Flat Planes vs. Solid Boxes: Choosing the Right Modular Geometry for Environment Design and for UE5

When creating modular environment assets for UE5, you'll face an important decision:

• Should you use flat plane geometry or solid box geometry?

Each approach has advantages and limitations, especially when working with UE5's features like Nanite and Lumen.

In this post and video I'll break down the differences between flat planes and solid boxes. Their pros and cons and why solid box geometry is often the better choice for modular assets.

While the focus is on UE5, these principles can apply to other game engines as well.

Video Tutorial

Understanding Flat Planes and Solid Boxes

To illustrate the differences, I have a simple scene and asset example of modular geometry using flat planes and solid boxes.

• Flat Plane Geometry: Single-sided plane, with the size of 300x300 units will be rendered only on one side. The back face will be invisible in-game.
• Solid Box Geometry: 300x300x20 unit cube will be rendered on all sides (top, bottom, front, back, left, and right), providing a fully enclosed, solid geometry.

Key Differences: Flat Planes vs. Solid Boxes

1. Triangle and Vertex Count

• Flat Planes: A single plane consists of only two triangles, making it highly efficient in terms of rendering cost. If you are prioritizing low triangle counts, flat planes are the way to go.
• Solid Boxes: A cube has 12 triangles, as it renders all six sides. While this increases the polygon count, geometry cost is cheap now days. Especially on next-gen platforms and if using UE5's Nanite system.

Takeaway: If minimizing triangle count is critical, flat planes are the better choice. However, with geometry being relatively inexpensive in modern engines, solid boxes will be a better option and you'll understand even more of why in a minute.

2. Light Occlusion and Shadow Casting

One of the most significant differences between flat planes and solid boxes is how they interact with light, particularly in UE5's Lumen dynamic lighting system.

• Flat Planes: Because flat planes are single-sided, they only block light on the rendered face. From the back, they are transparent, allowing light to pass through and failing to cast proper shadows. This can lead to unrealistic lighting in environments, especially in enclosed spaces. To deal with this, you must duplicate and rotate these planes or add other additional geometry to block light.
• Solid Boxes: Solid boxes block light and cast shadows correctly from all angles, as they are fully enclosed. This ensures consistent lighting behavior, making them ideal for dynamic lighting systems like Lumen.

Example in UE5: In a test scene, a flat plane wall casts a shadow only when viewed from the front. Rotating it 180 degrees reveals no shadow, as the back face lacks geometry. In contrast, a solid box wall consistently casts shadows and occludes light, regardless of the viewing angle.

Takeaway: Solid boxes are superior for realistic light occlusion and shadow casting and simplifying environment design.

3. Light Leaking Issues

Light leaking is a common problem in UE5 when using flat planes, particularly with Lumen. Because flat planes are paper-thin, light can seep through gaps or corners where geometry doesn't fully enclose a space.

• Flat Planes: In a test room built with single-plane geometry, light leaking was evident in dark corners, even when the room was fully enclosed. This occurs because Lumen struggles with thin geometry, leading to unwanted light artifacts.

• Solid Boxes: A similar room built with solid box geometry showed no light leaking, as the thicker, enclosed geometry properly occluded light.

According to Epic Games' Lumen technical doc, geometry for walls, floors, and ceilings should be at least 10cm thick to prevent light leaking and ensure proper distance field calculations. Single-plane geometry often fails to meet this requirement, while solid boxes naturally work.

Takeaway: Solid boxes prevent light leaking, making them the ideal choice for UE5 environments using Lumen.

4. Construction Efficiency

Building environments with modular assets can be time-consuming and the choice of geometry will impact this workflow.

• Flat Planes (Left): Constructing with flat planes can be time-consuming. To avoid see-through geometry or light leaking, you must duplicate planes, rotate them and position them with precision (e.g., creating at least 10-units of thickness as recommended by Epic Games). This requires constant adjustments, such as switching to orthographic viewports to align geometry, which slows down the process.
• Solid Boxes (Right): Solid boxes are far easier to work with. A single cube can be duplicated and placed without worrying about invisible faces or light issues, significantly reducing your construction time.

Example: A building constructed with flat planes took considerably longer to creatre due to the need for manual alignment and duplication. But similar structure built with solid boxes was completed in a fraction of the time.

Takeaway: Solid box geometry streamlines environment construction, saving time and preventing lighting issues.

5. Collision Handling

Collisions in UE5 are another area where solid boxes outperform flat planes.

• Flat Planes: UE5 struggles to generate automatic collisions for single-plane geometry. On import, enabling "Auto-Generate Collisions" failed to produce collisions for flat planes. I had to manually add simplified collision boxes, which may still cause issues due to the thin nature of the geometry.
• Solid Boxes: UE5 automatically generates collisions for solid geometry without issues.

Takeaway: Solid boxes work better with collisions and with auto-generated collisions in UE5.

When to Use Flat Planes

Despite their limitations, flat planes can still be good choice for certain scenarios:

• Low-Poly Environments: If triangle count is a primary concern (e.g., for mobile games or older engines), flat planes reduce rendering costs.
• Careful Design: By duplicating planes and adding small gaps (e.g., 10-20 units of thickness), you can minimize light leaking and occlusion issues.
• Hybrid Approaches: Combining flat planes for non-important geometry (e.g., distant scenery) with solid boxes for key structural elements, you'll be able to balance performance and visuals.

Why Solid Boxes Are Recommended for UE5

For most UE5 projects, solid box geometry is the superior choice due to its compatibility with Nanite and Lumen, as well as its ease of use:

• Nanite Compatibility: Solid boxes leverage UE5's Nanite system, which efficiently handles high-poly geometry, making triangle count less of a concern.
• Lumen Optimization: Solid boxes prevent light leaking and ensure proper light occlusion.
• Simplified Workflow: Solid boxes reduce the need for manual adjustments, speeding up environment construction.
• Reliable Collisions: Automatic collision generation works seamlessly with solid geometry, reducing setup time.

Free Project Files to Learn From

If you want to grab the assets that were used in this tutorial, download the FBX files, Maya scene file and UE5 assets from WoLD Patreon here.

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Now, if you are looking to learn and master the full modular environment design pipeline, then look no further than this complete tutorial course I've released, "Modular Environment Design Masterclass".

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