CSG System

CSG System

Constructive Solid Geometry system for rapid level greyboxing with boolean mesh operations on primitive shapes.

Source: SparkEngine/Source/Engine/LevelDesign/CSGSystem.h, SparkEditor/Source/Panels/CSGEditorPanel.h

Overview

The CSG system enables level designers to rapidly prototype environments using primitive shapes and boolean operations. Instead of modeling in external tools, designers create boxes, cylinders, spheres, wedges, and cones directly in the engine, then combine them with union, subtraction, and intersection operations to carve doorways, create windows, build complex room shapes, and more.

The system operates on CSGSolid objects -- collections of convex polygon faces with normals and texture coordinates. Boolean operations use BSP-based face clipping: each face of solid A is split by the planes of solid B's faces, keeping either the inside or outside portions depending on the operation. The result is a new solid that can be triangulated into a renderable CSGMesh with vertex and index buffers.

The editor provides CSGEditorPanel, an ImGui-based interface for creating brushes, selecting shapes and operations, adjusting transforms, and previewing the combined result in real time with auto-rebuild support.

Architecture

CSGSystem (singleton)
  |
  +-- CreateBrush(shape, size) --> uint32_t brushId
  |     Shapes: Box, Cylinder, Sphere, Wedge, Cone
  |
  +-- SetBrushTransform(id, transform)
  +-- SetBrushOperation(id, Additive|Subtractive|Intersect)
  |
  +-- Boolean Operations:
  |     Union(A, B)     --> A + B (combined volume)
  |     Subtract(A, B)  --> A - B (carve B from A)
  |     Intersect(A, B) --> A n B (overlap only)
  |
  +-- GenerateMesh(solid) --> CSGMesh (vertices + indices)
  +-- BuildAll() --> combined mesh from all brushes

CSGEditorPanel (ImGui)
  +-- Shape selector, size controls, operation picker
  +-- Brush list with per-brush delete
  +-- Auto-rebuild toggle, manual rebuild button
  +-- Statistics display (brush count, triangle count)

Key Classes

Class	Description
CSGSystem	Singleton managing brushes and boolean operations
CSGEditorPanel	ImGui editor panel for interactive CSG editing
CSGSolid	Collection of convex faces representing a solid volume
CSGFace	Single polygon face with vertices and normal
CSGVertex	Vertex with position, normal, and UV coordinates
CSGMesh	Output triangle mesh (vertex buffer + index buffer)
CSGPlane	BSP plane used for face splitting during boolean ops
BrushShape	Enum: Box, Cylinder, Sphere, Wedge, Cone
CSGOperation	Enum: Additive, Subtractive, Intersect

Usage

Creating and Combining Brushes

auto& csg = Spark::LevelDesign::CSGSystem::GetInstance();
csg.Initialize();

// Create a room (large box)
uint32_t room = csg.CreateBrush(Spark::LevelDesign::BrushShape::Box, {10, 3, 10});

// Create a doorway (small box positioned at wall)
uint32_t door = csg.CreateBrush(Spark::LevelDesign::BrushShape::Box, {1.2f, 2.5f, 0.5f});
csg.SetBrushTransform(door, {{5, 0, 0}, {0, 0, 0}, {1, 1, 1}});
csg.SetBrushOperation(door, Spark::LevelDesign::CSGOperation::Subtractive);

// Create a cylindrical pillar
uint32_t pillar = csg.CreateBrush(Spark::LevelDesign::BrushShape::Cylinder, {0.5f, 3.0f, 0});

// Build combined mesh from all brushes
Spark::LevelDesign::CSGMesh mesh = csg.BuildAll();
// mesh.vertices, mesh.indices, mesh.triangleCount

Direct Boolean Operations

auto& csg = Spark::LevelDesign::CSGSystem::GetInstance();

// Create two solids manually
auto boxSolid = /* ... */;
auto cylinderSolid = /* ... */;

// Subtract cylinder from box (e.g., create a hole)
auto result = csg.Subtract(boxSolid, cylinderSolid);

// Generate renderable mesh
auto mesh = csg.GenerateMesh(result);

Tessellation Control

// Increase cylinder smoothness (default: 16 segments)
csg.SetCylinderSegments(32);

// Increase sphere detail (default: 8 rings, 16 segments)
csg.SetSphereTessellation(16, 32);

API Reference

CSGSystem

Method	Description
Initialize()	Clear brushes, reset tessellation defaults
Shutdown()	Release all brushes
CreateBrush(shape, size)	Create a primitive brush, returns brush ID
SetBrushTransform(id, transform)	Set position, rotation, scale
SetBrushOperation(id, op)	Set Additive, Subtractive, or Intersect
RemoveBrush(id)	Delete a brush
Union(A, B)	Boolean union of two solids
Subtract(A, B)	Boolean subtraction (A minus B)
Intersect(A, B)	Boolean intersection (overlap only)
GenerateMesh(solid)	Triangulate a solid into vertex/index buffers
BuildAll()	Combine all brushes and return final mesh
SetCylinderSegments(n)	Set cylinder tessellation (min 3)
SetSphereTessellation(rings, segs)	Set sphere detail (min 2 rings, 3 segments)

CSGEditorPanel

Method	Description
Initialize()	Set up default brush shape, size, and auto-rebuild
Render()	Draw the ImGui interface (shape picker, brush list, build controls)
Update(dt)	Per-frame update

Configuration

Setting	Default	Description
Cylinder segments	16	Number of sides for cylinder/cone primitives
Sphere rings	8	Latitude subdivisions for sphere primitives
Sphere segments	16	Longitude subdivisions for sphere primitives
Auto-rebuild	On	Automatically rebuild combined mesh when brushes change

Primitive Shapes

Shape	Parameters	Description
Box	width, height, depth	Axis-aligned box centered at origin
Cylinder	radius, height, --	Vertical cylinder with configurable segments
Sphere	radius, --, --	Tessellated sphere
Wedge	width, height, depth	Right-angle wedge (ramp)
Cone	radius, height, --	Vertical cone with base at bottom

Related Systems

• Physics System -- CSG meshes can be used as collision geometry
• Render Graph -- CSG output meshes feed into the rendering pipeline
• Editor Architecture -- CSGEditorPanel is one of the specialized editor panels