If you’ve ever watched your game’s frame rate nose-dive during an intense scene, you’re not alone.
Chances are, your engine is still rendering objects the player can’t even see. Hidden rooms, blocked hallways, entire areas behind a wall—if your GPU is wasting cycles on them, you’re sacrificing performance for visuals that don’t matter.
That’s where occlusion culling comparison comes in.
In this article, we break down some of the most effective methods used in modern game development—like Portals, Hardware Occlusion Queries, and Software Rasterization. You’ll learn not only how each technique works, but which ones are best suited for your game’s world structure, whether you’re building tight indoor corridors or expansive outdoor maps.
We’ve based this guide on proven best practices and tested implementations across commercial engines. It’s a focused, real-world breakdown aimed at developers who want real performance gains—not just theory.
Smarter rendering starts here. Let’s find the occlusion strategy that fits your project.
The Core Concept: What is Occlusion Culling?
At its heart, occlusion culling is all about efficiency. It’s the process of figuring out which objects in a 3D scene are completely hidden behind others—and then skipping the work of rendering those hidden elements. If the camera can’t see it because something else is in the way, why waste resources drawing it?
To be clear, occlusion culling isn’t the same as frustum culling. Occlusion culling vs. frustum culling: frustum culling removes objects outside the camera’s view cone, while occlusion culling removes objects inside the view cone that are blocked. (Think of it like not painting furniture that’s behind a curtain—you know it’s there, but no one else needs to see it.)
This matters because reducing the number of draw calls—commands the CPU sends to the GPU—can massively improve performance. In modern game engines, a well-optimized occlusion system can make dense, layered environments feel smooth and responsive, even during fast-paced action sequences.
Pro tip: Smart culling means you can pack more detail into your world without bogging down your frame rate.
Technique 1: Portals and Cells – The Architectural Approach
Start with an anecdote about building your first multiplayer map in a retro-style shooter.
I still remember the first time I built a level using portals and cells—painstakingly carving out every room, placing each portal by hand like a digital bricklayer. Sure, it was tedious, but the performance gains? Worth it. I went from dropped frames in chokepoints to silky-smooth gameplay—with just some clever spatial planning.
Here’s how it works.
This technique divides a game level into discrete zones called cells (think rooms or hallways), which are connected by portals (basically, doorways). The game engine then checks which cells are visible from the player’s current location, only rendering what can be seen through portals. That means fewer draw calls and better frame rates.
Ideal Use Cases
- Works best in tightly structured environments like dungeons, space stations, or corridor shooters.
- Classic games like DOOM 3 and Thief thrived on this system (gloomy halls never looked so efficient).
Of course, it’s not all good news.
- Heavy manual setup. Level designers need to define each cell and portal explicitly. No freebies here.
- Rigid structure. It’s a nightmare in open-world or destructible scenes where layouts change dynamically.
Some might argue that occlusion culling is more versatile since it dynamically hides objects blocked by others at runtime, requiring less manual layout. Fair—but portals and cells hit different: they’re faster, leaner, and built for environments where precision pays off.
Pro Tip: If your level design is prone to modular reuse (copy/paste corridor sections), portals can help squeeze massive gains from minimal effort—once the layout’s done, reuse becomes your friend.
(Creating a labyrinth made of cloned hallways? Surprisingly efficient. Surprisingly terrifying.)
Technique 2: Hardware Occlusion Queries – The GPU’s Verdict
Ever wonder if your seemingly invisible enemy characters are actually chewing up rendering time? That’s where hardware occlusion queries step in, acting like the GPU’s bouncer—checking who really gets to be on the screen.
Here’s how it works: instead of rendering every 3D model in full detail, the engine asks the GPU a smart question—Is this object even visible? To do this, it first renders the big occluders (like walls, rocks, buildings), then sends a test using the object’s bounding box (a lightweight, simplified version of the object). If the GPU reports back with zero visible pixels, the full render for that object is skipped. Simple concept. Massive savings.
Ideal for: Games with lots of motion—think chaotic battle royales, bustling medieval towns, or sprawling open-world explorers where players peek around every corner. Not your slow, static museum walkthrough.
But it’s not all roses. The biggest issue? Latency. The CPU may have to wait a frame or two for the GPU to respond (kind of like texting someone who only replies after dinner). And if you send too many queries per frame, it may bog down rendering instead of speeding it up.
Still, for many developers, the pros trump the trade-offs:
- Pixel-level precision, meaning it culls what’s actually hidden.
- Runs live, no need for pre-baked visibility data you have to bake again each time your level changes.
Pro Tip: Use bounding boxes wisely. Over-complicating test geometry just defeats the purpose. Keep it lean.
Compared to other methods in occlusion culling comparison, hardware queries offer unmatched accuracy but come at a small cost in latency.
Want to optimize your frame rates even more? Check out our guide on tips to reduce frame drops in unity and unreal. Spoiler: a few smart tweaks can go a long way.
Technique 3: Software Rasterization – The CPU-Side Solution
“This is one of those rare cases where doing more work on the CPU can actually save you time,” said lead engine developer Merielle Chan during a recent AMA. She was referring to software rasterization — a CPU-based visibility check that mimics what the GPU’s Z-buffer does, but entirely in system memory.
Here’s how it works: the engine rasterizes a simplified, low-res version of large occluders into a software-managed depth buffer. Then, before any object gets shipped off to the GPU (which is like sending packages through customs), it checks bounding boxes against this buffer. If something’s fully hidden? Don’t bother rendering it.
You’ll hear some devs roll their eyes: “But why not just use the GPU’s occlusion queries?” Fair question. But there’s a catch.
Unlike those hardware-based checks, which can cause delays while waiting for the GPU to confirm visibility (cue the sync stalls), this method keeps it all on the CPU. That’s a win for CPU-heavy games, especially when frames per second are flirting with danger zones.
Pro tip: If your game has lots of AI or simulation logic that barely touches the GPU, this method can buy back valuable performance headroom.
Of course, it’s not perfect. Accuracy takes a hit — low-res buffers aren’t crystal-clear judges — and implementation? One dev described it as “threading a needle while the fabric is on fire.” Still, in the right situation, it sings.
Think of it like an occlusion culling comparison between precision and control — GPU may be sharper, but CPU gives you speed and autonomy.
Choosing the Right Culling Strategy for Your Game
You came here to make sense of occlusion culling comparison—and now, you’ve got the full picture.
From Portals to Occlusion Queries, we broke down the strengths and trade-offs of each method. Whether you’re building tight corridor shooters or sprawling open worlds, you now know which techniques play to each environment’s strengths.
Your challenge was clear: improve frame rates without sacrificing visual fidelity. Now you know how to do that—by matching your culling strategy to your level design and performance bottlenecks.
Start with strong frustum culling. Then, analyze your bottlenecks and test different solutions until your game runs smoother under pressure.
