Hello everybody,

I'm trying to implement collision and knock-back(?) for the particles in my game. I've got it sort of working, but--as you can see from the gif above--some particles behave oddly. Some drip down the tiles, others get caught in mid-air.

I was hoping some kind soul could take a look at my code and tell me if there is a better way of going about this. I'm guessing my issue is a result of how I'm checking for collisions. Thanks!

parts = {}

function make_px()
	for i=1,10 do
		p={}
		if not p1.left then
			p.x=p1.x+7
			else
			p.x=p1.x
		end
		p.y=p1.y+7
		p.c=4
		if not p1.left then
			p.dx=1+rnd(2)
			else
			p.dx=-1-rnd(2)
		end
		p.dy=rnd(1)-rnd(.5)
		p.age=rnd(2)
		add(parts,p)
	end
end

function update_px()
         local g = .05   

         --check pixels left, right, above, and below. If there is a collision, add knock-back

	for p in all(parts) do         
	 if (fget(mget((p.x-1)/8,p.y/8),0)) p.dy*=-.5 p.dx*=-.5 
	 if (fget(mget((p.x+1)/8,p.y/8),0)) p.dy*=-.5 p.dx*=-.5 
	 if (fget(mget((p.x)/8,p.y-1/8),0)) p.dy*=-.5 p.dx*=-.5 
	 if (fget(mget((p.x)/8,p.y+1/8),0)) p.dy*=-.5 p.dx*=-.5 

		p.x+=p.dx
		p.dy-=g
		p.y-=p.dy
		p.age+=.1

		if (p.age>5) del(parts,p) 		--delete old particles

	end
end

function draw_parts()
	for p in all(parts) do
		pset(p.x,p.y,p.c)
	end
end

hers an improved approach function update_px()
local g = 0.05

for p in all(parts) do
    -- Predict new position
    local new_x = p.x + p.dx
    local new_y = p.y + p.dy - g

    -- Check horizontal collision
    if fget(mget((new_x)/8, p.y/8), 0) then
        -- Collided horizontally
        p.dx *= -0.5
        new_x = p.x -- revert horizontal movement
    end

    -- Check vertical collision
    if fget(mget(p.x/8, (new_y)/8), 0) then
        -- Collided vertically
        p.dy *= -0.5
        new_y = p.y -- revert vertical movement
    else
        -- No collision, apply gravity and move
        p.x = new_x
        p.y = new_y
        p.dy -= g
    end

    p.age += 0.1
    if p.age > 5 then del(parts, p) end
end

end

Thank you @AtomicPico! Works beautifully! Just had to make sure gravity was being added rather than subtracted.

glad to hear it