# Help with rotation issue

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Hi everyone! I guess I can say I'm new to the community, so I guess I'll introduce myself as a recent game developer. I mainly do it in my free time, but I've been stumbling on this same problem in which I can't get the rotation on the arm of my character to line up to the mouse. Here is the exact issue though...

First of all let me show visually on what it is I have the problem with.

Let's say I have an arm holding a weapon:

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||||     ===={]

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There are two main vector2 points on this arm, the shoulder (p1), and the firing point for the weapon (p2)

||||||| -(p1)

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||||     ===={] -(p2)

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And then I have a third point (p3), which is basically the mouse.

||||||| -(p1)

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||||     ===={] -(p2)                            { } -(p3)

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The firing point must always be facing the mouse at all times. But, if the mouse were to move...

||||||| -(p1)

||||

||||     ===={] -(p2)

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{ } -(p3)

I don't know how to change the angle to move the arm to make p2 perfectly align with the new p3.

So far I have these variables:

Vector2 variables

p1 [point 1]           = origin

Op2 [old point 2]  = The firing point before the change of p3

Np2 [new point 2] = The firing point that has adjusted to align with Np3

Op3 [old point 3]  = The mouse before it moved (it is perfectly aligned with Op2)

Np3 [new point 3] = The mouse after it moved (it is now not aligned with Op2)

I'm not sure what angles to look for in comparison to each of these variables, so I thought I could ask the community for help on this problem. If anyone can help with this, that'd be fantastic!

Thanks!

Edited by Techoh

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I think you are just going to get an angle between two vectors to deduce another vector so you will be doing more work than necessary.

So don't use angles and just work directly with vectors

The problem...

It looks like the distance between p1 and p2 will be always the same, lets say its 2 meters

now I have to deduce the new direction the arm will have and put p2 in that path, it will be the direction from p1 to p3

to get this direction I make this operation    p3-p1/ length(p3-p1)=newdirection

now newdirection=newdirection*2;  to make newdirection as large as the distance that will always be between  p1 and p2

now I add p1+newdirection and I get the new position of p2

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Well, if you simply want to rotate the arm around its shoulder. You can use the following:

var m = Mouse.GetState();
Vector2 mousePos = new Vector2(m.X, m.Y);

Vector2 distance = mousePos - p1;
// calculate angle to rotate
float angle =
(float)Math.Atan2(distance.Y, distance.X);

spriteBatch.Begin();
spriteBatch.Draw(armAndGunTexture, p1.X, p.Y, null, Color.White, angle, Vector2.Zero,1f, SpriteEffects.None, 0);
spriteBatch.End();

I hope this helped :)

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...to get this direction I make this operation    p3-p1/ length(p3-p1)=newdirection

now newdirection=newdirection*2;  to make newdirection as large as the distance that will always be between  p1 and p2

now I add p1+newdirection and I get the new position of p2

Can I ask the logic behind this? I haven't taken any calculus or triginometry class (I don't know if it matters), but I guess to clear things up in my mind, I'm assuming that the term direction refers to the angle? Along with that, when you use the operation p3-p1/ length(p3-p1)=newdirection, are you finding the angle of p3 to p1 and then dividing that by the length between the two times the angle? I'd also like to ask the reason for multiplying newdirection by 2 if possible. Finally, are you adding the new angle to p1? (assuming direction equals angle)

If you have the time, it'd be really helpful to understand this. I seem to learn best when I understand the full mechanics of what's happening behind the math. That'd be great, thanks for the help!

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Well, if you simply want to rotate the arm around its shoulder. You can use the following:

var m = Mouse.GetState();
Vector2 mousePos = new Vector2(m.X, m.Y);

Vector2 distance = mousePos - p1;
// calculate angle to rotate
float angle =
(float)Math.Atan2(distance.Y, distance.X);

spriteBatch.Begin();
spriteBatch.Draw(armAndGunTexture, p1.X, p.Y, null, Color.White, angle, Vector2.Zero,1f, SpriteEffects.None, 0);
spriteBatch.End();

I hope this helped

Oh, the thing is, when I do that, it doesn't quite line up correctly with the gun. For instance, if I were to shoot something, it wouldn't quite line up with the mouse. (unless of course I line up the firing direction to a new angle to match up, but with I'm doing, it may be better overall if I have it all line up at the same angle if that makes sense. Thanks for the help though!)