Working with Gravity

Questions
1

Can anyone point me to some code or documentation that can get me going with Gravity?

I’ve tried working backward from Dungeon Roller but I can’t find the exact piece of code that lets a sprite move around via tilting the iPad. The in-app documentation is pretty sparse too.

I’m new to lua and couldn’t find anything via a google search either.

Thanks!
Nick

2

Hi @nsbarr

Here’s the relevant code from Dungeon Roller with some explanation

-- Assume we have some variable, self.position, which represents
-- the position of our object

self.position = vec2( 0, 0 )

-- Here we figure out the direction to move given the current gravity
-- The "+ vec2(0, 0.6)" simply offsets our Y-component
-- we do this to allow the game to be played with the iPad slightly tilted
-- you can think of it as a calibration amount

moveVec = vec2(Gravity.x, Gravity.y) + vec2(0,0.6)

-- Update our position using moveVec
-- We multiply by 20 to increase the strength of the gravity vector

self.position = self.position + moveVec * 20

Here it is put into an example project (Note: untested, may not work)

function setup()
    -- Start in the center of the screen
    position = vec2( WIDTH/2, HEIGHT/2 )
    
    -- Create two parameters to play with
    parameter( "yCalibration", 0, 2.0, 0.6 )
    parameter( "tiltStrength", 1, 40, 10 )
end

function draw()
    background(0)
    fill(255,0,0)
    
    moveVec = vec2(Gravity.x, Gravity.y) + vec2( 0, yCalibration )
    
    position = position + moveVec * tiltStrength
    
    ellipse( position.x, position.y, 100 )
end

You can experiment with this by changing the yCalibration and tiltStrength parameters to see how it effects the tilt controls.

3

Thanks Simeon! This is really helpful.

Unfortunately when I put this code into my project, it crashes and quits the app when I try to run it. I actually had the same problem when I was working backward from Dungeon Roller, so there may be a specific bug here.

I made sure I had the latest version of the app and also tried deleting/reinstalling it, with no luck.

Happy to provide any more useful information!

Thanks,
N

4

Here’s a silly little program I wrote for figuring out how gravity works.

-- Use this function to perform your initial setup
function setup()
    x = vec2(WIDTH/2,HEIGHT/2)
    v = vec2(0,0)
    c = color(255,0,0,255)
    r = 30
    g = vec2(0,0)
    h = 0.1
    gx = 0
    gy = 0
    gz = 0
    gl = 0
    watch("gx")
    watch("gy")
    watch("gz")
    watch("gl")
end

-- This function gets called once every frame
function draw()
    background(0,0,0,255)
    fill(c)
    ellipse(x.x,x.y,r)
    g = 2*vec2(Gravity.x,Gravity.y)
    v = v + h * ( g - 0.001 * v:len() * v )
    x = x + h * v
    if x.x < r or x.x > WIDTH - r then
        v.x = - v.x
    end
    if x.y < r or x.y > HEIGHT - r then
        v.y = - v.y
    end
    gx = Gravity.x
    gy = Gravity.y
    gz = Gravity.z
    gl = math.sqrt(gx*gx + gy*gy + gz*gz)
end

It’s a ball that rolls around the screen as you tip the iPad. There’s some built-in “friction” and it bounces off the sides. It also displays the components and length of the gravity vector (the latter being a little tedious; it was mainly because I see reports that the iPad can measure the strength of the gravitational field, which seems absurd.)

5

Nice, thanks Andrew! Interestingly this snippet didn’t crash the app…

6

Sorry @nsbarr I think that code I posted exhibits the vec2 bug in the current version of Codea. The new version awaiting approval fixes the bug.

7

Cool, looking forward to the fix. In the meantime will continue to play with the gravity stuff, thanks for all the help!