Inverse of math.atan2?

math lua trigonometry
SatheeshJM picture SatheeshJM · Jul 9, 2012 · Viewed 12.9k times · Source

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.
But I am lost here.

EDIT

OK, let me give you more details.

I needed to calculate angle between 2 points (x1,y1) and (x2,y2) and I did,

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

Now if I have the angle, is it possible to get back dy and dx?

Answer

andand picture andand · Jul 9, 2012

Given only the angle you can only derive a unit vector pointing to (dx, dy). To get the original (dx, dy) you also need to know the length of the vector (dx, dy), which I'll call len. You also have to convert the angle you derived from degrees back to radians and then use the trig equations mentioned elsewhere in this post. That is you have:

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

Given angle (in degrees) and the vector length, len, you can derive dx and dy by:

local theta = angle * pi / 180
local dx = len * cos(theta)
local dy = len * sin(theta)

Related questions

Sort points in clockwise order?

Given an array of x,y points, how do I sort the points of this array in clockwise order (around their overall average center point)? My goal is to pass the points to a line-creation function to end up with …

Read More
Lua replacement for the % operator

I want to check, if a number is divisible by another number: for i = 1, 100 do if i % 2 == 0 then print( i .. " is divisible.") end end This should work without any problems, but with the Lua in my server the script doesn't …

Read More
In Lua, how can I tell if a number divides evenly into another number?

In Lua, how can I tell if a number divides evenly into another number? i.e with no remainder? I'm just looking for a boolean true or false. 12/6 = 2 (true) 18/6 = 3 (true) 20/6 = 3.(3) (false)

Read More