2d Local coords

Say I have a 2d 21X19 Image, and I have to start a bullet from the coords 4,2. Now thats no problem if the image is not rotated, but how do I figure out where to put the bullet if its like at say 10 degrees rotation?

Hi

You have to know wich is the center of rotation, suppose, it's the center of the image (10.5 , 9.5 use real numbers, round at final stage)

then, the coords 4,2 relative to the center: -6.5 , 7.5
that's are OldX = -6.5, OldY = 7.5 after a rotation of "alfa" degrees NewX and NewY (respect to the center) are:

NewX = (OldX*Cos(alfa)) - (OldY*Sin(alfa))
NewY = (OldY*Cos(alfa)) + (OldX*Sin(alfa))

if alfa = 10 degrees then: NewX = -7.70361, NewY = 6.25735
(10 degrees anticlockwise, -10 if you want clockwise!)

I hope, someone has a better way (or easyer) but this should work

(remember to translate the newx and newy from center to the correct point)

Juan

Thanks, that worked perfectly.