Tutorial:PhysicsCollisionCallbacks (简体中文)
Contents
引言
如果你还不是太了解 love.physics,那么首先看看这个 physics tutorial。
在这个教程中我们会对两个物体进行碰撞,并用设置好的World:setCallbacks执行碰撞回调函数。
教程
main.lua 配置
首先配置 main.lua 中的3个 love 函数: love.load, love.update, and love.draw
function love.load()
end
function love.update(dt)
end
function love.draw()
end
World 配置
现在我们有个框架了,让我们用 newWorld 函数在 love.load 函数中配置 physics world 。 我们还会用到 World:update 函数
function love.load()
--用变量 "world" 存储创建好的新的物理世界
world = love.physics.newWorld(0, 200, true) --设置 X 方向上为 0,Y 方向上为 200 的重力。
end
function love.update(dt)
--永远别忘记了这玩意——更新!
world:update(dt)
end
现在,我们想创建一个球和一个地面。我们需要使用 newBody, newCircleShape,和 newRectangleShape 函数来创建它们。我们使用 newFixture 函数连接形状到物体上。
function love.load()
... -- don't include this, it's just indicating where the existing code for the function should be
ball = {}
ball.b = love.physics.newBody(world, 400,200, "dynamic") -- set x,y position (400,200) and let it move and hit other objects ("dynamic")
ball.b:setMass(10) -- make it pretty light
ball.s = love.physics.newCircleShape(50) -- give it a radius of 50
ball.f = love.physics.newFixture(ball.b, ball.s) -- connect body to shape
ball.f:setRestitution(0.4) -- make it bouncy
ball.f:setUserData("Ball") -- give it a name, which we'll access later
static = {}
static.b = love.physics.newBody(world, 400,400, "static") -- "static" makes it not move
static.s = love.physics.newRectangleShape(200,50) -- set size to 200,50 (x,y)
static.f = love.physics.newFixture(static.b, static.s)
static.f:setUserData("Block")
end
The objects are there now, but you can't yet see them. Let's draw them.
function love.draw()
love.graphics.circle("line", ball.b:getX(),ball.b:getY(), ball.s:getRadius(), 20)
love.graphics.polygon("line", static.b:getWorldPoints(static.s:getPoints()))
end
Now we should see a ball fall down and hit a solid rectangle.
World Callbacks
But what if we want more information on the two objects colliding? Now we will use World:setCallbacks to further dissect their collision(s).
First thing we do is set the world callbacks with World:setCallbacks. There are four callbacks for a collision: beginContact, endContact, preSolve, and postSolve.
- beginContact gets called when two fixtures start overlapping (two objects collide).
- endContact gets called when two fixtures stop overlapping (two objects disconnect).
- preSolve is called just before a frame is resolved for a current collision
- postSolve is called just after a frame is resolved for a current collision.
function love.load()
... -- substitute for the rest of love.load
world = love.physics.newWorld(0, 200, true)
--These callback function names can be almost any you want:
world:setCallbacks(beginContact, endContact, preSolve, postSolve)
text = "" -- we'll use this to put info text on the screen later
persisting = 0 -- we'll use this to store the state of repeated callback calls
Now define each function you just named.
function beginContact(a, b, coll)
end
function endContact(a, b, coll)
end
function preSolve(a, b, coll)
end
function postSolve(a, b, coll, normalimpulse, tangentimpulse)
end
These functions are called every time one of the collision actions happen. They pass in two fixtures and a collision object. The postsolve callback also contains the normal and tangent impulse for each collision contact point. These parameters can also be named to whatever you want. In this tutorial, we choose a, b, and coll.
- a is the first fixture object in the collision.
- b is the second fixture object in the collision.
- coll is the contact object created.
- normalimpulse is the amount of impulse applied along the normal of the first point of collision. It only applies to the postsolve callback, and we can ignore it for now.
- tangentimpulse is the amount of impulse applied along the tangent of the first point of collision. It only applies to the postsolve callback, and we can ignore it for now.
Say we want to print to screen whenever a callback is called. We just need to modify the text variable we added to love.load() earlier by appending a string every time a collision action happens. We need a bit of extra code to keep the output clean too.
A list of functions you can use on contacts can be found at the Contact page.
function love.update(dt)
... -- substitute for the rest of love.update
if string.len(text) > 768 then -- cleanup when 'text' gets too long
text = ""
end
end
function love.draw()
... -- substitute for the rest of love.draw
love.graphics.print(text, 10, 10)
end
function beginContact(a, b, coll)
x,y = coll:getNormal()
text = text.."\n"..a:getUserData().." colliding with "..b:getUserData().." with a vector normal of: "..x..", "..y
end
function endContact(a, b, coll)
persisting = 0 -- reset since they're no longer touching
text = text.."\n"..a:getUserData().." uncolliding with "..b:getUserData()
end
function preSolve(a, b, coll)
if persisting == 0 then -- only say when they first start touching
text = text.."\n"..a:getUserData().." touching "..b:getUserData()
elseif persisting < 20 then -- then just start counting
text = text.." "..persisting
end
persisting = persisting + 1 -- keep track of how many updates they've been touching for
end
function postSolve(a, b, coll, normalimpulse, tangentimpulse)
-- we won't do anything with this function
end
And now you know how to use world callbacks!
To better explore how this world behaves and see when the callbacks are invoked, add some controls to allow you to push around the ball:
function love.update(dt)
world:update(dt)
if love.keyboard.isDown("right") then
ball.b:applyForce(1000, 0)
elseif love.keyboard.isDown("left") then
ball.b:applyForce(-1000, 0)
end
if love.keyboard.isDown("up") then
ball.b:applyForce(0, -5000)
elseif love.keyboard.isDown("down") then
ball.b:applyForce(0, 1000)
end
if string.len(text) > 768 then -- cleanup when 'text' gets too long
text = ""
end
end
Finished
截图
main.lua
function love.load()
world = love.physics.newWorld(0, 200, true)
world:setCallbacks(beginContact, endContact, preSolve, postSolve)
ball = {}
ball.b = love.physics.newBody(world, 400,200, "dynamic")
ball.b:setMass(10)
ball.s = love.physics.newCircleShape(50)
ball.f = love.physics.newFixture(ball.b, ball.s)
ball.f:setRestitution(0.4) -- make it bouncy
ball.f:setUserData("Ball")
static = {}
static.b = love.physics.newBody(world, 400,400, "static")
static.s = love.physics.newRectangleShape(200,50)
static.f = love.physics.newFixture(static.b, static.s)
static.f:setUserData("Block")
text = "" -- we'll use this to put info text on the screen later
persisting = 0 -- we'll use this to store the state of repeated callback calls
end
function love.update(dt)
world:update(dt)
if love.keyboard.isDown("right") then
ball.b:applyForce(1000, 0)
elseif love.keyboard.isDown("left") then
ball.b:applyForce(-1000, 0)
end
if love.keyboard.isDown("up") then
ball.b:applyForce(0, -5000)
elseif love.keyboard.isDown("down") then
ball.b:applyForce(0, 1000)
end
if string.len(text) > 768 then -- cleanup when 'text' gets too long
text = ""
end
end
function love.draw()
love.graphics.circle("line", ball.b:getX(),ball.b:getY(), ball.s:getRadius(), 20)
love.graphics.polygon("line", static.b:getWorldPoints(static.s:getPoints()))
love.graphics.print(text, 10, 10)
end
function beginContact(a, b, coll)
x,y = coll:getNormal()
text = text.."\n"..a:getUserData().." colliding with "..b:getUserData().." with a vector normal of: "..x..", "..y
end
function endContact(a, b, coll)
persisting = 0
text = text.."\n"..a:getUserData().." uncolliding with "..b:getUserData()
end
function preSolve(a, b, coll)
if persisting == 0 then -- only say when they first start touching
text = text.."\n"..a:getUserData().." touching "..b:getUserData()
elseif persisting < 20 then -- then just start counting
text = text.." "..persisting
end
persisting = persisting + 1 -- keep track of how many updates they've been touching for
end
function postSolve(a, b, coll, normalimpulse, tangentimpulse)
end
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