title: square to trapezoid
body: <p>I know that transforming a square into a trapezoid is a linear transformation, and can be done using the projective matrix, but I'm having a little trouble figuring out how to construct the matrix.</p>
  
  <p>Using the projective matrix to translate, scale, rotates, and shear is straightforward.  Is there a simple projective matrix which will transform a square to a trapezoid?</p>