Thread: Angle of rotation formula depending on sprite position

Originally Posted by tabstop

If you go further to say 120 degrees, cos(120) is -0.5 and sin(120) is 0.866, so when your arrow points to 11 o'clock you get the point (-0.5, 0.866) which matches.

yep x is -0.5 and y is -0.866 at 11
but at 12 y = 1 x = 0
i get y = -cos i get why we are using cos..but not why negative cos
i know we're going left from 3,2,...12,11 0,90,120,180.. but how do you know going left is negative?

Originally Posted by Sipher

Hey, stop abusing my image!

ha ha sorry! It was begging to be abused :P

that was why I asking if maybe that -1 was relevant....
I was looking for a site to see if it could explain further
I found this:
Quadrant trigonometry facts
all positive from 0-90 on the first quadrant...
and since the ships heading on the first picture is 90 degrees (the arrow is at 12 o'clock) ..sin and cos are positive..yeah?
so why -cos(angle)..it doesn't make sense unless we're reading from 90-180 degrees (12 to 9 o'clock) then cos is -
if so at which point do we decide we are at 12-9 instead of 3-12 when the arrow is facing at 12 o'clock?
But I think I'm close to understanding this...

Originally Posted by Eman

it doesn't make sense unless we're reading from 90-180 degrees (12 to 9 o'clock) then cos is -

Exactly! Now you're talking! Remember what I said? That your second sprite's angle is +90 the first?

Originally Posted by Eman

if so at which point do we decide we are at 12-9 instead of 3-12 when the arrow is facing at 12 o'clock?

Well, it really doesn't matter. Both give the same result, but i'm not sure i understood your question...

Originally Posted by Sipher

Exactly! Now you're talking! Remember what I said? That your second sprite's angle is +90 the first?

yep, that you did but then I didn't know about the quadrant's rule my bad .

Well, it really doesn't matter. Both give the same result, but i'm not sure i understood your question...

What I mean is since 0 to 90 is +cos
and 90 to 180 is -cos
how do I know which of the rule to choose from if it is 90 degrees on the dot?

Yes, as I said, it doesn't matter because both yield the same result. cos90 is 0.

Originally Posted by Sipher

Yes, as I said, it doesn't matter because both yield the same result. cos90 is 0.

Yeah, except it won't work until I use a negative cos to find the y axis..i know it yields the same result on the unit circle..but to apply which rule to use for the ships heading makes all the difference

unless the answer is if the angle is exactly on the dot..if the arrow is pointing straight at 90 we cannot know if it is positive or negative cos..we can only try the one or the other...and whichever works tells us which quadrant in the unit circle we're dealing with.

Nononono...

Look, for your first sprite:
velX = cos(angle)
velY = sin(angle)

And for your second sprite:
velX = sin(angle)
velY = -cos(angle)

That's all there is to is!

aaah, but why for the 2nd one especially?

Ok what if the ship was facing 180 degrees? (will check it now)

lol I tried the formula
x = -Math.cos(angle)
y = -Math.sin(angle)
for 180 degrees
it worked perfectly for the up why is it not working when going down?..trig is beautiful and confusing as hell..I wish I understood it in secondary school :O

EDIT:
it worked with that formula..was editing the wrong file..nifty stuff..
Thanks lads..gave me a good trig lesson ha ha..sorry I was slow