6. Physics & Collision Systems
# 6. Physics & Collision Systems
Defold's physics and collision systems provide the foundation for creating realistic interactions in your platformer game. This section explores how to implement and fine-tune physics for your platformer characters and environment.
## Physics Components in Defold
Defold uses a 2D physics engine based on Box2D that provides several key components:
- **Collision Objects**: Define physical properties and collision shapes
- **Collision Shapes**: Define the geometry for collision detection
- **Joints**: Connect collision objects together
- **Physics Materials**: Define physical properties like friction and rebound
## Collision Object Types
Defold offers different collision object types to suit various game elements:
| Type | Description | Use Case |
|------|-------------|----------|
| **Dynamic** | Objects affected by gravity and forces | Player character, enemies, moving platforms |
| **Kinematic** | Objects that move but aren't affected by physics | Moving platforms with predefined paths |
| **Static** | Immovable objects | Ground, walls, fixed platforms |
| **Trigger** | Non-physical areas that detect collision | Checkpoints, power-up zones, death zones |
## Setting Up Collision Groups
Collision groups determine which objects can collide with each other. In a platformer, you might use groups like:
```lua
-- collision.lua
local M = {}
M.GROUP_PLAYER = hash("player")
M.GROUP_ENEMY = hash("enemy")
M.GROUP_GROUND = hash("ground")
M.GROUP_ITEM = hash("item")
M.GROUP_PROJECTILE = hash("projectile")
return M
```
## Collision Messages
When collisions occur, Defold sends messages that you can handle in your scripts:
```lua
function on_message(self, message_id, message, sender)
if message_id == hash("collision_response") then
-- Handle collision
if message.group == hash("ground") then
-- Player has hit ground
self.grounded = true
end
elseif message_id == hash("trigger_response") then
-- Handle trigger
if message.group == hash("item") then
-- Player has collected an item
end
end
end
```
## Physics Properties
Configure physics properties in your game.project file:
```
[physics]
gravity_y = -1000
scale = 0.01
debug = 0
```
## Ray Casting for Ground Detection
Ray casting is useful for detecting ground beneath the player:
```lua
function check_ground(self)
local ray_start = go.get_position()
local ray_end = vmath.vector3(ray_start.x, ray_start.y - 20, 0)
local result = physics.raycast(ray_start, ray_end, {hash("ground")})
if result then
self.grounded = true
else
self.grounded = false
end
end
```
## Implementing One-Way Platforms
One-way platforms are a common platformer feature. Here's how to implement them:
```lua
function on_message(self, message_id, message, sender)
if message_id == hash("collision_response") then
if message.group == hash("one_way_platform") then
-- Only collide if coming from above
if self.velocity.y <= 0 and message.normal.y > 0.7 then
-- Allow collision
self.grounded = true
else
-- Ignore collision
msg.post(sender, "ignore_collision")
end
end
end
end
```
## Visualizing Collision Shapes
For debugging, enable physics visualization in your game.project file:
```
[physics]
debug = 1
```
## Collision Flow Diagram
```mermaid
flowchart TD
A[Physics Update] --> B{Check for Collisions}
B -->|Collision Detected| C[Generate Collision Message]
C --> D[Call on_message in Scripts]
D --> E{Process Collision Response}
E -->|Accept| F[Apply Physics Response]
E -->|Ignore| G[Disable Collision]
B -->|No Collision| H[Continue Physics Update]
F --> I[Update Game State]
G --> I
H --> I
```
## Physics Calculations
The physics engine calculates forces and movement using equations like:
$$F = m \cdot a$$
Where $F$ is force, $m$ is mass, and $a$ is acceleration.
For jumping mechanics, the initial velocity needed to reach a height $h$ is:
$$v_0 = \sqrt{2 \cdot g \cdot h}$$
Where $g$ is gravity and $h$ is the desired jump height.
## Optimizing Physics Performance
1. **Use simple collision shapes** - Prefer circles and rectangles over complex polygons
2. **Limit dynamic objects** - Too many dynamic objects can slow down your game
3. **Use appropriate scale** - Keep physics values within a reasonable range
4. **Disable collisions between irrelevant objects** - Use collision groups effectively
5. **Consider using triggers** instead of collision objects when physical response isn't needed
## Common Physics Issues and Solutions
| Issue | Solution |
|-------|----------|
| Character sticking to walls | Implement sliding or add a small horizontal force |
| Falling through platforms | Increase collision shape size or use continuous collision detection |
| Jittery movement | Smooth positions or use fixed timestep for physics |
| Tunneling through objects | Increase collision shape size or decrease maximum velocity |
| Unstable stacking | Add friction or use constraints instead of stacking |
By mastering Defold's physics and collision systems, you can create responsive, realistic platformer mechanics that feel satisfying to players while avoiding common physics-related bugs and performance issues.