Issues detecting direction of collision

ClaudioBA

Hello, first time using Gideros! (and building apps for mobile devices)
im trying to get the collision and the direction of it but it doesnt work...
this is part of my code:

function GCchar:onBeginCollision(event)
local bodyA, bodyB = event.fixtureA:getBody(), event.fixtureB:getBody();

if self:collision(bodyA.xSprite) == "B" then
print("Down")
end

end

function Sprite:collision(sprite2)

-- self bottom < other sprite top
if self:getY() + self:getHeight() < sprite2:getY() then
return "T";
end
-- self top > other sprite bottom
if self:getY() > sprite2:getY() + sprite2:getHeight() then
return "B";
end
-- self left > other sprite right
if self:getX() > sprite2:getX() + sprite2:getWidth() then
return "R";
end
-- self right < other sprite left
if self:getX() + self:getWidth() < sprite2:getX() then
return "L";
end

return "N";
end

is this approach correct???
anyone has done this??

Thanks!

atilim

Hi @ClaudioBA, welcome here

You can use event.contact information like

local worldManifold = event.contact:getWorldManifold() print(worldManifold.normal.x, worldManifold.normal.y)

ClaudioBA

Thanks for the fast response!

There is a way to detect the bodies making contact?
for example, my character is walking against a wall (made by a pile of squares), and i get 2 lectures.... i supose one of the lectures is the square of the wall and the other is the square under the wall (on the floor)???

atilim

After creating your bodies, set a field like "type" (or any other name as you want):

local body1 = world:createBody(....) body1.type = "wall"   local body2 = world:createBody(...) body2.type = "character"

And then you can check

local bodyA = event.fixtureA:getBody()   if bodyA.type == "wall" then -- this is wall else if bodyA.type == "character" -- oh no, it's character end

ClaudioBA

Thanks, i'll try that!!!

ClaudioBA

Another question related to contact....
if i have my character against a wall at his right, how can i get the body at his right if they dont collide at the touch event?

ar2rsawseen

@ClaudioBA did not really understand the problem, but if the bodies are not colliding, but are near, you could use b2.World:queryAABB(lowerx, lowery, upperx, uppery) to get the fixture and then the body, where lowerx, lowery, upperx, uppery is bounding box where to check for box2d bodies

ClaudioBA

sorry my English is not very good...
i was trying to get the body colliding with the character, but after the onBeginCollision event take place....

EDIT: i'm doing it right now, but doing something like this:

charH is char Height

if ( bodyBY < bodyAY+charH/2 and bodyBY+32> bodyAY+charH/2) then

is pretty ugly and im sure there is a better way to acomplish this

Thanks!

ar2rsawseen

if onBeginCollision event is raised, then it means that bodies are already colliding, so I can't really understand where the problem is

You can check this tutorial: http://appcodingeasy.com/Gideros-Mobile/Animating-Box2d-objects

where collision detection happens here:

--define begin collision event handler function function scene:onBeginContact(e) --getting contact bodies local fixtureA = e.fixtureA local fixtureB = e.fixtureB local bodyA = fixtureA:getBody() local bodyB = fixtureB:getBody()   --check if first colliding body is a ball if bodyB.type and bodyB.type == "ball" then --switch animation using our created function bodyB.object:switchAnimation() end end

ClaudioBA

i mean i need to check contact after another action happens, not collition, so i need to identify the bodies in actual contact with it

ar2rsawseen

oh ok I think I get it now.
Well there are two ways, you can implement two methods for Event.BEGIN_CONTACT and Event.END_CONTACT

inside first one you can set some value self.isColliding = true and on second event set it to false. Then any time you can check if self.isColliding then.

Second not tested but should work, is to retrieve contact object from contact event and store it for later use

--define begin collision event handler function function scene:onBeginContact(e) self.contact = e.contact end   function scene:isColliding() return self.contact:isTouching() end