If you've ever tried building a character system from scratch, you know that setting up a roblox custom leg filter script can be a total nightmare if you don't know where to start. It's one of those things that sounds simple on paper—you just want the legs to ignore certain parts of the environment—but once you get into the weeds of Luau and physics, things get messy fast. Whether you're working on a custom procedural animation system or just trying to stop your character's feet from clipping through the floor, getting the filtering right is the difference between a polished game and a glitchy mess.
The reality is that Roblox's default physics engine does a lot of the heavy lifting for us, but as soon as you step outside the "standard" R15 or R6 movement, you have to take control of the reins. You might be building a spider-mech, a giant boss, or even just a highly detailed human character that uses Inverse Kinematics (IK). In all these cases, your script needs to know exactly what the legs should interact with and what they should ignore.
Why you actually need a filter script
Most developers realize they need a roblox custom leg filter script when their characters start doing weird things. Maybe the legs are trying to step on the character's own torso, or perhaps they're reacting to invisible triggers and decorative grass that should have no physical impact. If your leg raycasts are hitting the character itself, the legs will likely crumple, jitter, or fly off into space.
By implementing a custom filter, you're essentially giving your script a "blacklist" or a "whitelist." You're telling the game, "Hey, when you're looking for the ground, ignore everything inside this folder," or "Only look for parts labeled as Floor." This level of control is vital for performance too. If your script is constantly checking every single part in a 100-player server, your frame rate is going to tank. A well-optimized filter keeps things snappy.
Setting up the RaycastParams
To get started with a roblox custom leg filter script, you have to understand RaycastParams. This is the modern way to handle collisions and line-of-sight in Roblox. Back in the day, we used FindPartOnRayWithIgnoreList, but that's been deprecated for a while now.
When you create your params, the most important property is FilterDescendantsInstances. This is where you feed the script an array of objects it should ignore. Usually, you'll want to include the character's own model in this list. If you don't, the leg will "hit" the character's hip or knee, and the math for the foot placement will break instantly. You can also toggle the FilterType between Enum.RaycastFilterType.Exclude (the old ignore list style) or Enum.RaycastFilterType.Include (only hit these specific things).
Dealing with Collision Groups
While raycast filtering handles where the leg "looks," collision groups handle where the leg "touches." If you're writing a roblox custom leg filter script to handle procedural walking, you'll probably want to use the PhysicsService to set up specific groups.
For instance, you might have a group called "Legs" and a group called "IgnoreLegs." This allows you to have decorative items in your world—like small pebbles or translucent area triggers—that the player can walk through without the leg-stepping logic trying to climb over them. It keeps the movement smooth. Without this, your character might look like they're tripping over every tiny pebble on the road, which is frustrating for the player and looks pretty bad visually.
Handling R6 vs R15 differences
One thing that catches people off guard when writing a roblox custom leg filter script is the difference between R6 and R15 rigs. R6 rigs are simple—just two blocks for legs. R15 is a whole different beast with multiple joints (UpperLeg, LowerLeg, Foot).
If your script is designed for R15, you have to ensure that your filter list covers all fifteen parts of the body. If you miss one—say, the "LeftFoot"—the raycast might hit that foot while the leg is moving forward, causing the leg to think it's already found the ground. This results in the leg snapping to its own foot. It's a hilarious bug to see, but a pain to fix if you aren't paying attention to your filter arrays.
Optimizing the script for performance
You don't want your roblox custom leg filter script running on every single heartbeat if it doesn't have to. If the character is sitting in a vehicle or floating in the air, you can probably throttle the script or turn it off entirely.
Another trick is to simplify the geometry the legs are looking at. Instead of raycasting against high-poly meshes, many developers create an invisible "NavMesh" or a simplified collision floor. Your filter script can be set to only see this invisible floor. This makes the math way faster and prevents the legs from getting stuck on weird jagged edges of a complex 3D model. It's all about being clever with what the script is allowed to perceive.
Common pitfalls and how to avoid them
One of the biggest mistakes I see is people forgetting to update the filter list when the character changes. If you equip a sword or a piece of armor, that new part is now a "descendant" of the character. If your roblox custom leg filter script was only set to ignore the original body parts, it might suddenly start hitting the new sword.
Always make sure your script is dynamic. Instead of just listing parts, point it to the entire Model of the player. That way, anything added to the player—hats, tools, capes—is automatically ignored by the leg logic. Also, keep an eye on your CanQuery property. If you have parts that should never be hit by a raycast anyway, toggling CanQuery to false is a great way to bake that "filtering" directly into the game world without needing extra lines of code.
Fine-tuning the leg placement logic
Once your filter is solid, the next step is usually the actual placement math. A roblox custom leg filter script is really just the foundation. You take the result of that filtered raycast (the Position and Normal of the hit) and use it to position the foot.
If the raycast hits a steep slope, the "Normal" tells you the angle of that slope. A good script will tilt the foot to match that angle. If your filter is working correctly, this looks incredibly realistic. If the filter is weak, the foot might tilt based on a nearby wall or the character's own hand, leading to some very "broken-ankle" looking animations.
Final thoughts on implementation
Building a robust roblox custom leg filter script isn't just about writing ten lines of code and calling it a day. It's about understanding how your character interacts with the world you've built. You have to think about the edge cases—what happens if the player stands on a moving platform? What if they're standing on another player?
If you're using a whitelist approach (Include), make sure all your ground parts are tagged correctly or put into a specific folder. If you're using a blacklist (Exclude), keep it updated as the character's inventory changes. It sounds like a lot of maintenance, but once you get the logic down, it becomes second nature.
Ultimately, the goal is immersion. When a player walks across uneven terrain and their legs perfectly adjust to every rock and dip without glitching through their own body, they don't think about the script running in the background. They just think the game feels "right." And honestly, that's the best compliment a programmer can get. It takes a bit of trial and error to get the raycast params and collision groups perfectly synced, but the result is a much more professional-feeling experience for anyone playing your game. Keep testing, keep tweaking those filters, and eventually, your character physics will be rock solid.