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[BlackDragon777]

Hey everyone. I have a simple math question for you. I know how to rotate a point about the origin by using matricies. I also know how to do it using the formula.

The way I do it is

x = x*cos(theta) - y*sin(theta)
y = x*sin(theta) + y*cos(theta)

However I want to rotate my triangle about an arbitrary point. I read in my graphics book that the way to do that using a formula would be,

x = x*cos(theta) - y*sin(theta) + ORIGIN_VARIABLE
y = x*sin(theta) + y*cos(theta) + ORIGIN_VARIABLE

HOwever that doesn't seem to work. I am not great at math. I can do it with matricies but I think it is easier and faster to use a formula if at all possible. can anyone correct my latter formula? Thanks!

[AudioMonster]

I am not good a 3D maths too but i can try :

Your first formula rotates the point [X Y] around [0 0]
Your second formula does the same but then translates the rotated point by [OriginX OriginY].

What you want to do is to rotate around [OriginX OriginY], so you have to translate each point by [-OriginX -OriginY] and do a rotation around [0 0] by using your first formula, then you retranslate the rotated point by [OriginX OriginY].

Like:

x' = x - OriginX;
y' = y - OriginY;

x = x'*cos(theta) - y'*sin(theta) + OriginX;
y = x'*sin(theta) + y'*cos(theta) + OriginY;

That SHOULD work, but i don't guarantee anything ;)
There is probably a way to simplify that though.

[ector]

[AudioMonster]

Would that achieve less multiplication / additions ?

[BlackDragon777]

Hey! Thanks! I wasn't subtracting the origin points away. Thanks, that fixed my problem!

GOD Bless you Always!!!!