General Discussion

I'm 20ish years removed from trig, so I've forgotten most of what I knew.

I've searched the net, but can't find a "simple" answer, everything I've found deals with problems more complicated than mine...

All I need to do is to be able to find the angle to a specific point on a grid.

For instance, the angle to (5, 5) is 45 degrees. I can do that.  acos(5 / sqrt(5*5 + 5*5)) / (2pi) * 360.

And the angle to (2, 20) is acos(2 / sqrt(2 * 2 + 20 * 20)) / (2pi) * 360 = 84 degrees.

But... my problem comes in when I want to find, say, (2, -20). I still come up with 84 degrees.

What am I overlooking (something with signs I'm sure)?

Thanks in advance.

2, -20 would still be 84 degrees from the x-axis.

You could find the 'actual' angle by

Degrees are always positive unless you've moved clockwise from the origin. So this point would be -84 degrees. or, if you were moving counterclockwise from origin, it would be 276 degrees (360-84)

There are four quadrants, and 2, -20 is in the fourth (the bottom right one).

So the only thing I can really do is just check the signs of the x and y coords to find the quadrant and work it from there? Poopy, I hate inelegant solutions like that :).

I wanted some clean way to come up with the 276 :).

I would prefer a clean way too, but the arccos has no negative range :(. (I think)

Originally posted by tylerthedrui

I would prefer a clean way too, but the arccos has no negative range :(. (I think)

Your using length to calculate angles.
The concept of a negative length is nonsensical, it purely defines direction (well thats my understanding in this highly excitable pre-greenbay game state)

Ah, to be young and work with numbers in math... *sigh* I guess it's back to practicing my sideways 8's.

Hehe, that's the problem, I'm not young anymore, and I've forgotten how to do the easy stuff LOL. I have a super fancy computer and a super fancy calculator, but I can barely remember how to do this stuff.

Originally posted by Nasica

Originally posted by tylerthedrui

I would prefer a clean way too, but the arccos has no negative range :(. (I think)

Your using length to calculate angles.
The concept of a negative length is nonsensical, it purely defines direction (well thats my understanding in this highly excitable pre-greenbay game state)

GB lost because we were calling stupid plays and trying to be heroes rather than jump on the fumble :'(

Originally posted by tylerthedrui

 

Originally posted by Nasica

Originally posted by tylerthedrui

I would prefer a clean way too, but the arccos has no negative range :(. (I think)

Your using length to calculate angles.
The concept of a negative length is nonsensical, it purely defines direction (well thats my understanding in this highly excitable pre-greenbay game state)

Wasn't talking about a negative length. Negative angles are measured clockwise from the origin.

 

GB lost because we were calling stupid plays and trying to be heroes rather than jump on the fumble :'(

My apologies, i actually didnt mean to quote you, but to quote the message above yours by the OP

  Remember, the Tangent of an angle is the opposite over the adjacent.  Divide the Y coordinate by the X coordinate, then take the inverse tangent of that number, and boom, you have the angle.  Its a joke, really, when you remember the method.

Originally posted by Sharajat

  Remember, the Tangent of an angle is the opposite over the adjacent.  Divide the Y coordinate by the X coordinate, then take the inverse tangent of that number, and boom, you have the angle.  Its a joke, really, when you remember the method.

I thought about that way, but it still suffers the "lack of inelegance" (unless I'm making a mistake somewhere).

(20, -2) comes up with -5.7 degrees (I'd prefer it to be 354 without having to modify it, but I can deal with it)...

(-20, -2) comes up with 5.7 degrees, which is the same result as (20, 2), which is 180 degrees opposite it.

So I'm still stuck looking at the signs to figure out which quadrant it is and working from there. Bleh... :)

What I guess I am wanting is a "clockface" type of function/algorithm that knows I want the number of degrees from the positive X-axis counterclockwise around to the point on the grid. /sigh