Question

I have a pretty annoying problem. I would like to create a drawing program, using winform + XNA combo.

The most important part would be to transform the mouse position into the XNA drawn grid – I was able to make it for the translations, but it only work if I don’t zoom in – when I do, the coordinates simply went horrible wrong.

And I have no idea what I doing wrong. I tried to transform with scaling matrix, transform with inverse scaling matrix, multiplying with zoom, but none seems to work.

In the beginning (with zoom value = 1) the grid starts from (0,0,0) going to (Width, Height, 0). I was able to get coordinates based on this grid as long as the zoom value didn’t changed at all. I using a custom shader, with orthographic projection matrix, identity view matrix, and the transformed world matrix.

Here are the two main methods:

internal void Update(RenderData data)
{
    KeyboardState keyS = Keyboard.GetState();
    MouseState mouS = Mouse.GetState();

    if (ButtonState.Pressed == mouS.RightButton)
    {
        camTarget.X -= (float)(mouS.X - oldMstate.X) / 2;
        camTarget.Y += (float)(mouS.Y - oldMstate.Y) / 2;
    }

    if (ButtonState.Pressed == mouS.MiddleButton || keyS.IsKeyDown(Keys.Space))
    {
        zVal += (float)(mouS.Y - oldMstate.Y) / 10;
        zoom = (float)Math.Pow(2, zVal);
    }

    oldKState = keyS;
    oldMstate = mouS;

    world = Matrix.CreateTranslation(new Vector3(-camTarget.X, -camTarget.Y, 0)) * Matrix.CreateScale(zoom / 2);
}

internal PointF MousePos
{
    get
    {
        Vector2 mousePos = new Vector2(Mouse.GetState().X, Mouse.GetState().Y);

        Matrix trans = Matrix.CreateTranslation(new Vector3(camTarget.X - (Width / 2), -camTarget.Y + (Height / 2), 0));

        mousePos = Vector2.Transform(mousePos, trans);
        return new PointF(mousePos.X, mousePos.Y);
    }
}

The second method should return the coordinates of the mouse cursor based on the grid (where the (0,0) point of the grid is the top-left corner.).

But is just don’t work. I deleted the zoom transformation from the matrix trans, as I wasn’t able to get any useful results (most of the time, the coordinates were horribly wrong, mostly many thousands when the grid’s size is 500×500).

Any ideas, or suggestions? I’ve been trying to solve this simple problem for two days now :\

Answer 1

Take a look at the GraphicsDevice.Viewport.Unproject method for converting screen space locations in to world space, it basically goes through your world, view, projection transformations in reverse order.

as for your zooming issue, instead of scaling the world transform why not move the camera closer to the object that you’re viewing?