The Gray Level Difference Vector (GLDV)

Dear scikit-image Users: It is so nice to find The Gray Level Co-occurrence Matrix (GLCM) in scikit-image. http://scikit-image.org/docs/dev/auto_examples/plot_glcm.html However, I am interested in using The Gray Level Difference Vector (GLDV). Is not it available in scikit-image? Can someone guide me how to calculate it? Thanks in the advance. Artur Bercik