What Raycasting Actually Does in Roblox

Raycasting is just a line-draw operation that checks what it hits along the way. You give it an origin point, a direction, and optionally a maximum distance, and the engine returns the first surface it intersects. That result tells you distance, impact position, surface normal, and which part was hit. It sounds simple because it is simple, but the way Roblox implements it has quirks that trip people up constantly. I spent weeks debugging a stealth mechanic where enemies detected the player through walls, only to realize the raycast was hitting geometry on the other side of the corridor because I forgot that rays through transparent parts unless you explicitly filter them. The fix was adding an OriginPart parameter to the RaycastParams so the ray treats the starting volume as solid.

Implementing Roblox Raycasting Correctly

The basic call looks like this, though the real implementation requires more scaffolding than most tutorials show: workspace:Raycast(origin, direction, raycastParams) The origin is a Vector3 position in workspace space. The direction is a normalized Vector3. The raycastParams object controls what gets ignored and what you want returned. Without params, the ray hits everything including parts marked as children of the same model. Here is what a functional params object actually needs: local params = RaycastParams.new() params.FilterDescendantsInstances = {player, enemy} params.FilterType = Enum.RaycastFilterType.Exclude params.IgnoreWater = true FilterDescendantsInstances takes a table of parts or models. FilterType determines whether you include or exclude those objects. Setting it to Exclude means the ray passes through enemies but stops at walls. Include does the opposite, which is useful for line-of-sight checks between specific units. The return value is a RaycastResult object containing HitPart, Position, Normal, and Distance. Position is where the ray actually touched the surface in workspace coordinates. Normal points outward from that surface, which matters for ricochet calculations or determining if a wall faces toward the shooter. Distance is measured in studs from origin to impact point. I learned the hard way that Distance can return 0 when the ray starts inside a part. This happens frequently with proximity-based triggers when the player walks into a volume. The workaround is checking if Distance is less than 0.1 and treating it as a fail state rather than a valid hit. I added a minimum distance threshold that skips any result under half a stud to avoid false positives. The direction vector needs normalization before passing it in. If you skip this step, the ray uses the raw magnitude as its effective distance multiplier, which produces inconsistent results across different weapon types. A unit vector guarantees distance maps directly to studs regardless of input. For movement-based detection, cache the last frame position and cast from there to the current frame. This catches objects the player passed through between renders, which is impossible with position-only checks. The delta vector becomes your direction and the magnitude becomes your distance. local function castForward(part) local origin = part.Position local direction = part.Velocity.Unit local distance = part.Velocity.Magnitude * dt local result = workspace:Raycast(origin, direction * distance, params) return result end This pattern handles fast-moving projectiles without tunneling. The dt value comes from RunService.Heartbeat and gives you frame-accurate timing without relying on physics updates. For line-of-sight verification between two units, use the normalized direction between them and set the distance to their separation plus a small buffer. The buffer accounts for floating point precision errors that sometimes cause rays to miss by microscopic amounts. A 0.01 stud buffer usually covers this without introducing false positives. Raycasting against terrain works differently than against parts. Terrain returns a normal but no HitPart, since the terrain mesh is a single continuous object rather than discrete geometry. I wasted an afternoon checking HitPart:IsA("Terrain") before realizing terrain just doesn't populate that field. The solution is checking if HitPart is nil and assuming terrain hit when the ray returns a result without a part reference. Water filtering is another gotcha. Setting IgnoreWater to true makes the ray pass through water surfaces, which is usually what you want for underwater weapons. Leaving it false causes the ray to stop at the water-air boundary, breaking any mechanic that needs to detect targets through liquid. Performance matters when casting multiple rays per frame. Reuse a single RaycastParams object across calls instead of allocating new ones every frame. The engine caches the query structure, and reallocation adds measurable GC pressure during heavy raycasting scenarios like volumetric fog or dense foliage visibility checks. If you need multiple hits along a single ray, implement your own binary search or use region queries instead. Roblox raycasting stops at the first intersection by design, so there is no built-in multi-hit mode. My workaround for detecting all walls in a corridor was casting at incremented distances and collecting results until the ray exited the volume entirely. The main bottleneck with raycasting is frame budget, not calculation cost. A single ray takes microseconds. Firing hundreds per frame in a loop with fresh params objects is what slows things down. Batch your queries when possible, and use spatial partitioning to avoid unnecessary casts against empty space. For projectile weapons, consider using physical bullets at close range and raycasting only beyond a threshold distance. Raycast hits don't visually match trajectory in the same way as simulated physics, and players notice when bullets appear to teleport rather than arc naturally. The hybrid approach gives you performance for long-range sniping while keeping close combat feeling responsive.