[Solved] objects twitching when moving / removing from table?

[unixfreak]

I have a side-scrolling project (like a space shooter) where enemies come in waves from the right side of the screen, and i have encountered a problem i haven't been able to solve.

Basically, as an enemy goes off the left side of the screen, it is removed from the table -- however, the next enemy in the table after it, will twitch backwards. Also when an enemy is removed (for example, colliding with a projectile) the enemy behind it, or after it in the table order, will twitch backwards aswell.

In the example below, the squares would be enemies.

Here is a simplified example showing the bug; (watch the left side of the screen)

squares.love

(556 Bytes) Downloaded 80 times

The code, if you cannot test the attachment:

Code: Select all

square = {}
square.w = 40
square.h = 40
square.x = love.graphics.getWidth()
square.y = math.random (0,love.graphics.getHeight()-20)
square.xvel = 200
square.yvel = 0
square.cycle = 0
square.delay = 0.5

square.objects = {}

function love.update(dt)
        square.cycle = math.max(0, square.cycle - dt)

        if square.cycle <= 0 then
                table.insert(square.objects, {
                        x = love.graphics.getWidth(),
                        y = math.random (0,love.graphics.getHeight()-square.h),
                        r = math.random(100,255),
                        g = math.random(100,255),
                        b = math.random(100,255),
                })

                square.cycle = square.delay
        end

        for i,s in pairs(square.objects) do
                s.x = s.x - square.xvel *dt

                if s.x < 0-square.w then
                        table.remove(square.objects, i)
                end
        end
end


function love.draw()
        for _,s in pairs(square.objects) do

                love.graphics.setColor(s.r,s.g,s.b,255)
                love.graphics.rectangle("line", s.x,s.y,square.w,square.h)
        end
end


function love.keypressed(key)
        if key == "escape" then love.event.quit() end
end

Does anyone know what is happening? And how to fix it?

[zorg]

For me, they're constantly twitching, but that's an issue with my window manager.
Either try math.floor-ing the x and y coordinates of your squares in your love.draw, or try running it in fullscreen, whether the issue exists there as well, or not.

[unixfreak]

For me, they're constantly twitching, but that's an issue with my window manager.
Either try math.floor-ing the x and y coordinates of your squares in your love.draw, or try running it in fullscreen, whether the issue exists there as well, or not.

Unfortunately i've tried all of that. I did also disable vsync and manually capped the fps to 200 or so. The effect is less visible, but the positions move when they are not supposed to.

The reason that it's a problem, is i might have multiple sprites moving at the same speed whilst overlapping to form a larger object, and the slight twitchyness doesn't look very nice.

[unixfreak]

Here's a screenshot from the game i am working on, which hopefully makes the problem more clear;

screen1.jpg (88.31 KiB) Viewed 2272 times

The sprites should stay in formation, like the ones at the top right of the screen. But as an enemy goes off the left side (it is just removed from the table) which causes the next enemy in that table, to move backwards? You can see the overlap caused in the red circle i added.

I feel it's as if the ID of the table entry is shifted 1 value, as the enemy in front has been removed from the table... hence the loop thinks it has already moved that enemy? I'm not sure how to fix it though.

[unixfreak]

Well, I think i've just found a workaround.

In love.update(dt) i move the table.remove( .. ) to another loop after shifting the x position, which seems to fix the problem... i'm not sure it it's ideal though -- is there a way to get that behaviour in a single loop?

Original:

Code: Select all

        for i,s in pairs(square.objects) do
                s.x = s.x - square.xvel *dt

                if s.x < 0-square.w then
                        table.remove(square.objects, i)
                end
        end

Workaround:

Code: Select all

        for i,s in pairs(square.objects) do
                s.x = s.x - square.xvel *dt

        end

        for i,s in pairs(square.objects) do
                if s.x < 0-square.w then
                        table.remove(square.objects, i)
                end
        end

[s-ol]

the problem is probably that you use ipairs or a regular for loop with forward-iteration, but doing table.remove changes the indices:

Code: Select all

1: A
2: B
3: C
4: D

1: A - updating
2: B
3: C
4: D

1: A
2: B - updating: collision so remove!
3: C
4: D

1: A
2: C NOT UPDATED!
3: D - updating

When you do table.remove all the elements further down the table are moved upwards one slot, but the for loop doesn't "know" so it just keeps going regularily, skipping the next item. The trick is to instead use a regular for loop and iterate backwards

Code: Select all

for i=#items,1,-1 do
  local item = items[i]
  if item.remove then
    table.remove(items, i)
  end
end

[unixfreak]

When you do table.remove all the elements further down the table are moved upwards one slot, but the for loop doesn't "know" so it just keeps going regularily, skipping the next item. The trick is to instead use a regular for loop and iterate backwards

Code: Select all

for i=#items,1,-1 do
  local item = items[i]
  if item.remove then
    table.remove(items, i)
  end
end

Thats clear to me now. Many thanks for that example, that seems to work fine.