Vectors to angles?

[Killerguppy101]

I need to know if there is a formula that lets me take a vector, say 12,8 and convert it to degrees.

PS- does anyone know of a DirectX tutorial that starts from the beginning, for someone who has never programmed in DX before?

[SLH]

I assume that 12 is along the x axis, and 8 is along the y axis.

If it is, then the angle would be from the xaxis, anti-clockwise, to the vector direction.

magnitude = sqr((X^2)+(Y^2))
angle = atn(Y/X)

[Killerguppy101]

i kinda understand, but could you clarify:
1 What is the magnitude formula for?
2 What does atn do?
3 Is there any way to measure the angle from the y-axis, clockwise (vector 0,10=0 degrees; 10,0=90 degrees, etc.)?

[kedaman]

1. magnitude is the length of the vector
2. atn is short for arcus tangent, which is the name of a function to retrieve the angle a in radians in this triangle of any x and y:

a
|\
| \
|x \
|_y_\

3. 270-a*180/pi

[SapphireGreen]

Isn't that just inverse TAN? Anyway.

If I have a D3D camera looking at pont 0,0,0 (origin), and I want to move it 15 degrees to the right on the X-axis (which is positive), how do I calculate what point it's looking at now?

Likewise, how do I do the reverse (make it look at point 5,0,0 and then calculate the degree of rotation)?

[riis]

Atn is the inverse of tan, that's right.

Atn is fine, if you don't need the direction of the angle, since Atn will always give a result between -1/2 pi and +1/2 pi.

Use this formula if you want to take the direction of the vector into account:

Public Function Arctan2(y As Double, x As Double) As Double
 
    If x = 0 Then
        If y > 0 Then
            Arctan2 = HALFPI
        Else
            Arctan2 = -HALFPI
        End If
    ElseIf x > 0 Then
        Arctan2 = Atn(y / x)
    Else
        If y < 0 Then
            Arctan2 = Atn(y / x) - PI
        Else
            Arctan2 = Atn(y / x) + PI
        End If
    End If
 
End Function

SapphireGreen, what's the location of the camera? If you only have a lookat point, then you can't calculate anything.

[Killerguppy101]

Thnx. It sorta works, but I still sometimes get negative angles. I need something that will go 0-359 degrees, starting at 12 o' clock and going clockwise.

(I'm not to good at this kind of math, or else I'd probably be able to fix this myself.)