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Unity Collision, Physics, and Jumping for a 2D Controller

Set up collision flags, stop gravity buildup on the ground, add jumping, and tune jump height and apex time with Rigidbody2D-style platformer movement in Unity.

GuideKit EditorialEditorial team
6 min read
Unity Editor in play mode with the Inspector open on the right and a simple platformer scene visible in the Scene and Game views.
Unity is running in play mode with the Inspector visible and the player and jump settings shown on the right. — footage: Sebastian Lague

Use collision flags to check when the player is grounded, clear the Y velocity when grounded so gravity does not build up, and base jumping math on jumpHeight and timeToJumpApex for consistent results. Smooth X movement further by tuning separate acceleration settings for grounded and airborne movement.

Set up collision flags in the controller script

MonoDevelop Unity is open with Controller2D.cs displayed and the CollisionInfo struct visible near the bottom of the code editor.

The controller script is open in MonoDevelop with the CollisionInfo struct area visible near the bottom. — Sebastian Lague

steps

  1. In your controller script, define a CollisionInfo struct with booleans above, below, left, and right.
  2. Add a Reset method in the struct to set all flags to false. This prepares the collision state before each move.
  3. Store one CollisionInfo instance on your controller, and call its Reset() method at the top of the Move method.
  4. Update these flags within your horizontal and vertical collision checks: set left or right in horizontal checks depending on direction, and above or below in vertical checks.

Stop gravity from building up while grounded

Unity Editor with a 2D platformer scene open, the Controller2D script displayed in the Inspector on the right, and a simple level preview in the center.

Unity shows a 2D platformer scene with the Controller2D script open in the Inspector and a simple level in the Scene view. — Sebastian Lague

Add jumping from the grounded state

MonoDevelop Unity is open to Player.cs, showing C# code with a grounded collision check and a space key jump input branch.

MonoDevelop Unity displays the Player.cs script with grounded collision handling and a jump input branch in view. — Sebastian Lague

steps

  1. In the player script, use the jump input (typically space bar) and check that the below collision flag is true to confirm the player is grounded.
  2. When both are true, apply an upward velocity by assigning velocity.y = jumpVelocity.
  3. Store jumpVelocity in a variable so it is easy to tweak and not buried in the movement code.
  4. Test the jump in play mode to confirm that the character only jumps while on the ground.

Drive jump height with jumpHeight and timeToJumpApex

Unity editor with the Game view running, a Player object selected in the Inspector, and script fields for jump height and time to jump apex visible.

Unity editor shows the Player Inspector while the game is running, with jump height and time to jump apex fields visible. — Sebastian Lague

before after

Before

Direct gravity and velocity values (less intuitive)

// Old approach—hard to tune
float gravity = -2f;
float jumpVelocity = 8f;
After

Inspector-friendly jump tuning (recommended)

// Public fields, easier to adjust
public float jumpHeight = 4f;
public float timeToJumpApex = 0.4f;

void Start()
{
    gravity = -(2 * jumpHeight) / Mathf.Pow(timeToJumpApex, 2);
    jumpVelocity = Mathf.Abs(gravity) * timeToJumpApex;
}

With this approach, you can adjust jumpHeight and timeToJumpApex directly in the Inspector for quick iteration. This setup makes jump behavior more predictable and easier to tweak.

Smooth platformer movement with grounded and airborne acceleration

MonoDevelop-Unity shows a C# script for player movement with collision checks and autocomplete suggestions open over the code.

MonoDevelop-Unity displays a player movement script with grounded collision checks and acceleration logic visible. — Sebastian Lague

steps

  1. Replace direct assignment of horizontal velocity with Mathf.SmoothDamp, using a float variable to store the smoothing velocity reference.
  2. Define two new float fields: accelerationTimeGrounded (for when the player is standing on a surface) and accelerationTimeAirborne (for in-air control).
  3. When the player is grounded (collisions.below), use accelerationTimeGrounded; otherwise, use accelerationTimeAirborne as the smoothing time.
  4. In play mode, you should now see smoother left and right movement, with tighter response on the ground and looser, more floaty movement in the air for natural-feeling platform controls.

Also worth knowing

  • Physics sync options: If the player seems to sink into or bounce off surfaces, enable Auto Sync Transforms in the Physics and Physics 2D Project Settings for greater stability.
  • Verlet integration for stable jumps: Euler-style physics integration can introduce minor jump errors. For especially repeatable jump apex heights, try a Verlet-based calculation for improved physical accuracy.
  • Collision ray length tip: When building collision rays, apply skinWidth after taking the absolute value of velocity. Adding skinWidth to a negative value can cause subtle bouncing issues against surfaces.

FAQ

how to add collision in Unity

Set up a struct with flags for above, below, left, and right collisions; reset these at each move; update them in your collision checks; and zero out the Y velocity whenever the player is grounded to prevent unwanted movement through surfaces.

Unity Rigidbody2D tutorial

Control movement using a Rigidbody2D-style pattern: check grounded state, assign jump velocity on jump input, and apply velocity smoothing for stable, responsive character movement.

Unity 2D physics basics

Handle gravity, collision flag management, and velocity tweaking separately, using collision info to decide when to apply or zero velocity changes.

Unity jumping and gravity guide

Let jumpHeight and timeToJumpApex decide the math for gravity and jump velocity rather than using hard-coded numbers, making jump tuning predictable and intuitive.

Unity platformer movement tutorial

Get smooth, responsive movement by using different acceleration smoothing values for grounded and airborne movement, making controls both tight and natural for players.

Unity collision detection script

Store per-side collision states (above, below, left, right) in a dedicated struct, reset them every frame, and update them in the controller’s collision handlers for reliable detection.

Official reference: Unity manual.

Related guides

Sources

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