The Horizontal Slice

tim-cain game-development production level-design quest-design outer-worlds

Abstract

Problem: How do you determine the right balance between main quest content and side quests, and how do you know how much space you have to fill in your game world?

Approach: Tim Cain explains the "horizontal slice" — a production technique he developed across multiple projects — where you build out all the maps needed for the main quest line from start to finish, even in rough/gray-box form, to get a visceral sense of the game's scope.

Findings: The horizontal slice reveals how much space the main quest uses, how much remains for side quests and exploration, and forces the team to confront the true scale of production work needed. Combined with XP budgeting rules (like the 80/20 rule), it becomes a powerful planning tool.

Key insight: Building every map the main quest touches — even as gray boxes — gives the team a concrete, visceral understanding of scope that no design document can match, and naturally reveals where side quests and exploration content should go.

Source: https://www.youtube.com/watch?v=HeauMfrHbvU

1. Vertical Slice Recap

Before explaining the horizontal slice, Tim recaps the vertical slice: you take one area of your game and complete it to shippable quality. All art is final, creatures are animated and balanced, the player has appropriate abilities — everything looks and plays like it could ship. For The Outer Worlds, the vertical slice excluded the player's ship (so no companion-swapping or crafting stations were needed), but everything else in that slice was fully polished. This is what publishers typically ask for because many people can't look at gray-box levels and imagine the final product.

2. What Is the Horizontal Slice?

The horizontal slice is the opposite approach in scope. Instead of finishing one area to perfection, you create all the maps needed for the main quest line from beginning to end. Most of these maps aren't finished — they might be gray boxes, NPCs might be standing in T-poses with stubbed dialogue, caves might just be transitions into big gray rooms, and lockpicking might auto-succeed with a text message instead of a minigame.

The point isn't polish. The point is instantiation — making the entire main quest path physically exist in the game so you can walk through it.

3. Why It Matters

3.1. Confronting the True Scale

Putting all the maps together in their relative sizes and configurations, with all NPCs and quests placed, gives the team a concrete idea of how much content needs to be made. Even if everything is written in a design document somewhere, the horizontal slice is where people suddenly realize: "Wow, we have a lot of maps to make, a lot of art to make, a lot of dialogue to write, a lot of quests to script."

3.2. Revealing Space for Side Content

Once the main quest path is laid out, you can see which parts of each map it actually uses. If a player leaves a castle and travels across an outdoor area to reach a cave, the main quest only touches the path between those two points. Everything else is available for side quests, exploration content, or environmental storytelling.

3.3. XP and Content Budgeting

Tim connects this to his 80/20 rule (discussed in a separate video on XP): 80% of XP should come from the main quest, 20% from side quests. The horizontal slice makes this tangible — if the main quest uses a certain percentage of map space, you know roughly how much room you have for side content, and how much is left for pure exploration (ruins to climb, creatures to fight, loot to discover — content with no quest attached).

The ratios are flexible. You might go 90/10, 60/40, or 50/50 depending on your game. The key rule Tim follows: a player who does only the main quest should still be able to finish the game, reaching sufficient power level without any side quests. Side quests then serve as optional ways to gain extra power, better items, and easier late-game encounters — especially important for harder character builds that lack dialogue solutions to the final encounter.

4. Lessons from Experience

Tim traces the evolution of this technique across his career:

  • Fallout and Arcanum had no formal process — content distribution was "feel it as you go," resulting in uneven quest density with big chunks of the world having nothing in them.
  • Temple of Elemental Evil, based on a tabletop module, had areas that were too dense in some places.
  • Vampire: The Masquerade – Bloodlines ran out of time, leaving areas like the Warrens as large spaces with very little content.
  • Carbine (WildStar) was where Tim first tried the horizontal slice, but it failed — partly because artists wanted to fully art the levels instead of accepting gray boxes, which defeated the purpose.
  • The Outer Worlds was where the horizontal slice finally worked well, giving the team a clear picture of scope and content distribution.

5. The Gray Box Problem

One of the biggest challenges with the horizontal slice is that some people cannot look at a gray-box level and imagine what it will be like in the finished game. This is the same reason publishers demand vertical slices — they need to see the final product to understand it. On a development team, artists in particular may resist gray-box levels, wanting to fully art them before anyone evaluates the space, which undermines the speed and flexibility the horizontal slice is meant to provide.

6. References