Offsetting image position when scaling in XNA

Question

What Im trying to do is pretty simple, offset the rendering position of my image when scaled up/down. I honestly can't remember how to-do this :(

if (newAnim.texture != null)
{
    // Set Source Rectangle
    sourceRect.X = newAnim.imageRectArray[newAnim.CurrentFrame].X;
    sourceRect.Y = newAnim.imageRectArray[newAnim.CurrentFrame].Y;
    sourceRect.Width = newAnim.imageRectArray[newAnim.CurrentFrame].Width;
    sourceRect.Height = newAnim.imageRectArray[newAnim.CurrentFrame].Height;

    // Get new center when scaled.
    Vector2 center = new Vector2(
        (newAnim.imageVector2Array[newAnim.CurrentFrame].X * scale) / 2,
        (newAnim.imageVector2Array[newAnim.CurrentFrame].Y * scale) / 2
    );

    // Only Render if we have Something to use
    if (newAnim.CurrentFrame >= 0)
        spriteBatch.Draw(
            newAnim.texture, new Vector2(100, 100), sourceRect,
            Color.White, 0f, center, 1f, SpriteEffects.None, 0f
        );

I'm unsure what I need to do to the Vector2. Hopefully getting the center/origin position is correct? I think so.

Edit:

http://dl.dropbox.com/u/13874083/screenshots/screen120107-160918.png

The bigger image is the scaled image (pretty obvious :P), but both are rendered at 100, 100.

Answer 1

Since we reached the conclusion that you're simply trying to implement a zooming behavior, here's the most simple camera class possible with Zoom and Origin. By default this will be the middle of the screen:

public class Camera
{
    public Camera(Viewport viewport)
    {
        Origin = new Vector2(viewport.Width / 2.0f, viewport.Height / 2.0f);
        Zoom = 1.0f;
    }

    public float Zoom { get; set; }
    public Vector2 Origin { get; set; }

    public Matrix ViewMatrix
    {
        get 
        { 
            return Matrix.CreateTranslation(new Vector3(-Origin, 0.0f)) * 
                   Matrix.CreateScale(Zoom, Zoom, 1) * 
                   Matrix.CreateTranslation(new Vector3(Origin, 0.0f)); 
        }
    }
}

And here's an example of how to use it. Notice both sprites are being drawn exactly with the same parameters (except for the color):

Camera camera = new Camera(GraphicsDevice.Viewport);
camera.Origin = Vector2.Zero;

Vector2 position = new Vector2(100, 100);
Vector2 origin = new Vector2(texture.Width, texture.Height) / 2f;

camera.Zoom = 1f;
spriteBatch.Begin(0, null, null, null, null, null, camera.ViewMatrix);
spriteBatch.Draw(texture, position, null, Color.LightBlue, 0f, origin, 1f, SpriteEffects.None, 0f);
spriteBatch.End();

camera.Zoom = 3f;
spriteBatch.Begin(0, null, null, null, null, null, camera.ViewMatrix);
spriteBatch.Draw(texture, position, null, Color.LightCoral, 0f, origin, 1f, SpriteEffects.None, 0f);
spriteBatch.End();

Which results in the following picture:

In this example I set the camera origin to be Vector2.Zero since it matches the behavior on your screenshot. But normally, it would be better to zoom around the center of the screen, in which case just don't set any Origin.

Give both a try, and if you decide that you really don't need an Origin, then simply use the scale matrix Matrix.CreateScale(Zoom, Zoom, 1) directly. You don't need to calculate new positions or origins for the sprites!

---

PS - About the origin of your sprites

Hopefully getting the center/origin position is correct?

Not really. If you want the origin to be the center of the sprite, and since you're using source rectangles, your origin should be:

Vector2 origin = new Vector2(sourceRect.Width, sourceRect.Height) / 2f;

The origin is defined in local (or model) space so it doesn't matter if you're scaling the entire scene or even the individual sprites, the origin still won't change.