Sprite Collision Detection

Questions
1

Anyone had done a function to detect the collision of two different shaped and sized Sprites?

function CollisionDetect(x1,y1,w1,h1,x2,y2,w2,h2)
…
end

2

If you’re satisfied with overlap of the bounding rectangles, it’s a snap. If you need pixel level accuracy… It’s less snappy, but doable.

3

No problem With a small overlap

4

You could do this with less code, but my usual solution is to give each item bounds, like this

Frame = class()

function Frame:init(left, bottom, right, top)
    self.left = left
    self.right = right
    self.bottom = bottom
    self.top = top
end

Then, you want to test collisions use

function Frame:overlaps(f)
    if self.left > f.right or self.right < f.left or
    self.bottom > f.top or self.top < f.bottom then
        return false
    else
        return true
    end
end

That one if statement does all the work of checking for overlap.

Does that make sense?

5

I’d also suggest bringing the bounding box in a few pixels so it’s just within the bounds of your sprite, that way you give the player a little leeway in deciding if a collision occurs or not.

Also if you have a big complex shape you can create a hierarchy of bounding boxes, by defining each portion of the shape as a separate box and then having a “master” bounding box that is generated from the min / max extents of all the child boxes. You can then get fairly accurate / quick detection by checking first to see if the master box collides and if it does then check each of the child boxes in turn.

6

Thanks guys; I used the approach posted by Mark. and also I extended a little the bounding box 5 pixels on every direction

7

… @edwin809 Don’t know if you need anything like this anymore, but here is a program I wrote showing the collision of 2 irregular shaped objects. Just touch the white dot and move it around to move the smaller object so it collides with the larger one.


-- polygon collision

displayMode(FULLSCREEN)
supportedOrientations(PORTRAIT)

function setup()
    -- fixed points for polygons
    verts1={vec2(250,600),vec2(260,700),vec2(280,750),vec2(460,800),vec2(400,660),vec2(360,620)}
    verts2={vec2(400,400),vec2(360,500),vec2(450,450),vec2(500,400)}     
    tx=0
    ty=0  
end

function draw()
    background(40, 40, 50)
    stroke(255)
    strokeWidth(4)
    
    j=#verts1    -- draw 1st polygon at a fixed location
    for z=1,#verts1 do
        line(verts1[z].x,verts1[z].y,verts1[j].x,verts1[j].y)
        j=z
    end
    
    j=#verts2    -- draw 2nd polygon moving with touch x,y
    for z=1,#verts2 do
        line(verts2[z].x+tx,verts2[z].y+ty,verts2[j].x+tx,verts2[j].y+ty)
        j=z
    end
    
    check2()    -- check if polygons are colliding
    
    fill(255)
    text(str,350,700)  
    ellipse(400+tx,150+ty,20)              
end

function check2()    -- calculate x,y points between verticies
    j=#verts2
    for z=1,#verts2 do
        m=(verts2[j].y-verts2[z].y)/(verts2[j].x-verts2[z].x)
        b=verts2[z].y-m*verts2[z].x
        dir=1
        if verts2[j].x>verts2[z].x then
            dir=-1
        end
        str=""
        for x=verts2[j].x,verts2[z].x,dir do
            y=m*x+b
            if check(tx+x,ty+y) then
                str="collision"
                return
            end
        end
        j=z
    end
end  
        
function check(px,py)    -- check if points px,py are within polygon
    j=#verts1
    c=-1
    for i=1,#verts1 do
        a1=false
        if verts1[i].y>py then
            a1=true
        end
        a2=false
        if verts1[j].y>py then
            a2=true
        end
        b1=verts1[j].x-verts1[i].x
        b2=py-verts1[i].y
        b3=(verts1[j].y-verts1[i].y)
        if a1~=a2 and px<(b1*b2/b3+verts1[i].x) then
            c = c *-1
        end
        j=i
    end 
    if c==1 then
        return true
    else
        return false
    end
end

function touched(t)    -- used just to move 2nd polygon around
    if t.state==BEGAN then 
        sx=t.x
        sy=t.y
    elseif t.state==MOVING then
        tx=t.x-sx
        ty=t.y-sy
    elseif t.state==ENDED then
        tx=0
        ty=0
    end
end