Creating Physics Nodes II

In this tutorial, we will dive into the advanced features of physics nodes, including:

1. Sensor Functionality: Sensors do not participate in physical collisions but can detect when objects enter or leave a specified area.
2. One-Way Platform: Using the onContactFilter callback to create a platform that only allows objects to pass through from one direction.
3. Detecting Collision Events: How to listen for and handle object collision events.

Through the following code examples and explanations, you'll learn how to implement these features in Dora SSR.

1. Creating the Physics World

First, initialize the physics world and set basic parameters.

Lua

local Vec2 <const> = require("Vec2")
local PhysicsWorld <const> = require("PhysicsWorld")

local gravity <const> = Vec2(0, -10) -- Define gravity direction and magnitude

local world = PhysicsWorld() -- Create the physics world
world:setShouldContact(0, 0, true) -- Set collision rules between groups
world.showDebug = true -- Show debug information

2. Creating a Sensor

A sensor is a special type of physics object that doesn’t participate in collisions but can detect other objects entering or leaving its area.

2.1 Defining the Sensor

Lua

local BodyDef <const> = require("BodyDef")

-- Define a hollow circular static body using a series of vertices
-- This limits the activity area for this example
local terrainDef = BodyDef()
local count <const> = 50
local radius <const> = 300
local vertices = {}
for i = 1, count + 1 do
	local angle = 2 * math.pi * i / count
	vertices[i] = Vec2(radius * math.cos(angle), radius * math.sin(angle))
end
-- Add a chain-shaped static body definition
terrainDef:attachChain(vertices, 0.4, 0)

-- Add a circular sensor with a tag number 99
terrainDef:attachDiskSensor(99, Vec2(80, 80), 100)

• attachChain: Creates a chain shape composed of multiple vertices.
• attachDiskSensor: Adds a circular sensor at the specified location that does not participate in collisions but can detect objects entering the area.

2.2 Listening for Sensor Events

Lua

local terrain = Body(terrainDef, world)
terrain:onBodyEnter(function(other, sensorTag)
	if sensorTag == 99 then
		-- Triggered when an object enters the sensor area
		other.velocity = other.velocity * 0.5 -- Reduce the object's speed by half
	end
end)
terrain:addTo(world)

• onBodyEnter: Called when another object enters the sensor area.
• Callback parameters:
  • other: The object that enters the sensor.
  • sensorTag: The tag used to identify different sensors.

3. Creating a One-Way Platform

By using onContactFilter, we can control how objects interact with the platform, creating a one-way passage.

3.1 Defining the Platform Body

Lua

local platformDef = BodyDef()
-- Define a rectangular platform
platformDef:attachPolygon(
	Vec2(0, -80), -- Position
	120, 30, -- Width and height
	0, -- Angle
	1, 0, 1.0 -- Density, friction, and restitution
)

3.2 Setting the Collision Filter

Lua

local platform = Body(platformDef, world)
platform:onContactFilter(function(body)
	return body.velocityY < 0 -- Only collide when the object is moving downward
end)
platform:addTo(world)

• onContactFilter: A custom collision detection rule.
• The callback returns true to allow the collision or false to ignore it.
• By checking the object’s vertical velocity (velocityY), we achieve the one-way collision effect.

4. Detecting Collision Events

By listening to collision events, we can execute specific logic, such as playing sound effects or triggering animations.

4.1 Creating a Dynamic Body

Lua

local BodyDef <const> = require("BodyDef")
local diskDef = BodyDef()
diskDef.type = "Dynamic"
diskDef.linearAcceleration = gravity
diskDef:attachDisk(20, 5, 0.8, 1) -- Create a disk-shaped dynamic body with a radius of 20

local disk = Body(diskDef, world, Vec2(100, 200))
disk.angularRate = -1800 -- Set the angular rotation rate
disk:addTo(world)

• type: Set the body as dynamic so it’s affected by the physics engine.
• attachDisk: Creates a circular-shaped rigid body.

4.2 Listening for Collision Events

Lua

local Line <const> = require("Line")
local App <const> = require("App")

-- Create a marker to draw at the collision point
local drawNode = Line({
	Vec2(-20, 0),
	Vec2(20, 0),
	Vec2.zero,
	Vec2(0, -20),
	Vec2(0, 20)
}, App.themeColor)
drawNode:addTo(world)

disk:onContactStart(function(other, point)
	-- Set the marker's position at the collision point
	drawNode.position = point
end)

• onContactStart: Called when the body begins contact with another object.
• Callback parameters:
  • other: The other object involved in the collision.
  • point: The world coordinates of the collision point.
• drawNode: Used to display a marker at the collision point for visual observation.

5. Running the Code

With the above code, you will achieve the following effects:

• Sensor functionality: When an object enters the sensor area, its speed is halved, simulating a slow-down zone.
• One-way platform: Objects can only fall through the platform from above, preventing passage from below.
• Collision events: When the circular body collides with another object, a marker appears at the collision point.

6. Conclusion

This tutorial covered advanced physics node features, including using sensors, creating a one-way platform, and detecting and handling collision events. These features can enrich your game's interactivity and enhance player experience.

By mastering sensors and collision events, you can:

• Design complex trigger mechanisms: Such as traps, switches, and teleporters.
• Optimize game physics effects: Achieve more realistic physical interactions, such as elastic collisions and speed control.
• Enhance gameplay: Create unique level designs and challenges.

We hope this tutorial helps you deepen your understanding of Dora SSR's physics system and inspires you to explore more advanced game functionalities!

tip

In actual development, utilizing the advanced features of the physics engine can make your game more creative and fun. Experiment with different parameters and functions to discover more possibilities!