[RESOLVED] VB:6.0 - Rotate Points on a 2D X,Y Graph? (need math help)

[Tontow]

I have no clue about the math needed for this. Please help me.

I have several unknown points on a 2D graph. IE: -1,5 3,3 -7,-8 4,-9

How would I rotate them X number of degrees? (A function would be appreciated.)

[wy125]

you can derive a formula from the sine and cosine sum formulas which would give you:

Xr = X * cos(theta) - Y * cos(theta)
Yr = Y * cos(theta) + X * cos(theta)

where, Xr is the rotated x coordinate,Yr is the rotated y coordinate, and X and Y are the original coords. This assumes that you are rotating about the origin.

[Caskbill]

you can derive a formula from the sine and cosine sum formulas which would give you:

Xr = X * cos(theta) - Y * cos(theta)
Yr = Y * cos(theta) + X * cos(theta)

where, Xr is the rotated x coordinate,Yr is the rotated y coordinate, and X and Y are the original coords. This assumes that you are rotating about the origin.

Be aware that the angle theta must be in radians, not degrees since VB works in radians.

To convert degrees to radians, multiply degrees by pi/180. To convert radians to degrees, multiply radians by 180/pi.

[Tontow]

So I get theta by multiplying degrees by pi/180?

[wy125]

So I get theta by multiplying degrees by pi/180?

Well as Caskbill points out, VB assumes the angle is in radians for its trig functions such as sine and cosine. If you wish to use degrees as your basis, then you would will need to convert. If you wish to use radians, then no conversion is necessary. So, let's say you want to rotate your point by 30 degrees. Then you would need to convert as follows:

theta = 30 * pi / 180

Now theta has a value in radians equivalent to 30 degrees and now the formulas are valid.