How To: Draw a Sprite Over a Model
Demonstrates how to draw a sprite so that it obscures a model. In this example, we are drawing an animated sprite representing an explosion over the current screen position of a 3D model.
The Complete Sample
The code in this 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.
For this sample, the camera is a standard arc ball camera, implemented by camera.cs. The 3D model file is a simple ring, implemented by ring16b.x. The animated explosion sprite is implemented by explosion.dds. These files can be found in the complete sample. See How To: Animate a Sprite for an example of the AnimatedTexture class.
Drawing a Sprite Over a Model
To draw a sprite over a model
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In your Update method, handle the input to move your camera, then call UpdateFrame on the AnimatedTexture.
protected override void Update(GameTime gameTime) { GamePadState PlayerOne = GamePad.GetState(PlayerIndex.One); // Move the camera using thumbsticks MoveCamera(PlayerOne); // Start or stop the animated sprite using buttons if (PlayerOne.Buttons.A == ButtonState.Pressed) explosion.Play(); if (PlayerOne.Buttons.B == ButtonState.Pressed) explosion.Stop(); // Update the animated sprite explosion.UpdateFrame((float)gameTime.ElapsedGameTime.TotalSeconds);
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Use CreateMerged to create a BoundingSphere that contains all the BoundingSphere values for each ModelMesh in the Model.
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Use Viewport.Project to find the centerpoint of that sphere, which is is the center of the model in screen coordinates.
// Create a total bounding sphere for the mesh BoundingSphere totalbounds = new BoundingSphere(); foreach (ModelMesh mesh in Ring.Meshes) { totalbounds = BoundingSphere.CreateMerged(totalbounds, mesh.BoundingSphere); } // Project the center of the 3D object to the screen, and center the // sprite there Vector3 center = GraphicsDevice.Viewport.Project(totalbounds.Center, projectionMatrix, Camera1.ViewMatrix, Matrix.Identity); explosionpos.X = center.X; explosionpos.Y = center.Y;
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Take the BoundingSphere for the model and use that to create a BoundingBox with CreateFromSphere.
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Find the corner of the box that is farthest from the center using Project, and use that to scale the sprite appropriately.
// Create a bounding box from the bounding sphere, and find the corner // that is farthest away from the center using Project BoundingBox extents = BoundingBox.CreateFromSphere(totalbounds); float maxdistance = 0; float distance; Vector3 screencorner; foreach (Vector3 corner in extents.GetCorners()) { screencorner = GraphicsDevice.Viewport.Project(corner, projectionMatrix, Camera1.ViewMatrix, Matrix.Identity); distance = Vector3.Distance(screencorner, center); if (distance > maxdistance) maxdistance = distance; } // Scale the sprite using the two points (the sprite is // 75 pixels square) explosion.Scale = maxdistance / 75; base.Update(gameTime); } -
In your Draw method, draw the Model normally, then draw the animated sprite using the position calculated in Update.
protected override void Draw(GameTime gameTime) { GraphicsDevice.Clear(Color.CornflowerBlue); //Draw the model, a model can have multiple meshes, so loop foreach (ModelMesh mesh in Ring.Meshes) { //This is where the mesh orientation is set, as well as our camera and projection foreach (BasicEffect effect in mesh.Effects) { effect.EnableDefaultLighting(); effect.World = Matrix.Identity * Matrix.CreateRotationY( RingRotation ) * Matrix.CreateTranslation( RingPosition ); effect.View = Camera1.ViewMatrix; effect.Projection = projectionMatrix; } //Draw the mesh, will use the effects set above. mesh.Draw(); } // Draw the sprite over the 3D object spriteBatch.Begin( SpriteBlendMode.AlphaBlend, SpriteSortMode.Deferred, SaveStateMode.SaveState ); explosion.DrawFrame( spriteBatch, explosionpos ); spriteBatch.End(); base.Draw( gameTime ); }
