Rotations and Origins

Question

I was hoping someone could explain to me, or help me understand, the math behind rotations and origins. I'm working on a little top down space sim and I can rotate my ship just how I want it.

Now, when I get my blasters going it'd be nice if they shared the same rotation.

Here's a picture.

and here's some code!

blast.X = ship.X+5;
blast.Y = ship.Y;
blast.RotationAngle = ship.RotationAngle;
blast.Origin = new Vector2(ship.Origin.X,ship.Origin.Y);

I add five so the sprite adds up when facing right. I tried adding five to the blast origin but no go.

Any help is much appreciated

Answer 1

if you see the draw, I think is easier to understand.

You should have a ship position that is the same for the blast, and you have two sprites with different sizes. You should realize the rotation is related to the origin, so you have to pair the rotation origins.

The yellow circles are the origins for each sprite:

a) Ship: You have to sure that the origin point to the center of the ship. It seems to be the center of the texture.

   o1 = new Vector2(ship.Width, ship.Height) * 0.5f;

b) Blast: The blast orign is not the texture center. It seems something like:

   o2 = new Vector2(-10, ship.Height*0.5f);

That way draw them is pretty easy:

batch.Draw(ship, ship_pos, null, color, o1, angle,....)
batch.Draw(blast, ship_pos, null, color, o2, angle,...)

Answer 2

You have to rotate first, then move to rotated direction. Also, you have to use position vectors.

Set positions:

Vector2 shipPosition = new Vector2(100,100);
Vector2 blastPosition = shipPosition;

Set origins to middle of image

ship.origin = new Vector2(ship.Heigth / 2, ship.Width / 2);
blast.origin = new Vector2(blast.Heigth / 2, blast.Width / 2);

Change rotation:

blast.RotationAngle = ship.RotationAngle;

Move blast x,y forward from ship position, towards direction its rotated

float x = shipPosition.X+(float)(Math.Cos(blast.RotationAngle - MathHelper.PiOver2) * 5f);
float y = shipPosition.Y+(float)(Math.Sin(blast.RotationAngle - MathHelper.PiOver2) * 5f);

Apply new position

blastPosition  = new Vector2(x,y);

Draw ship

    spriteBatch.Draw(ship                   //the texture
                , shipPosition              //it's location as a Vector2
                , null                      //no source rectangle - use entire Texture2D
                , Color.White               //with the image's own colors
                , ship.RotationAngle        //at the current XNA radians
                , ship.origin               //rotation point of the image, origin
                , 1                         //use the original scale
                , SpriteEffects.None        //don't flip or otherwise manipulate image
                , 0                         //no changes to layer depth
                );

Draw blast

    spriteBatch.Draw(blast                  //the texture
                , blastPosition             //it's location as a Vector2
                , null                      //no source rectangle - use entire Texture2D
                , Color.White               //with the image's own colors
                , blast.RotationAngle       //at the current XNA radians
                , blast.origin              //rotation point of the image, origin
                , 1                         //use the original scale
                , SpriteEffects.None        //don't flip or otherwise manipulate image
                , 0                         //no changes to layer depth
                );

Hope this works, i can't test this, but it should work.

Difference in this is, that if you rotate first ( you are still in the spot, inside ship, but rotated correctly ), then you move blast forward ( along the rotation its facing ) 5 steps. It should now be perfectly aligned with the ship.

---

NOTE:

Position is x,y where texture is located on screen.

Origin is x,y in Texture, that is used as central point when rotating it.

If you move origin from center of picture, rotation gets weird ( atleast in symmetric textures).

EDIT: Added the draw commands also, to make sure its used correctly.