🏗️ Blockout Geometry: Modeling Mode & BSP

Blocking Out Levels in Unreal

Every level starts as a rough draft. Before artists build detailed environments, designers block out the space with simple placeholder geometry to test layout, flow, and pacing. This grey, untextured first pass is also called greyboxing, and it is one of the most important stages of level design.

Unreal lets you block out geometry directly in the editor, with no external software. In modern Unreal (UE5) the tool for this is Modeling Mode, a full set of in-editor modeling tools that produce real Static Meshes. Before Modeling Mode existed, the same job was done with BSP (Binary Space Partitioning) geometry, built from additive and subtractive "brushes." BSP is still in the engine, but it is now a legacy tool.

Adding and Subtracting Space

Think of it like working with clay or foam blocks. You either:

• Add material to create solid shapes (walls, floors, pillars)
• Remove material to carve out hollow spaces (rooms, doorways, alcoves)

Modeling Mode does this with mesh Booleans; BSP did it with additive and subtractive brushes. Either way, you are sculpting space itself.

Figure: The core idea shared by both tools · additive volumes create solid geometry (left), subtractive volumes carve out hollow spaces (right).

Why Blockout Comes First

Whichever tool you use, blocking out early pays off:

1. Rapid iteration: Simple geometry lets you test the flow and pacing of a level before committing to detailed art.
2. No external tools: Everything happens inside Unreal, so there is no export and re-import round trip.
3. Greyboxing: Building with plain grey shapes to prove out gameplay is standard practice at every studio.
4. Quick edits: Resizing a room or moving a wall takes seconds.

Where BSP Stands in Modern Unreal

It is worth being clear about BSP's status. Epic's own documentation says the role of Geometry Brushes "has been passed on to Static Meshes," and that Geometry Brushes are not recommended as a final method of level design. They remain handy for a quick throwaway test, but Modeling Mode has replaced them as the primary blockout tool because it:

• Produces real Static Meshes that render efficiently and support Nanite (UE5's virtualized geometry)
• Avoids the cracks, Z-fighting, and lightmap problems BSP is prone to
• Keeps you in one workflow from rough blockout to polished mesh

Additive vs. Subtractive Brushes

Additive and subtractive operations are the foundation of both blockout tools. In Modeling Mode they appear as Boolean operations (Union to add, Subtract to carve); in legacy BSP they are the two brush types, Additive and Subtractive. The terms below use BSP's brush language, but the same add-and-subtract thinking drives Modeling Mode's Booleans.

Additive Brushes

An Additive Brush creates solid geometry—it adds matter to the world. When you place an additive brush, you're saying "this space is now filled with solid material."

Think of additive brushes as the LEGO bricks you stack to build structures. Common uses include:

• Walls: Vertical planes that block movement and sight
• Floors: Horizontal surfaces players walk on
• Ceilings: Overhead surfaces that define room boundaries
• Pillars and columns: Vertical supports and decorative elements
• Platforms: Raised areas players can climb onto

When you first place a BSP brush in Unreal, it defaults to Additive. In the editor, additive brushes typically appear with a subtle blue tint (when in Wireframe or BSP mode) to distinguish them from the world.

Subtractive Brushes

A Subtractive Brush does the opposite—it carves out space, removing geometry to create hollow areas. When you place a subtractive brush, you're saying "this space should be empty air."

Think of subtractive brushes like a sculptor's chisel—they remove material to reveal the form underneath. Common uses include:

• Rooms: Carving interior spaces from solid blocks
• Doorways: Cutting openings in walls for passage
• Windows: Creating openings for visibility and light
• Alcoves and niches: Indentations in walls for decoration or pickups
• Tunnels and passages: Carving paths through terrain

Subtractive brushes appear with a reddish/pink tint in the editor, making them easy to distinguish from additive brushes.

How They Work Together

The real power of BSP comes from combining additive and subtractive brushes. Here's a typical workflow:

1. Start with a large additive brush to create a solid block of geometry
2. Use subtractive brushes to carve out rooms and corridors within that block
3. Add more additive brushes for internal walls, pillars, or details
4. Use additional subtractive brushes for doorways, windows, or decorative cutouts

Figure: The typical BSP workflow - start with additive blocks, carve out interiors with subtractive brushes, then add details.

Brush Order and Priority

When multiple brushes overlap, Unreal processes them in a specific order. Understanding this order helps you predict how your geometry will look:

1. Base assumption: The world starts as empty space (void)
2. Additive brushes are processed first, creating solid geometry
3. Subtractive brushes are processed second, carving away from the additive geometry
4. If brushes of the same type overlap, the order they were created matters (later ones take priority)

Think of it like working with layers in Photoshop or painting on a canvas—earlier layers can be covered or modified by later ones.

Creating Blockout Geometry with Modeling Mode

Now let's build something. In modern Unreal the fastest way to block out geometry is Modeling Mode: enter the mode, create a box, and shape it into a room. (The legacy BSP route is covered at the end of this section for reference.)

Entering Modeling Mode

Modeling Mode is one of the editor's modes, alongside Selection, Landscape, and Foliage:

1. Click the Selection Mode dropdown at the top-left of the main toolbar and choose Modeling (or, with the viewport focused, press Shift+5)
2. A column of tool categories appears on the left: Create, Select, XForm, Deform, Model, Mesh, and more
3. Click the Create category to reveal the primitive shapes: Box, Sphere, Cylinder, Cone, Torus, Stairs, and others
4. When you're finished modeling, switch back to Select mode (Shift+1) to return to normal editing

Figure: Modeling Mode's Create category · the modern in-editor blockout palette (Box, Sphere, Cylinder, Cone, Torus, Stairs, and more). Each shape you create becomes a real Static Mesh.

Building Your First Room

Let's block out a simple room. The workflow is: create a solid box, then carve the interior and a doorway with Boolean subtractions.

Step 1: Create the Shell

1. In Create, click Box. Click once in the viewport to drop it, then move the mouse and click again to set its footprint and height (or just click Complete for a default box)
2. In the Modeling panel on the left, set exact dimensions: Dimensions X 400, Y 600, Z 300 (centimeters)
3. Click Complete (or press Enter). Unreal saves a new Static Mesh asset and drops an instance in your level

Figure: A box created with Modeling Mode's Box tool · unlike a BSP brush, this is a genuine Static Mesh asset the moment you complete it.

Step 2: Carve the Interior

1. Create a second, smaller Box inside the first (for example X 340, Y 540, Z 240, which leaves 30cm walls, floor, and ceiling), centered in the shell
2. Select both boxes, then in the Model category choose Boolean and set the operation to Subtract (the inner box removes its volume from the shell)
3. Click Complete. You now have a hollow room with walls, a floor, and a ceiling

Step 3: Add a Doorway

1. Create a small Box sized like a doorway (about X 100, Y 90, Z 200) positioned so it passes through one wall at floor level
2. Select the room and the doorway box, run Model → Boolean → Subtract again, and Complete
3. The opening is cut clean through the wall

You now have a complete room with an entrance!

Common Modeling Mode Shapes

Legacy Route: BSP Brushes

For completeness, here is the older BSP workflow. The concepts (additive and subtractive volumes) are identical; only the tool differs. BSP brushes live in the Place Actors panel under the Geometry category:

1. Open Window → Place Actors
2. Expand the Geometry category to find Box, Cone, Cylinder, Sphere, and Stair brushes
3. Drag a brush into the viewport; a brush defaults to Additive, and you switch it to Subtractive in the Details panel to carve
4. Because BSP is legacy, prefer Modeling Mode for anything you intend to keep

Figure: The legacy BSP brushes still live under Place Actors → Geometry in 5.8 · useful to recognize, but Modeling Mode is the modern choice.

Shaping and Combining Geometry

Placing a box is just the start. Both Modeling Mode and legacy BSP give you ways to combine volumes and reshape them into rooms, arches, and details. The concepts below apply to both; Modeling Mode is where you'll use them in practice.

CSG Operations

CSG (Constructive Solid Geometry) is the technical term for Boolean operations on solid volumes. Legacy BSP applied them automatically through brush types; in Modeling Mode you invoke them directly with the Boolean tool (Model category), choosing Union, Subtract, or Intersect. Same three operations, either way.

Figure: The three main CSG / Boolean operations. In Modeling Mode you pick them explicitly (Model → Boolean); legacy BSP applied them by brush type.

Modeling Mode Shaping Tools

Once a shape exists, the PolyModel and Model categories let you turn a plain box into real architecture:

• Extrude: Push a selected face in or out. The workhorse for pulling a room out of a wall or raising a platform.
• Inset: Shrink a face inward to create a frame, ledge, or recessed panel.
• Bevel: Round or chamfer edges so corners read as trim rather than hard blocks.
• Boolean: Union, Subtract, or Intersect two meshes (the CSG operations above).
• PolyEd: Full control over individual vertices, edges, and faces when a shape needs hand-tuning.

Snapping and Alignment

Precise placement keeps blockouts clean in either tool:

• Grid snapping: The grid button in the viewport toolbar sets the snap increment (common values: 10, 25, 50, 100). Hold Ctrl while dragging to snap.
• Transform typing: Enter exact Location and Dimensions in the Details or Modeling panel for pixel-perfect walls.
• Vertex snapping: Hold V while dragging to snap a mesh to another mesh's vertices, so walls meet cleanly with no gaps.

Good alignment prevents the gaps and seams that plague sloppy blockouts.

Why BSP Became Legacy

Modeling Mode outputs Static Meshes, so it sidesteps the problems that made BSP a legacy tool. It is still worth understanding those limitations, both to know why the field moved on and in case you open an older BSP level.

What BSP Gave Up

In early Unreal Engine versions (UE1, UE2), BSP was the primary way to build levels. Because Modeling Mode produces Static Meshes directly, it wins on every row of the table below, which is exactly why it replaced BSP as the default blockout tool:

Common BSP Problems

When working with BSP, you'll likely encounter these issues:

1. BSP Holes / Cracks

Thin black lines appearing at brush intersections. Causes: Floating-point precision errors, non-coplanar surfaces, overlapping brushes. Fix: Rebuild geometry, align brushes to grid, ensure surfaces are flush.

2. Z-Fighting

Flickering between two overlapping surfaces. Causes: Two brush faces occupying the same space. Fix: Offset one surface slightly, or merge brushes into one.

3. Invalid Brush

Error message after editing geometry. Causes: Creating non-planar faces, self-intersecting geometry, degenerate triangles. Fix: Undo changes, rebuild from simpler brushes, avoid extreme vertex editing.

4. Lighting Artifacts

Splotchy shadows, light leaks, or black surfaces. Causes: BSP doesn't have explicit lightmap UVs, small surfaces, overlapping geometry. Fix: Increase lightmap resolution, rebuild lighting, convert to Static Mesh with proper UVs.

Figure: Common BSP issues - cracks at seams, Z-fighting flicker, and lighting artifacts.

Best Practices (If You Use Legacy BSP)

If you do work with legacy BSP brushes, these guidelines keep problems to a minimum (with Modeling Mode, most of them simply don't arise):

1. Keep it simple: Avoid complex intersecting geometry—use clean, orthogonal shapes
2. Align to grid: Always snap brushes to grid (10, 25, 50, 100cm increments)
3. Rebuild frequently: Press Ctrl+Shift+; often to catch errors early
4. Avoid small details: Don't use BSP for trim, molding, or fine details—use Static Meshes
5. Limit brush count: Fewer, larger brushes perform better than many small ones
6. Test collision: Add a player character and physically walk through your spaces
7. Plan for conversion: Design with the knowledge you'll convert to Static Meshes later

From Blockout to Production

Blockout geometry eventually becomes finished art. How you get there depends on the tool you used. Modeling Mode is already a Static Mesh, so there is nothing to convert , so you refine it in place or hand it to an artist. Legacy BSP must first be converted to a Static Mesh, described below.

Why Convert?

Converting BSP to Static Meshes gives you several advantages:

• ✅ Better performance: Static Meshes render more efficiently
• ✅ Cleaner visuals: No more BSP cracks or Z-fighting
• ✅ Nanite support: Can enable UE5's virtualized geometry for extreme detail
• ✅ Better lighting: Proper lightmap UVs eliminate artifacts
• ✅ Simplified collision: Can create custom, optimized collision meshes
• ✅ Reusability: Converted meshes become assets you can use elsewhere

The Conversion Process

If your blockout is legacy BSP, here is how to convert it to a Static Mesh (Modeling Mode output is already a mesh, so you can skip this):

1. Select the BSP brushes you want to convert (you can select multiple)
2. Go to Brush → Create Static Mesh in the menu (or right-click and choose "Convert to Static Mesh")
3. A dialog will appear asking where to save the new mesh
4. Choose a location in your Content Browser (create a "Geometry" folder if needed)
5. Name the mesh descriptively (e.g., "SM_Room_Prototype_01")
6. Click Create Static Mesh
7. The original BSP brushes remain in the level—you can delete them or keep them as reference
8. A new Static Mesh Actor is created in the same location with the geometry

Figure: Blockout to production · Modeling Mode meshes skip straight to refinement; legacy BSP adds a convert-to-Static-Mesh step first.

Post-Conversion Steps

After converting to Static Mesh, you'll want to refine the mesh:

1. Open the Static Mesh Editor: Double-click the new mesh in Content Browser
2. Generate Lightmap UVs: Go to Details → LOD0 → Generate Lightmap UVs (fixes lighting artifacts)
3. Create collision: Collision → Add Box/Capsule Simplified Collision (or auto-generate)
4. Apply materials: Replace the default BSP material with proper materials
5. Set up LODs: For distant viewing, create Level of Detail meshes (optional but recommended)
6. Enable Nanite (UE5): In Details, enable "Nanite" for massive performance gains with complex geometry

Alternative: Replace with Modular Assets

Instead of converting BSP directly, you might choose to replace it with modular mesh pieces:

1. Keep your BSP as a reference (make it invisible or put in a "Reference" folder)
2. Create or obtain modular wall, floor, ceiling, and door pieces
3. Place these meshes to match your BSP layout
4. Delete the original BSP once replacement is complete

This approach gives you more control over the final look and often results in better visual quality, but takes longer than simply converting.

Summary

Blocking out geometry in-editor is the first step of level design, and in modern Unreal that means Modeling Mode. Legacy BSP taught the field the additive/subtractive way of thinking, which still underlies today's tools. Let's recap:

Key Takeaways

• 🏗️ Blockout (greyboxing) is the first pass of level design: simple placeholder geometry to test layout and gameplay
• 🔧 Modeling Mode is UE5's modern in-editor blockout tool; every shape you create is a real Static Mesh
• ➕➖ Additive and subtractive (Union / Subtract Booleans) is the shared idea behind both Modeling Mode and legacy BSP
• 🧱 Shaping tools like Extrude, Inset, and Bevel turn a plain box into real architecture
• ⚠️ BSP is legacy: performance issues, visual artifacts, and no Nanite support , and Epic recommends against it for final geometry
• 🔄 Straight to production: Modeling Mode meshes need no conversion; legacy BSP must be converted to a Static Mesh first
• ✅ Best practice: Block out with Modeling Mode, keep shapes simple and grid-aligned, then refine into finished meshes

What's Next?

Now that you can block out geometry with Modeling Mode, the next lesson introduces Lesson 2.4: Static Meshes and Assets. You'll learn how to import 3D models from external software and build levels from modular mesh sets, the backbone of modern level construction.