On differentiable area-preserving maps of the plane

F: ℝ² → ℝ² is an almost-area-preserving map if: (a) F is a topological embedding, not necessarily surjective; and (b) there exists a constant s > 0 such that for every measurable set B, μ(F(B)) = sμ(B) where μ is the Lebesgue measure. We study when a differentiable map whose Jacobian determinant is nonzero constant to be an almost-area-preserving map. In particular, if for all z, the eigenvalues of the Jacobian matrix DF_z are constant, F is an almost-area-preserving map with convex image.