Problem with atan2

[VirusX]

I'm having bit of a problem with atan2. Scratching my head for last couple days and can't understand why it does not work.

I need simple lookAt() function which I found here in forums, but it works arseways for me ):

I have two actors. First actor1 should look at actor2. I also added two white lines: line1 actors r0 position and line2 where it should look at.

Screen Shot 2014-06-02 at 15.36.10.png (34.12 KiB) Viewed 4356 times

Code :

Code: Select all

actor1 = { x = 200, y = 200}
actor2 = { x = 500, y = 50}

function love.load()
	actorImg = love.graphics.newImage("actor.png") --64x64
	ox = actorImg:getWidth() / 2
	oy = actorImg:getHeight() / 2
end

function love.mousepressed( x, y, button )
	if button == 'l' then
		angle = lookAt(actor1, actor2)
		print(angle..' Deg: '..math.deg(angle))
	end
end

function lookAt(obj, target)
  local a = math.atan2(target.y - obj.y, target.x - obj.x)
  return a < 0 and a + math.pi * 2 or a
end

function love.draw()
	-- actor1
	love.graphics.draw(actorImg, actor1.x, actor1.y, angle, 1,1, ox, oy )
	-- actor2
	love.graphics.draw(actorImg, actor2.x, actor2.y, 0, 1,1, ox, oy)
	
	-- line1 r0 position
	love.graphics.line(actor1.x, actor1.y, actor2.x, actor2.y)
	-- line2 lookAt
	love.graphics.line(actor1.x, actor1.y, actor1.x, 0)
end

Also console output: 5.8195376981788 Deg: 333.43494882292

Not sure what I'm doing wrong, so any help more then welcome.

[HugoBDesigner]

Oh, yeah, I ALWAYS have problems with math.atan2. The system of angles they use there is different. Here's what I mean:

shitty_math_atan2.png (16.65 KiB) Viewed 4345 times

I made this some time ago even. The way I found to work on this is to have a conversion function. You can use this, if you want:

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function convertAngle(a)
	local a = a
	a = math.pi-a
	return math.offset(math.abs(a)+math.pi/2, 0, math.pi*2, math.pi*2)
end

It may not work 100%, but where I used it it worked fine. I hope I helped!

[Roland_Yonaba]

JUst wanted to point out something.
Love system coordinates is not the same as the one assumed in a classical euclidian space.
In LÖVE, x is positive and grows when moving right, and y is positive and grows when moving down. The center is the left-upper bottom of the screen. This is not odd, most of game frameworks uses the same convention.
But, in a euclidian space, the classical one taught at school, we often assume x is positive and grows when moving right, and y is positive and grows when moving up (as opposed to LÖVE's system of coordinates).
Therefore, I think flipping the dy sign in your function fixes everything.

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local function getAngleBetween(obj, target)
  local dx = target.x - obj.x
  local dy = -(target.y - obj.y) -- same as obj.y - target.y
  local rad = math.atan2(dy, dx)
  return rad < 0 and rad + math.pi*2 or rad
end

[VirusX]

It may not work 100%, but where I used it it worked fine. I hope I helped!

In your code you use math.offset which is afaik not Lua builtin function? Can you let me know what it does.

JUst wanted to point out something.
Love system coordinates is not the same as the one assumed in a classical euclidian space.
In LÖVE, x is positive and grows when moving right, and y is positive and grows when moving down. The center is the left-upper bottom of the screen. This is not odd, most of game frameworks uses the same convention.
But, in a euclidian space, the classical one taught at school, we often assume x is positive and grows when moving right, and y is positive and grows when moving up (as opposed to LÖVE's system of coordinates).
Therefore, I think flipping the dy sign in your function fixes everything.

Sorry, I forgot to mention that I suck at maths, but I'm trying by best so don't beat me with the stick yet (: Anyway, I tried your example and it's still working arseways:

Screen Shot 2014-06-02 at 18.10.16.png (34.61 KiB) Viewed 4332 times

It is "looking" to that direction but not where I expect... it should be about 60deg (where second white line is), but I'm getting ~26deg

[Roland_Yonaba]

My bad. Just use the return value of math.atan2(dy, dx) straight as the rotation argument to love.graphics.draw.

[VirusX]

My bad. Just use the return value of math.atan2(dy, dx) straight as the rotation argument to love.graphics.draw.

Code: Select all

function lookAt2(obj, target)
  local dx = target.x - obj.x
  local dy = -(target.y - obj.y) -- same as obj.y - target.y
  return math.atan2(dy, dx)
  --return rad < 0 and rad + math.pi*2 or rad
end

It still gives me same result. rad: ~0.46 (deg: ~26.5). Not sure what I'm doing wrong :/

[HugoBDesigner]

It may not work 100%, but where I used it it worked fine. I hope I helped!

In your code you use math.offset which is afaik not Lua builtin function? Can you let me know what it does.

Oh, sorry, I just copy-pasted it, so I haven't really checked it. It's a function that pushes a value into a limit. If it's n more than the maximum value, it'll return the minimum value plus n. Kind of what math.mod does, except that you can have custom values. I just made that to be easier. Here it is:

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function math.offset(n, mn, mx, f)
	local n = n
	local f = f or mx
	if math.abs(mn-mx) > f then f = mx end
	while n >= mx do
		n = n - f
	end
	while n < mn do
		n = n + f
	end
	return n
end

[Roland_Yonaba]

My bad. Just use the return value of math.atan2(dy, dx) straight as the rotation argument to love.graphics.draw.

Code: Select all

function lookAt2(obj, target)
  local dx = target.x - obj.x
  local dy = -(target.y - obj.y) -- same as obj.y - target.y
  return math.atan2(dy, dx)
  --return rad < 0 and rad + math.pi*2 or rad
end

It still gives me same result. rad: ~0.46 (deg: ~26.5). Not sure what I'm doing wrong :/

Hi,
Well, I guess I am getting rusty. There is been a long time I have played with LÖVE, and I forgot another minor detail. Added to the fact that y coordinates are flipped upside down, rotation is clockwise, and I was assuming it was anti-clockwise.
This should hopefully work.

Code: Select all

function lookAt(obj, target)
  local dx = target.x - obj.x
  local dy = target.y - obj.y
  local a = math.atan2(-dy, dx)
  return math.pi/2 - a
end

[VirusX]

Hi,
Well, I guess I am getting rusty. There is been a long time I have played with LÖVE, and I forgot another minor detail. Added to the fact that y coordinates are flipped upside down, rotation is clockwise, and I was assuming it was anti-clockwise.
This should hopefully work.

Code: Select all

function lookAt(obj, target)
  local dx = target.x - obj.x
  local dy = target.y - obj.y
  local a = math.atan2(-dy, dx)
  return math.pi/2 - a
end

You are a star mate!! Thank you so much that works bang on.