Simple BoxNormalCollision function

gemboy100 · April 2019

Maybe someone find it useful, I used it in my game that I never finished ..

function BoxNormalCollision(ObjA,ObjB)
    local x, y = 0,0 
    local a = {x= ObjA:getX(), y= ObjA:getY(), w= ObjA:getWidth(), h= ObjA:getHeight()}
    local b = {x= ObjB:getX(), y= ObjB:getY(), w= ObjB:getWidth(), h= ObjB:getHeight()}
 
    if -- first test for any collision
    (
        a.x < b.x+b.w and
        b.x < a.x+a.w and
        a.y < b.y+b.h and
        b.y < a.y+a.h
    ) 
    then -- test from where does collision come
        local ax0 = a.x + a.w /2
        local ay0 = a.y + a.h /2
        local bx0 = b.x + b.w /2
        local by0 = b.y + b.h /2
        --desno dolje
        if bx0 > ax0 and by0 > ay0 then
            if (a.x+a.w) - b.x > (a.y+a.h) - b.y then
                y = -1
                ObjA:setY(b.y-a.h)
            else
                x = -1
                ObjA:setX(b.x-a.w)
            end		
            return x,y
        --desno gore
        elseif bx0 > ax0 and by0 < ay0 then
            if (a.x+a.w) - b.x > (b.y+b.h) - a.y then
                y = 1
                ObjA:setY(b.y+b.h)
            else
                x = -1
                ObjA:setX(b.x-a.w)
            end		
            return x,y
        --lijevo dolje
        elseif bx0 < ax0 and by0 > ay0 then
            if (b.x+b.w) - a.x > (a.y+a.h) - b.y then
                y = -1
                ObjA:setY(b.y-a.h)
            else
                x = 1
                ObjA:setX(b.x+b.w)
            end
            return x,y
        --lijevo gore
        elseif bx0 < ax0 and by0 < ay0 then
            if (b.x+b.w ) - a.x > (b.y+b.h) - a.y then
                y = 1
                ObjA:setY(b.y+b.h)
            else
                x = 1
                ObjA:setX(b.x+b.w)
            end
            return x,y
        --
        end  
    end
    return x,y
end

-demo proj is attached

SinisterSoft · April 2019

Gideros has a plugin for a C version of Bump - it does this sort of collision detection (and more) and may be faster - you should take a look.

http://wiki.giderosmobile.com/index.php/Special:MyLanguage/bump

Here is the official Bump documentation:
https://github.com/kikito/bump.lua

oleg · April 2019

Another option for checking the collision

function Sprite:collision(sprite2)
        -- self bottom < other sprite top
        if self:getY() + self:getHeight()/3 < sprite2:getY() then
                return false
        end
        -- self top > other sprite bottom
        if self:getY() > sprite2:getY() + sprite2:getHeight() then
                return false
        end
        -- self left > other sprite right
        if self:getX() > sprite2:getX() + sprite2:getWidth() then
                return false
        end
        -- self right < other sprite left
        if self:getX() + self:getWidth()/3 < sprite2:getX() then
                return false
        end
                return true
end

 
-- class of object
 
if self:collision(sprite5) then
 
--actions in a collision
end

SinisterSoft · April 2019

Yes, that works great too. I usually use this method, but inline. Bump has a few advantages as it calculates the rebound and the actual collision point - even if an object is travelling faster than the size of the collision square.

antix · April 2019

Bump is basically a spatial hash with extra features (collision). You can query it with rectangles, points or segments. It is a fantastically robust addition to Gideros.

oleg · April 2019

I think we should be able to check the collision by various methods

gemboy100 · April 2019

Can someone convert my example project to cbump, I really would love to try it but don't have time to study it.

SinisterSoft said:
Bump has a few advantages as it calculates the rebound and the actual collision point - even if an object is travelling faster than the size of the collision square.

btw. I had problem in my game similar to this example I attached, If i put spacing between blocks a bit larger and the ball was hitting from right side, for example, lands on a block, and hits just about the corner of the previous brick causing ball to switch unwanted direction

I know this happen cause it tests collision in 1,2,3 .. order from objs table, but for me it was weird enough to have this same issue with box2d

antix · April 2019

@gemboy100 head over to https://github.com/kikito/bump.lua and have a read. Gideros bump is a native (6 or so times faster) of that

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