A little geometry help for offset rotation

tatiang

I have a line that rotates in a circle on a fixed point (lineX, lineY). I want another line to be offset by about 10 pixels so that it is always on a tangential line to the circle that the initial line is creating when it rotates. A picture is worth a thousand words so:



Knowing the x,y values for the fixed point (the blue dot) and the rotation angle for the first line, how can I determine the x,y values for the second line (the red dot)?

The closest I've gotten is x=10*sin(lineAngle)+lineX; y=10*cos(lineAngle)+lineY. Unfortunately, it works okay but then the red line crosses over the blue line instead of staying on the tangent/circle.

MarkOnTheIron

You should see something like this:

http://dl.dropbox.com/u/8244920/LineRotation.zip

MarkOnTheIron

Try x=cos(blulineangle)*10+linex y=sin(bluelineangle)+liney

tatiang

@MarkOnTheIron It just does the same thing and overlaps.

tatiang

Ah, okay, so I just had my cos and sin mixed up. You're right... thanks for the demo!

tatiang

Sorry, one more question: this works if my actors are verticle to begin with (e.g. width=4, height=80), but not when they start out horizontal (e.g. height=4, width=80). How would I adjust the math to work with horizontal actors?

tatiang

@MarkOnTheIron So this is what happens when I make the actors horizontal: http://dl.dropbox.com/u/19602014/LineRotationHorizontal.zip

jckmcgraw

@tatiang

Ah, okay, so I just had my cos and sin mixed up. You're right... thanks for the demo!

Soh - Cah - Toa !! (Indian chanting or whatever :P)

Sine is opp/hyp
Cos is adj/hyp
Tan is opp/adj

That just helped me on my CST today

Jack McGraw

tatiang

Well, I did remember SohCahToa, I just somehow had the trig functions in the wrong relation to x and y. My initial sketch was this:

tatiang

LOL... actually, that worked. I just plugged in my equations with horizontal lines and it worked. I just had to broadcast game.Angle = self.rotation+180 because the angle that my center point line was reporting was a negative number.

jckmcgraw

@tatiang

Well... I didn't do much but glad it's working!

Jack McGraw

MarkOnTheIron

Without having too much complicated functions instead of changing the height and width of the actor to make them horizontal, just change the actor rotation to 90 that way my demo will still work.

Also if you don't want too many constraints you can unlock the red actor and access the blue actor positions and rotation directly without having to constrain them to a game attribute.

tatiang

@MarkOnTheIron I needed to have the rotation of my actors at 0, but you're right that does work.

And thanks for the suggestion about constraints. I try to use unlocked scene attributes whenever possible.