// pdev is the IDirect3DDevice9 pointer
D3DVIEWPORT9 viewport;
pdev->GetViewport(&viewport);

// Short variable names to make the matrix readable
int w = viewport.Width; int h = viewport.Height;
int vx = viewport.X; int vy = viewport.Y;

// The scaling factors
float sx = 1.0f / w;
float sy = 1.0f / h;

// The projection matrix
D3DMATRIX matProj = {
                    sx * 2.0f,                  0.0f, 0.0f, 0.0f,
                         0.0f,            -sy * 2.0f, 0.0f, 0.0f,
                         0.0f,                  0.0f, 1.0f, 0.0f,
       -sx * (w + 2 * vx + 1), sy * (h + 2 * vy + 1), 0.0f, 1.0f
};