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

This isn’t quite how you’re supposed to be manipulating the location of a model in world space and since you’re creating a new vertex array every single draw frame you’ll find that it performs pretty badly when you come to draw more than a few triangles.

declare the vertices and index list for your triangle just once in the LoadContent method.

VertexBuffer triangleVertexBuffer;
IndexBuffer triangleIndexBuffer;

protected override void LoadContent()
{
    // Setup a basic effect to draw the triangles with.
    mEffect = new BasicEffect(GraphicsDevice);

    // setup the triangle vertext buffer and load up it's content.
    triangleVertexBuffer = new VertexBuffer(GraphicsDevice, typeof(VertexPositionColor), 3, BufferUsage.WriteOnly);
    triangleVertexBuffer.SetData<VertexPositionColor>(new VertexPositionColor[] 
    {
        new VertexPositionColor (new Vector3 (0f, -1f, 0.0f), Color.Blue),  // Top Point
        new VertexPositionColor (new Vector3 (-1f, 1f, 0.0f), Color.White), // Bottom Left
        new VertexPositionColor (new Vector3 (1f, 1f, 0.0f), Color.Red),    // Bottom Right
    });

    // setup an index buffer to join the dots!
    triangleIndexBuffer = new IndexBuffer(GraphicsDevice, IndexElementSize.SixteenBits, 3, BufferUsage.WriteOnly);
    triangleIndexBuffer.SetData<short>(new short[]
    { 
        0, 
        1, 
        2, 
    });
}

After this assuming your effect takes in to account a world transformation (basic effect does) you can use that parameter to move the triangle.

 protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.CornflowerBlue);

            for (int i = 0; i < mRayPos.Length ; i++)
            {
                // This is the line that moves the triangle along your ray.
                mEffect.World = Matrix.CreateTranslation(new Vector3(mRayPos[i].X, mRayPos[i].Y, mRayPos[i].Z));
                mEffect.Techniques[0].Passes[0].Apply();

                // These lines tell the graphics card that you want to draw your triangle.
                GraphicsDevice.SetVertexBuffer(triangleVertexBuffer);
                GraphicsDevice.Indices = triangleIndexBuffer;
                GraphicsDevice.DrawIndexedPrimitives(PrimitiveType.TriangleList, 0, 0, 3, 0, 1);
            }

            base.Draw(gameTime);
        }

If you use this method it becomes a very simple operation to rotate or scale your trangle using Matrix.CreateRotation and Matrix.CreateScale.