How to shoot a triangle out of an asteroid which floats all of the way up to the screen?

Question

I currently have an asteroid texture loaded as my "test player" for the game I'm writing. What I'm trying to figure out how to do is get a triangle to shoot from the center of the asteroid, and keep going until it hits the top of the screen. What happens in my case (as you'll see from the code I've posted), is that the triangle will show, however it will either be a long line, or it will just be a single triangle which stays in the same location as the asteroid moving around (that disappears when I stop pressing the space bar), or it simply won't appear at all. I've tried many different methods, but I could use a formula here.

All I'm trying to do is write a space invaders clone for my final in C#. I know how to code fairly well, my formulas just need work is all.

So far, this is what I have:

Main Logic Code

        protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(ClearOptions.Target, Color.Black, 1, 1);

            mAsteroid.Draw(mSpriteBatch);

            if (mIsFired)
            {
                mPositions.Add(mAsteroid.LastPosition);
                mRay.Fire(mPositions);
                mIsFired = false;
                mRay.Bullets.Clear();
                mPositions.Clear();
            }

            base.Draw(gameTime);
        }

Draw Code

            public void Draw()
            {
                VertexPositionColor[] vertices = new VertexPositionColor[3];

                int stopDrawing = mGraphicsDevice.Viewport.Width / mGraphicsDevice.Viewport.Height;

                for (int i = 0; i < mRayPos.Length(); ++i)
                {
                    vertices[0].Position = new Vector3(mRayPos.X, mRayPos.Y + 5f, 10);
                    vertices[0].Color = Color.Blue;
                    vertices[1].Position = new Vector3(mRayPos.X - 5f, mRayPos.Y - 5f, 10);
                    vertices[1].Color = Color.White;
                    vertices[2].Position = new Vector3(mRayPos.X + 5f, mRayPos.Y - 5f, 10);
                    vertices[2].Color = Color.Red;

                    mShader.CurrentTechnique.Passes[0].Apply();
                    mGraphicsDevice.DrawUserPrimitives<VertexPositionColor>(PrimitiveType.TriangleStrip, vertices, 0, 1);

                    mRayPos += new Vector2(0, 1f);

                    mGraphicsDevice.ReferenceStencil = 1;
                }
            }

Answer 1

So you want to shoot a triangle bullet from an asteriod player at the bottom of the screen?

Firstly, why is your logic code in the Game.Draw() function?

You should maintain a list of triangles in your game:

//WARNING: psuedo code
// In Game.cpp

List<Triangle> _mTriangles = new List<Triangles>();

public override void Update()
{
    if(Keyboard.GetState() == SPACE_DOWN)
    {
       var triangle = new Triangle(mAsteroid.pos);
       _mTriangles.add(triangle);
    }

    if(Keyboard.GetState() == LEFT_RIGHT)
        mAsteroid.pos.x += Keyboard.GetState().X_OFFSET

    foreach(var t in mTriangles) t.y -= Triangle.Speed * elapsedTime;

    // TODO: another loop to remove triangles outside of screen.
}

public override void Draw()
{
   mAsteriod.Draw();
   foreach(var t in mTriangles) t.Draw();
}