Turning works for all but below object

[Skeletonxf]

In my game I initially had instant snap-to-new-angle turning for the enemies which worked fine. I then created a new enemy with turning that takes time to reach the direction at the player and I cannot work out why it turns the wrong way when the player passes underneath the enemy, especially when it works perfectly for all other angles???

This snippet of code in game/game.lua controls turning for missle ships which is the type of ship I'm having the bug with

Code: Select all

            if ship.shipType == "missle" then
              if not ship.angle then ship.angle = math.atan2(dx,dy) end
              ship.oldAngle = ship.angle
              ship.idealAngle = math.atan2(dx,dy)
              -- the code has no special case for turning to something beneath????
              if ship.idealAngle > ship.oldAngle then
                ship.angle = ship.angle + 0.1
              else
                ship.angle = ship.angle - 0.1
              end
              --ship.angle = (ship.oldAngle + adjustAngle) Uncomment this line and snap-to turning has no glitches
            end
            -- cap to unit circle (radius of 1)
            dx, dy = -math.sin(ship.angle), -math.cos(ship.angle)

The ships then move like this

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ship.dx = ship.dx + (dx or 0)*dt*window.sx*95*0.07
ship.dy = ship.dy + (dy or 0)*dt*window.sy*75*0.07
ship.dx = ship.dx*math.pow(0.001,dt)
ship.dy = ship.dy*math.pow(0.001,dt)

Bug

https://youtu.be/Umn2UOJsHxA

sensor.love

(182.94 KiB) Downloaded 37 times

[pgimeno]

I have only checked the video, but it seems clear that the problem happens in the transition from -180° to 180° or vice versa. See http://stackoverflow.com/questions/1878 ... n-2-angles

Edit: To be a bit more explicit, the sign of that angle difference is what you need to check instead of ship.idealAngle > ship.oldAngle.

[Ulydev]

Hey Skeletonxk,

Here's my function to lerp from one angle to another. It might help you with transitioning angles correctly.

Code: Select all

math.lerpAngle(old, new, speed) --speed = dt for instance

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function math.lerpAngle(a, b, k)
  local pi = math.pi
  if (math.abs(a - b) > pi) then
    if (b > a) then
      a = a + pi * 2
    else
      b = b + pi * 2
    end
  end

  local value = (a + ((b - a) * k))

  local rangeZero = pi * 2

  if (value >= 0 and value <= pi * 2) then
    return value
  end

  return (value % rangeZero)
end

[airstruck]

Here's a nice short one that was posted and/or written by some combination of Jasoco and/or Davidobot (sorry guys, memory not so clear on this one).

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local function angleDifference (t1, t2)
   return (t1 - t2 + math.pi) % (math.pi * 2) - math.pi
end

[pgimeno]

Yes, that's the translation to radians of the SO reply

[Skeletonxf]

Thanks the lerping function helps doubly because I had to make the turning frame rate independent next anyway, though I also had to add a math.min(k,1) to the line assigning value to prevent some really weird movement with high dt multipliers (probably over turning).

[Jasoco]

Here's a nice short one that was posted and/or written by some combination of Jasoco and/or Davidobot (sorry guys, memory not so clear on this one).

Code: Select all

local function angleDifference (t1, t2)
   return (t1 - t2 + math.pi) % (math.pi * 2) - math.pi
end

Haha, yeah, I was just coming in here to post this myself. I had this same problem at various times and this should fix it.