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Rotate sprites around a point in 2D

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#1 Dobbydoo   Members   -  Reputation: 174


Posted 06 January 2014 - 04:46 PM

I'm trying to make an entity system where the entity can have multiple components, like arms and legs, or weapon attachments, but I can't figure out how to get all of the components to stay in the right position when the entity is rotated.


With the code I currently have, the rotation works, but I can't place the components where they are supposed to be. When I try to place the component at the mouse position it doesn't go where it is supposed to, and rather seems to stick to a circle path.


                Vector2 newPos = new Vector2(Base.X, Base.Y);

                newPos.X += (float)(PositionRelativeToBase.X * Math.Cos(Rotation - MathHelper.ToRadians(PositionRelativeToBase.Y)));
                newPos.Y += (float)(PositionRelativeToBase.X * Math.Sin(Rotation - MathHelper.ToRadians(PositionRelativeToBase.Y)));


This is code that I found elsewhere, though, so I don't understand it entirely, so I can't really explains my thoughts behind it.


#2 xnafan   Members   -  Reputation: 159


Posted 08 January 2014 - 03:16 AM

Hi Dobbydoo smile.png

It sounds like there are two different tasks here:


1) Getting the mouse's position relative to the parent object (where you want the childobject attached, right?).

2) Drawing the child object in-game while being able to rotate its position and Texture.



Solving 1 is pretty simple:


To get direction from object to mouse as a Vector2, subtract object's position (a Vector2) from mouse's position (also a Vector2):


Vector2 distanceAndDirectionFromObjectToMouse = mousePosition - objectPosition;


This value you can store along with the child object, to know where to draw it in relation to the parent.


Solving 2) is more complicated.

You want to be able to rotate the child object's position around the parent.

To do this you need to:

  1. Find the current rotation of and distance to the child object
  2. change the rotation by the wanted amount
  3. calculate the new position using cos and sin (you may want to read up on these)
    basically cos(angle) gives you the X coordinate of where to position something, and sin(angle) gives you the Y coordinate.

Getting the rotation of a Vector2 is possible using the Atan2 function:


private float Vector2ToRadian(Vector2 direction)
    return (float)Math.Atan2(direction.X, -direction.Y);


Radian is  a type of measurement where there are 2 * PI degrees (approximately 6.283) around the full circumference of the circle as opposed to regular degrees where you have 360.


Getting the distance is a method on the Vector2 struct called Length().


float distanceToChild = distanceAndDirectionFromObjectToMouse.Length();


using the distance and rotation you can use the following method to find the new location of the child object (the "sattelite" in the methodname below):


public Vector2 GetPositionOfSatellite(Vector2 center, float distance, float directionInRadians)  


          float yDifference = (float)Math.Sin(directionInRadians);

          float xDifference = (float)Math.Cos(directionInRadians);

          Vector2 direction = new Vector2(xDifference, yDifference);

          Vector2 precisePositionOfSatellite = center + direction * distance;

          return precisePositionOfSatellite;      


If that is unclear, you may benefit from the two articles I've taken code from:





...otherwise, just ask smile.png


Kind regards - Jakob

Edited by xnafan, 08 January 2014 - 03:17 AM.

www.xnafan.net - tutorials, tools and code samples for making games :)

#3 Dobbydoo   Members   -  Reputation: 174


Posted 09 January 2014 - 07:10 AM

Thanks for the answer :)


I don't have time to code anything at the moment, though, but I think can figure it out from here, so thanks for the help :)

#4 Dobbydoo   Members   -  Reputation: 174


Posted 11 January 2014 - 06:51 AM

I got it working! Thanks a lot for your help! biggrin.png


I'm going to try to explain what I did, so that others trying to achieve the same might benefit from it smile.png


First I create a child object I place it at the mousepos

entity.childObjects.Add = new Rectangle(ms.X, ms.Y, size.X, size.Y);

Then I store the distance between the childObject and the Base of the parent (I use a list for this at the moment, but you should probably create a class for childObject and have all the information you need there)

entity.ConnectionGap.Add(new Vector2(ms.X - entity.Base.X, ms.Y - entity.Base.Y));

In my entity class, this is what I do to get the right position. As I mentioned, it's probably better to create a childObject class, so that you won't need lots of different lists.

for (int i = 0; i < childObjects.Count; i++)
                Vector2 newPos =
GetPositionOfSatellite(new Vector2(Base.X, Base.Y),
new Vector2(ConnectionGap[i].X, ConnectionGap[i].Y).Length(),
Vector2ToRadian(new Vector2(ConnectionGap[i].X, ConnectionGap[i].Y)) + Rotation); //I changed this method a little, because it inverted my coordinates for some reason

                childObjects[i] = new Rectangle((int)newPos.X, (int)newPos.Y, childObjects[i].Width, childObjects[i].Height);

private float Vector2ToRadian(Vector2 direction)
            return (float)Math.Atan2(direction.Y, direction.X);

I hope I explained the well enough, and that someone will benefit from it smile.png

Edited by Dobbydoo, 11 January 2014 - 06:52 AM.

#5 xnafan   Members   -  Reputation: 159


Posted 11 January 2014 - 06:55 AM

Glad to hear it! :)

Cudos on coming back with your solution for future reference :)



www.xnafan.net - tutorials, tools and code samples for making games :)

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