How to verify if the player can jump?

Question

I'm working on a small game using lua with Löve2D framework and his included binding of Box2D, I'm trying to verify if the player can jump, so I used this code (It's not the entire code, just the essential) :

function love.update(dt)
    world:update(dt)
    x,y = player.body:getLinearVelocity()
    if y == 0 then cantJump = false else cantJump = true end

    player.body:setAngle(0)
    player.x = player.body:getX() - 16 ; player.y = player.body:getY() - 16;

    if love.keyboard.isDown("d") then player.body:setX(player.body:getX() + 400*dt) ; player.body:applyForce(0,0) end
    if love.keyboard.isDown("q") then player.body:setX(player.body:getX() - 400*dt) ; player.body:applyForce(0,0) end
    if love.keyboard.isDown(" ") and not cantJump then player.body:setLinearVelocity(0,-347) end
 end

But my problem is that the detection is a little bit random, sometimes the player can jump when he's on the ground or some objects, sometimes he can't. How can I fix it ?

Answer 1

As Bartek says, you shouldn't be doing y == 0 because y is a floating point variable, and it's presumably unlikely that it will be exactly equal to 0, especially in a physics engine.

Use an epsilon value like this:

x,y = player.body:getLinearVelocity()
epsilon = 0.5 -- Set this to a suitable value
if math.abs(y) < epsilon then cantJump = false else cantJump = true end

This is saying "cantJump = true if the player's y position is within 0.5 of 0." Of course, you'll want to experiment to see what a good epsilon value is. I just arbitrarily picked 0.5.