Killing the Walk Monster

59K views · 6 years ago · talk · expert · evergreen

Casey Muratori of Molly Rocket explains the system he built for player movement and collision in Jonathan Blow's The Witness, the so-called walk monster. He describes the problems with conventional collision handling and the approach that replaced it.

HN people cite it as the definitive account of how custom movement code freed The Witness's level designers and artists to work without collision bugs.

“There's a talk that Casey gives where he explains how he implemented the movement system for The Witness”

8 comments from Hacker News

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@obl2021 · in “Player Movement System for the Witness”
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@jp31412020 · in “Killing the Walk Monster [video]”
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@seventh-chord2020 · in “Preventing entire classes of bugs in collision systems (2018) [video]”
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@jodrellblank2026 · in “Casey Muratori – The Root of the Root of All Evil – BSC 2026 [video]”
> "if walk monster manually trolls the map" Casey Muratori has a 50 minute video on "Killing the Walk Monster" which is about how they started looking for glitchy game geometry by pretending to be a player walking around, then by a flood-fill ("walk monster") probing for barriers, then the video is explaining how they designed something "exponentially faster" (both demonstrated starting at time 38:33). The better version was realtime enough that level designers could use its overlays interactively while working on the level, and during test playthroughs. He describes it as "exponentially faster". It doesn't seem like the earlier poster calling it "brute force" is a reasonable description. ▶ this video And the place the player can walk but shouldn't be able to (off the boat), is not a fault in the walk system, as explained in that video near the end, it's a place where level designers made a mistake.
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@wasmperson2025 · in “The Big Oops: Anatomy of a Thirty-Five-Year Mistake [video]”
A custom engine is never _needed_ technically speaking since COTS engines are often customizable to the point where you can do anything you want with them. That doesn't mean that they don't influence the design of games, though. There's a talk that Casey gives where he explains how he implemented the movement system for The Witness, in which he shows examples of Unity-based "walking simulator"-type games dealing with limitations of the engine in ways that The Witness was able to totally avoid. This allowed the game's artists to be creative with set design without worrying as much about performance issues or collision bugs, thus potentially opening up more design space. ▶ this video Here's some praise of The Witness by Fabien Giesen: > That’s where I am right now. I have never seen another game as cohesive as this. Not even close. Without getting into specifics or spoiler territory, I have never seen a game with such manifest intention behind every single detail, nor one where all the details cohere this well into a whole. This goes for the design and gameplay itself, but also across traditional hard boundaries. Game design and art play off each other, and both of these are tightly coupled with some of the engine tech in the game. It’s amazing. There is no single detail in the game that would be hard to accomplish by itself, but the whole is much more than the sum of its parts, with no signs of the endless compromise that are normally a reality in every project. link
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@sunrunner2025 · in “Splitting from Handmade Network”
I feel the same about Casey and his work, and like another commenter said, his simple, practical and honest approach to his work is refreshing in an era of languages, environments and tools that feel bloated, slow and ineffective (IMHO). > That being said the whole network idea, and the fetishisation of his particular coding philosophy was always cringe. And I agree with this too. The point of Handmade Hero to me was always to show that the complex software systems that you interact with can be understood despite their size and complexity, and that it's still possible (though perhaps in a lot of cases impractical) to recreate these from the ground up. Also that there's tremendous value in understanding how something works from first principles (see also Casey's 'How to Open a Black Box' lecture, link). Having said that, while I feel it's important to aspire to some level of simplicity and solving-the-actual-problem that Casey's coding style embodies, it also feels like an approach that works best only when a) the number of people working on a single program is relatively small and b) the expertise is deep and so the need for abstractions is low or non-existent. With Casey's background in game tools it makes a lot of sense, but this approach doesn't seem like it scales up, and he's essentially the only contributor to Handmade Hero too. > It took him and his team 8 years of dedicated work to make The Witness, and it's just not a very technically impressive game. I'm a big fan of the design of The Witness and Blow's work in general and I don't feel like being "technically impressive" was ever the goal, as it wasn't with Braid either. Jon is as much of a solo designer on his projects, and it's not clear how much of that total development time went into their custom rasteriser and navigation (Killing the Walk Monster, ▶ this video) versus puzzle design. At the end of the day I believe the end result was fantastic and I don't think too much about the development time, but it's true that Jon was in a unique position to be able to self-fund almost the entire project which a lot of other studios can't do. > There's clearly flaws in making software this way. As is always the case, being somewhere in the middle feels like the ideal place and extremes at either end have their own problems.
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@throwuxiytayq2022 · in “Unreal vs. Unity Opinion”
His contributions seem limited in scope but very significant: ▶ this video
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@db48x2021 · in “Why video game doors are so hard to get right [video]”
▶ this video Here’s a game developer talking about the problem and how they solved it in The Witness, but note well the limitations he describes. It’s not directly useful if the player can jump, for instance.