D — accepts QuadMesh3D, QuadGrid3D, ColoredTriangleMesh3D, or an IModel3D (flattened). Use this for single-mesh algorithms.

Any other exact type (e.g. LineMesh3D) is accepted only when the upstream object already is that type — no conversion is defined, so the graph must actually produce it.

Output (return type). Return a geometry type the host can render or pass on:

• TriangleMesh3D, QuadMesh3D, QuadGrid3D — meshes/surfaces.
• ColoredTriangleMesh3D — per-vertex colored mesh; this is the analyzer / heat-map pattern (compute a scalar per vertex/face, map to color). See Modifiers/ColorSharpEdges.cs.
• IModel3D / Model3D — instanced models with materials; build with Model3DBuilder.
• LineMesh3D — line overlays (boundaries, arrows, diagrams).

Model-level ops iterate model.Instances, remap meshes through a Model3DBuilder, and prefilter by model.GetInstanceBounds() — see the ModelHorizontalSlice half of the PlaneCut example.

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5. Two worked examples (read these first)

Generator — BlockMesh

examples/Ara3D.Studio.Examples/Generators/BlockMesh.cs

public class BlockMesh : IGenerator
{
    [Range(0f, 10f)] public float SizeX = 1;
    [Range(0f, 10f)] public float SizeY = 1;
    [Range(0f, 10f)] public float SizeZ = 1;

    public bool EmptyTop;
    public bool EmptyBottom;
    public bool EmptySides;

    public QuadMesh3D Eval()
    {
        var shape = new CellGridBuilder3D(3, 3, 3).Remove(1, 1, 1);
        if (EmptyTop)    shape.Remove(1, 1, 2);
        if (EmptyBottom) shape.Remove(1, 1, 0);
        if (EmptySides)  shape.Remove(1,0,1).Remove(1,2,1).Remove(0,1,1).Remove(2,1,1);
        return shape.ToMesh().Scale((SizeX, SizeY, SizeZ));
    }
}

Shows: marker interface, [Range] sliders, bool checkboxes, Eval() with no input returning a mesh. The whole tool is one method building geometry from parameters.

Modifier — MeshHorizontalSlice

examples/Ara3D.Studio.Examples/Modifiers/PlaneCut.cs

[Category(nameof(Categories.Meshes))]
public class MeshHorizontalSlice : IModifier
{
    [Range(0f, 1f)] public float Height { get; set; } = 0.5f;

    public TriangleMesh3D Eval(TriangleMesh3D mesh)
    {
        var bounds = mesh.Bounds;
        if (Height == 0) return ([], []);
        var z = bounds.Min.Z.Lerp(bounds.Max.Z, Height);
        var plane = new Plane(Vector3.UnitZ, -z);
        return mesh.ClipBelow(plane);
    }
}

Shows: [Category] grouping, a single [Range] parameter, Eval(TriangleMesh3D) returning a mesh, and reuse of an existing library op (ClipBelow). The same file's ModelHorizontalSlice is the IModel3D (instance-level) counterpart — read it to learn the Model3DBuilder pattern.

More examples worth copying

• Modifiers/WeldVertices.cs — read-outs + ctx.Application.RefreshUI(this).
• Modifiers/RefineAndCoarsen.cs — several modifiers + a helper class in one file; the ColorByTriangleQuality class is the heat-map/analyzer pattern (ColoredTriangleMesh3D).
• Modifiers/FaceNormalArrows.cs — building an IModel3D overlay with instanced arrow meshes.
• Studio's built-in coding prompt (host repo: src/Ara3D.Studio/Ara3DStudio/codingprompt.txt) contains a large gallery of small generators and deformers (Cylinder, Torus, TwistDeformer, NoiseDeformer, GridClone, …) — the canonical "cheat sheet" of idioms.

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6. Reusable idioms (don't reinvent)

• mesh.Bounds, new Topology(mesh), new MeshAttributes(mesh, topo) — topology/adjacency, normals.
• mesh.WeldVertices(), mesh.Deform(p => ...), mesh.WithPoints(...), mesh.Scale(v), mesh.Triangulate() — common mesh transforms.
• new Model3DBuilder() + AddInstance / AddMeshWithoutInstance / AddModel — assemble models.
• mesh.Clone(material, transforms) — instance a mesh across many transforms.
• Return ColoredTriangleMesh3D for analyzers; map a per-element scalar to color yourself (see ColorByTriangleQuality).
• The SDK ships list-preserving LINQ (Select, Map, Slice, Zip, …) that returns IReadOnlyList with O(1) count/indexing — prefer it over IEnumerable where it exists.
• Selection sets (Ara3D.Studio.API.SelectionSet, examples Selection/ folder): a selection-creator must return FlowObject (e.g. via the examples' ctx.SelectFaces(mesh, combine, predicate) helper) — returning a bare mesh loses the selection attribute at the pipeline boundary. A selection-consumer reads ctx.GetFaceSelection(mesh); treat null as "whole mesh" so the tool still works without a selection. Topology-changing modifiers drop selections automatically (WithNewContent default); pass an AttributeDomainMask if indexing was preserved. Pure index-list ops: mesh.ExtractFaces/DeleteFaces/SplitByFaces (MeshSelectionOps).

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7. Deploying a script

1. Scripts folder. Default Documents/Ara 3D/Scripts (override with the SCRIPTS_PATH environment variable). In Studio: menu Compiler → Open Scripts Folder.
2. Drop the .cs file in that folder (subfolders are watched too). A DirectoryWatcher recompiles automatically on every save; the tool appears/updates live.
3. No using directives needed. A GlobalUsings.cs in the Scripts folder pre-imports the common namespaces (Ara3D.Geometry, Ara3D.Models, Ara3D.Studio.API, Ara3D.PropKit, System.Linq, System.ComponentModel.DataAnnotations, …). Canonical list: examples/Ara3D.Studio.Examples/GlobalUsings.cs. Write scripts with no namespace and no usings, just the class(es) — matching codingprompt.txt.
4. Extra dependencies. Put additional referenced DLLs in the Scripts folder's Libraries subfolder (or list paths in refs.txt); they are loaded at compile time.
5. Compile errors surface in Studio's compilation panel; the offending tool just won't appear.

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8. Other component roles (when a modifier/generator isn't the right shape)

From src/Ara3D.Studio.API/Interfaces.cs:

• IScriptedCommand / IModelCommand — one-shot actions (Execute/CanExecute), the right home for reports and audits that don't return geometry (e.g. a mesh-statistics dump).
• ITool — modeless UI tool.
• IExporter / ILoader — custom file write/read.
• IAsset / IAssetSource — data that enters the graph (with optional attachments like BIM data).

There is no IAnalyzer interface: an analyzer is just a modifier returning ColoredTriangleMesh3D.

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9. Authoring checklist

• Class implements IGenerator or IModifier.
• Exactly one public Eval; generator takes no geometry arg, modifier takes the input type.
• Input type is IModel3D (model-level) or TriangleMesh3D (mesh-level) unless a specific upstream type is guaranteed.
• Return type is renderable (TriangleMesh3D/QuadMesh3D/QuadGrid3D/ColoredTriangleMesh3D/IModel3D/LineMesh3D).
• Parameters are public fields/props with [Range]/[Options]; class has [Category].
• Read-outs are { get; private set; } + ctx.Application.RefreshUI(this).
• Follow the csharp-style skill for the geometry code (functional-first, immutable, expression-bodied).
• No namespace/usings; drop in the Scripts folder; confirm it compiles and appears.

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Maintenance (keep this skill true to the code)

This skill documents a reflection-based contract. If any of these change, update the matching section here. Files marked (host repo) live in the Ara 3D Studio application repository, not this SDK.

Source of truth	Governs
src/Ara3D.Studio.API/Interfaces.cs	the marker interfaces / component roles (§1, §8)
src/Ara3D.Studio.WpfControls/EvaluatorWrapper.cs (host repo)	how Eval is found + args bound; RefreshUI (§1, §2, §3)
src/Ara3D.Studio.API/FlowObjectConverters.cs	valid input/output conversions (§4)
src/Ara3D.Studio.API/EvalContext.cs	what EvalContext exposes (§3)
src/Ara3D.Studio.API/Attributes.cs, src/Ara3D.PropKit/OptionsAttribute.cs	[OnDemand]/[Animated]/[Options] (§2)
Services/CompilerService.cs, Services/FolderService.cs (host repo)	Scripts folder, watching, references (§7)
examples/Ara3D.Studio.Examples/	the example files cited in §5