Help with setRestitution -- weird bugs?

[Clash]

So, I just followed the physics tutorial: http://love2d.org/wiki/Tutorial:Physics

I have three questions

1. Why does the ball bounce when it touches the ground if the default restitution is 0?

2. The code says

Code: Select all

  --let's create a ball
  bodies[1] = love.physics.newBody(world, 650/2, 650/2, 15, 0) --place the body in the center of the world, with a mass of 15
  shapes[1] = love.physics.newCircleShape(bodies[1], 0, 0, 20) --the ball has a radius of 20

But the ball appears from the top center of the screen. Actually at the beginning the ball isn't even showing up and I don't know why either.

3. Why does setting restitution of the ball shape to a value bigger than 0.2 results in the ball not appearing in the screen at all. (I believe this is related to #2)

Thanks in advance!

[kikito]

Please upload a sample .love file. Otherwise it's very difficult to test exactly what is wrong.

[Clash]

I didn't change anything in the tutorial, this is the exact code I'm using

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function love.load()
  world = love.physics.newWorld(-650, -650, 650, 650) --create a world for the bodies to exist in with width and height of 650
  world:setGravity(0, 700) --the x component of the gravity will be 0, and the y component of the gravity will be 700
  world:setMeter(64) --the height of a meter in this world will be 64px
  
  bodies = {} --create tables for the bodies and shapes so that the garbage collector doesn't delete them
  shapes = {}
  
  --let's create the ground
  --we need to give the ground a mass of zero so that the ground wont move
  bodies[0] = love.physics.newBody(world, 650/2, 625, 0, 0) --remember, the body anchors from the center of the shape
  shapes[0] = love.physics.newRectangleShape(bodies[0], 0, 0, 650, 50, 0) --anchor the shape to the body, and make it a width of 650 and a height of 50
  
  --let's create a ball
  bodies[1] = love.physics.newBody(world, 650/2, 650/2, 15, 0) --place the body in the center of the world, with a mass of 15
  shapes[1] = love.physics.newCircleShape(bodies[1], 0, 0, 20) --the ball has a radius of 20

  --initial graphics setup
  love.graphics.setBackgroundColor(104, 136, 248) --set the background color to a nice blue
  love.graphics.setMode(650, 650, false, true, 0) --set the window dimensions to 650 by 650
end


function love.update(dt)
  world:update(dt) --this puts the world into motion
  
  --here we are going to create some keyboard events
  if love.keyboard.isDown("right") then --press the right arrow key to push the ball to the right
    bodies[1]:applyForce(400, 0)
  elseif love.keyboard.isDown("left") then --press the left arrow key to push the ball to the left
    bodies[1]:applyForce(-400, 0)
  elseif love.keyboard.isDown("up") then --press the up arrow key to set the ball in the air
    bodies[1]:setY(650/2)
  end
end

function love.draw()
  local x1, y1, x2, y2, x3, y3, x4, y4 = shapes[0]:getBoundingBox() --get the x,y coordinates of all 4 corners of the box.
  --x1, y1 represent the bottom left corner of the bounding box
  --x2, y2 represent the top left corner of the bounding box
  --x3, y3 represent the top right corner of the bounding box
  --x4, y4 represent the top right corner of the boudning box
  local boxwidth = x3 - x2 --calculate the width of the box
  local boxheight = y2 - y1 --calculate the height of the box
  love.graphics.setColor(72, 160, 14) --set the drawing color to green for the ground
  --the rectangle is drawing from the top left corner
  --so we need to compensate for that
  love.graphics.rectangle("fill", bodies[0]:getX() - boxwidth/2, bodies[0]:getY() - boxheight/2, boxwidth, boxheight)
  love.graphics.setColor(193, 47, 14) --set the drawing color to red for the ball
  --the circle is drawing from the center
  --so we do not need to compensate
  love.graphics.circle("fill", bodies[1]:getX(), bodies[1]:getY(), shapes[1]:getRadius(), 20)
end

To test #3, just add at love.load()

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shapes[1]:setRestitution(0.3);

and the circle won't even appear

Thanks in advance!

[vrld]

I cannot reproduce bug 2 and 3. As for those bugs, try to cap dt in love.update, like so:

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function love.update(dt)
    dt = math.min(dt, 1 / 30)
    ...
end

dt can become very big in the first frame, which will in turn update the physics system with a way to big timestep.

[Clash]

I cannot reproduce bug 2 and 3. As for those bugs, try to cap dt in love.update, like so:

Code: Select all

function love.update(dt)
    dt = math.min(dt, 1 / 30)
    ...
end

dt can become very big in the first frame, which will in turn update the physics system with a way to big timestep.

Thanks! That did fix #2 and #3

Any idea why #1 happens? I also noticed something weird, when setting the restitution to high values like 0.9, when the ball is near the ground, it bounces a million times before stopping, very weird! I believe this is related with #1. Any idea?

Thanks in advance!

[Robin]

#1 is because the default restitution is not zero, it seems to be 0.1. So when you set it to 0.9, it bounces back nine times as strong as normally.

[Clash]

#1 is because the default restitution is not zero, it seems to be 0.1. So when you set it to 0.9, it bounces back nine times as strong as normally.

If I set it to 0 the same thing happens. I believe the default is 0.

[BlackBulletIV]

Make sure inertia is 0. That's just off the top of my head, but I think inertia would probably make a difference.

[Robin]

I believe the default is 0.

I tested, the default is 0.1.

I also tried your code, and after limiting dt, I found no problem. It didn't bounce when I set restitution to 0 and it didn't fly off the world for 0.9.

[Clash]

I believe the default is 0.

I tested, the default is 0.1.

I also tried your code, and after limiting dt, I found no problem. It didn't bounce when I set restitution to 0 and it didn't fly off the world for 0.9.

You're right, the default is 0.1.
About 0.9, I didn't say it flies off the world, it just bounces a million times close to the ground before stopping. Could you please try? Thanks for the help!

Edit: Oh about flying off the world, you're right, when limitting dt that fixed it. But it still bounces a million times on the ground