• Announcements

    • khawk

      Download the Game Design and Indie Game Marketing Freebook   07/19/17

      GameDev.net and CRC Press have teamed up to bring a free ebook of content curated from top titles published by CRC Press. The freebook, Practices of Game Design & Indie Game Marketing, includes chapters from The Art of Game Design: A Book of Lenses, A Practical Guide to Indie Game Marketing, and An Architectural Approach to Level Design. The GameDev.net FreeBook is relevant to game designers, developers, and those interested in learning more about the challenges in game development. We know game development can be a tough discipline and business, so we picked several chapters from CRC Press titles that we thought would be of interest to you, the GameDev.net audience, in your journey to design, develop, and market your next game. The free ebook is available through CRC Press by clicking here. The Curated Books The Art of Game Design: A Book of Lenses, Second Edition, by Jesse Schell Presents 100+ sets of questions, or different lenses, for viewing a game’s design, encompassing diverse fields such as psychology, architecture, music, film, software engineering, theme park design, mathematics, anthropology, and more. Written by one of the world's top game designers, this book describes the deepest and most fundamental principles of game design, demonstrating how tactics used in board, card, and athletic games also work in video games. It provides practical instruction on creating world-class games that will be played again and again. View it here. A Practical Guide to Indie Game Marketing, by Joel Dreskin Marketing is an essential but too frequently overlooked or minimized component of the release plan for indie games. A Practical Guide to Indie Game Marketing provides you with the tools needed to build visibility and sell your indie games. With special focus on those developers with small budgets and limited staff and resources, this book is packed with tangible recommendations and techniques that you can put to use immediately. As a seasoned professional of the indie game arena, author Joel Dreskin gives you insight into practical, real-world experiences of marketing numerous successful games and also provides stories of the failures. View it here. An Architectural Approach to Level Design This is one of the first books to integrate architectural and spatial design theory with the field of level design. The book presents architectural techniques and theories for level designers to use in their own work. It connects architecture and level design in different ways that address the practical elements of how designers construct space and the experiential elements of how and why humans interact with this space. Throughout the text, readers learn skills for spatial layout, evoking emotion through gamespaces, and creating better levels through architectural theory. View it here. Learn more and download the ebook by clicking here. Did you know? GameDev.net and CRC Press also recently teamed up to bring GDNet+ Members up to a 20% discount on all CRC Press books. Learn more about this and other benefits here.
Sign in to follow this  
Followers 0
Christian Thorvik

Rotate sprites around a point in 2D

4 posts in this topic

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.


Share this post

Link to post
Share on other sites

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

Share this post

Link to post
Share on other sites

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 :)


Share this post

Link to post
Share on other sites

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

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
Followers 0