Inverse of math.atan2?

Question

What is the inverse of the function

math.atan2

I use this in Lua where I can get the inverse of math.atan by math.tan

Answer 1

You'll probably get the wrong numbers if you use:

local dy = y1-y2 
local dx = x1-x2
local angle = atan2(dy,dx) * 180 / pi

If you are using the coordinate system where y gets bigger going down the screen and x gets bigger going to the right then you should use:

local dy = y1 - y2
local dx = x2 - x1
local angle = math.deg(math.atan2(dy, dx))
if (angle < 0) then
  angle = 360 + angle
end

The reason you want to use this is because atan2 in lua will give you a number between -180 and 180. It will be correct until you hit 180 then as it should go beyond 180 (i.e. 187) it will invert it to a negative number going down from -180 to 0 as you get closer to 360. To correct for this we check to see if the angle is less than 0 and if it is we add 360 to give us the correct angle.