Codea - vanishing point demo with tilt controls

-- Use this function to perform your initial setup
function setup()
    iparameter("VanishPointx",0,WIDTH,WIDTH/2)
    iparameter("VanishPointy",0,WIDTH,WIDTH/2)
    iparameter("VanishingPoint",0,1,0)
    iparameter("Horizon",0,1,0)
    iparameter("Midline",0,1,0)
    iparameter("HitBox",0,100,50)
    iparameter("Sticker",0,1,0)
    iparameter("Mobs",0,1,1)
    iparameter("zSpeed",0,30,0)
    iparameter("Mobtype",0,1,1)
    Mobz = {}
    print("Move the sliders to make something appear")
end

-- This function gets called once every frame
function draw()
    vp = tiltValue() --vanishing point
    background(0,0,0,0)
    stroke(0, 255, 39, 255)
    strokeWidth(3)
    fill(3, 253, 15, 255)
    if HitBox > 0 then
        ShowHitBox(HitBox)
    end
     if Midline == 1 then
        line(vp.x,0,vp.x,HEIGHT)
    end
    if Horizon == 1 then
        line(0,vp.y,WIDTH,vp.y)
    end
    if VanishingPoint == 1 then
        ellipse(vp.x,vp.y,5,5)
    end
    if Mobs == 0 then
        for k, v in pairs(Mobz) do
            table.remove(Mobz,k)
        end
    else
        nMobz = table.maxn(Mobz)
        for i = 1,10-nMobz do
            x=math.random()*WIDTH
            y=math.random()*HEIGHT
            z=WIDTH*i+(math.random()*56)
            d=256
            table.insert(Mobz,nMobz+1,{x=x,y=y,z=z,d=d})
        end
        for k, v in pairs(Mobz) do
            zd = l3Dto2d(Mobz[k].d,Mobz[k].z)
            e = c3Dto2d(vec3(Mobz[k].x,Mobz[k].y,Mobz[k].z))
            if Mobtype == 0 then
                ellipse(e.x,e.y,zd)
            else
                sprite("SpaceCute:Planet",e.x,e.y,zd)
            end
        end
    end
    if zSpeed > 0 and table.maxn(Mobz) > 0 then
        for k, v in pairs(Mobz) do
            Mobz[k].z = Mobz[k].z - zSpeed
            if Mobz[k].z < 0 then
                table.remove(Mobz,k)
            end
        end
    end

    if Sticker == 1 then --draw something full size
        --like a sticker on the window
        ellipse(100,100,100)
        --sprite("SpaceCute:Planet",300,300,256)
    end
end

function tiltValue()
    return vec2(
        (Gravity.x+1)*WIDTH/2,
        (Gravity.y+1)*HEIGHT/2
    )
end

function l3Dto2d(d,z)
    if z == 0 then
        --depth is zero, we're on the screen
        return (d)
    elseif z < 0 then
        --depth is negative
        --look out it's behind you!
        --now crash and burn them for passing this to us
        return (nil)
        --maybe after some thought this could be handled
    else --if z > 0
        --depth is positive
        --it's ahead of us (doesn't mean we can see it)
        dz = d * WIDTH / ( WIDTH + z)
        return(dz)
    end -- z depth
end

function c3Dto2d(p)
    --print(p)
    --convert 3D to 2D
    --accepts p a vec3 -- x,y,z
    --returns a vec2 --x,y
    -- p.x + (vp.x / 2)/WIDTH/p.z
    -- p.x - ((WIDTH - vp.x) /2)/WIDTH/p.z
    local x2, y2
    if p.z == 0 then
        --depth is zero, we're on the screen
        return (vec2(p.x,p.y))
    elseif p.z < 0 then
        --depth is negative
        --look out it's behind you!
        --now crash and burn them for passing this to us
        return vec2(nil,nil)
        --maybe after some thought this could be handled
    else --if p.z > 0
        --depth is positive
        --it's ahead of us (doesn't mean we can see it)
        x2 = p.x + ((vp.x - p.x)* (p.z/WIDTH) / ((p.z/WIDTH)+1) )
        y2 = p.y + ((vp.y - p.y)* (p.z/WIDTH) / ((p.z/WIDTH)+1) )
        --print(x2,y2)
        return(vec2(x2,y2))
    end -- z depth
end

function ShowHitBox(boxes)
    for i = 1,boxes do
        --the hit box is a box in front of the viewer
        --objects in the hit box will hit the screen
        sll = vec3(0,0,(i-1)*WIDTH)          -- screen lower left point
        sul = vec3(0,HEIGHT,(i-1)*WIDTH)     -- screen upper left point
        slr = vec3(WIDTH,0,(i-1)*WIDTH)      -- screen lower right point
        sur = vec3(WIDTH,HEIGHT,(i-1)*WIDTH) -- screen upper right point
        -- depth as in the depth of the box
        -- the box depth is set to it's width
        dll = vec3(0,0,i*WIDTH)          -- depth lower left point
        dul = vec3(0,HEIGHT,i*WIDTH)     -- depth upper left point
        dlr = vec3(WIDTH,0,i*WIDTH)      -- depth screen lower right
        dur = vec3(WIDTH,HEIGHT,i*WIDTH) -- depth screen upper right
        sll2 = c3Dto2d(sll)
        sul2 = c3Dto2d(sul)
        slr2 = c3Dto2d(slr)
        sur2 = c3Dto2d(sur)
        dll2 = c3Dto2d(dll)
        dul2 = c3Dto2d(dul)
        dlr2 = c3Dto2d(dlr)
        dur2 = c3Dto2d(dur)
        DrawBox(sll2,sul2,slr2,sur2,dll2,dul2,dlr2,dur2)
    end
end

function DrawBox(sll2,sul2,slr2,sur2,dll2,dul2,dlr2,dur2)
    --back
    line(dll2.x,dll2.y,dul2.x,dul2.y)
    line(dul2.x,dul2.y,dur2.x,dur2.y)
    line(dur2.x,dur2.y,dlr2.x,dlr2.y)
    line(dlr2.x,dlr2.y,dll2.x,dll2.y)
    --sides
    line(sll2.x,sll2.y,dll2.x,dll2.y)
    line(sul2.x,sul2.y,dul2.x,dul2.y)
    line(sur2.x,sur2.y,dur2.x,dur2.y)
    line(slr2.x,slr2.y,dlr2.x,dlr2.y)
    --front
    line(sll2.x,sll2.y,sul2.x,sul2.y)
    line(sul2.x,sul2.y,sur2.x,sur2.y)
    line(sur2.x,sur2.y,slr2.x,slr2.y)
    line(slr2.x,slr2.y,sll2.x,sll2.y)
end