Rotating a Group of Sprites
Demonstrates how to rotate a group of sprites around a single point.
The Complete Sample
The code in the topic shows you the technique. You can download a complete code sample for this topic, including full source code and any additional supporting files required by the sample.
Drawing a Rotated Group of Sprites
To draw a rotated group of sprites on screen
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Follow the steps of Drawing a Sprite.
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Create one set of Vector2 objects that represents the unrotated positions of the sprites and one set to hold the rotated values.
private Vector2[] myVectors; private Vector2[] drawVectors; protected override void Initialize() { myVectors = new Vector2[9]; drawVectors = new Vector2[9]; base.Initialize(); }
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After loading the sprite, calculate the positions of the unrotated group of sprites based on the sprite's size.
private Texture2D SpriteTexture; private Vector2 origin; private Vector2 screenpos; protected override void LoadContent() { // Create a new SpriteBatch, which can be used to draw textures. spriteBatch = new SpriteBatch(GraphicsDevice); SpriteTexture = Content.Load<Texture2D>("ship"); origin.X = SpriteTexture.Width / 2; origin.Y = SpriteTexture.Height / 2; Viewport viewport = graphics.GraphicsDevice.Viewport; screenpos.X = viewport.Width / 2; screenpos.Y = viewport.Height / 2; }
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In your Update method, copy the unrotated vectors and determine the screen position around which all the sprites will rotate.
private float RotationAngle = 0f; private Matrix rotationMatrix = Matrix.Identity; protected override void Update(GameTime gameTime) { ... float elapsed = (float)gameTime.ElapsedGameTime.TotalSeconds; RotationAngle += elapsed; float circle = MathHelper.Pi * 2; RotationAngle = RotationAngle % circle; // Copy and rotate the sprite positions. drawVectors = (Vector2[])myVectors.Clone(); RotatePoints(ref screenpos, RotationAngle, ref drawVectors); base.Update(gameTime); }
Transform each vector using a rotation matrix created for the rotation angle.
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To rotate around the origin, transform each vector relative to the origin by subtracting the origin vector.
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Add the origin vector to the transformed vector to create the final rotated vector.
private static void RotatePoints(ref Vector2 origin, float radians, ref Vector2[] Vectors) { Matrix myRotationMatrix = Matrix.CreateRotationZ(radians); for (int i = 0; i < 9; i++) { // Rotate relative to origin. Vector2 rotatedVector = Vector2.Transform(Vectors[i] - origin, myRotationMatrix); // Add origin to get final location. Vectors[i] = rotatedVector + origin; } }
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Draw each sprite using the rotated vectors as screen locations.
private void DrawPoints() { // Draw using manually rotated vectors spriteBatch.Begin(); for (int i = 0; i < drawVectors.Length; i++) spriteBatch.Draw(SpriteTexture, drawVectors[i], null, Color.White, RotationAngle, origin, 1.0f, SpriteEffects.None, 0f); spriteBatch.End(); }
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When all the sprites have been drawn, call End.
