def dImage_wrt_2dVerts(observed, visible, visibility, barycentric, image_width, image_height, num_verts, f):
    """Construct a sparse jacobian that relates 2D projected vertex positions
    (in the columns) to pixel values (in the rows). This can be done
    in two steps."""

    n_channels = np.atleast_3d(observed).shape[2]
    shape = visibility.shape

    # Step 1: get the structure ready, ie the IS and the JS
    IS = np.tile(col(visible), (1, 2*f.shape[1])).ravel()
    JS = col(f[visibility.ravel()[visible]].ravel())
    JS = np.hstack((JS*2, JS*2+1)).ravel()

    pxs = np.asarray(visible % shape[1], np.int32)
    pys = np.asarray(np.floor(np.floor(visible) / shape[1]), np.int32)

    if n_channels > 1:
        IS = np.concatenate([IS*n_channels+i for i in range(n_channels)])
        JS = np.concatenate([JS for i in range(n_channels)])

    # Step 2: get the data ready, ie the actual values of the derivatives
    ksize=1
    sobel_normalizer = cv2.Sobel(np.asarray(np.tile(row(np.arange(10)), (10, 1)), np.float64), cv2.CV_64F, dx=1, dy=0, ksize=ksize)[5,5]
    xdiff = -cv2.Sobel(observed, cv2.CV_64F, dx=1, dy=0, ksize=ksize) / sobel_normalizer
    ydiff = -cv2.Sobel(observed, cv2.CV_64F, dx=0, dy=1, ksize=ksize) / sobel_normalizer

    xdiff = np.atleast_3d(xdiff)
    ydiff = np.atleast_3d(ydiff)

    datas = []

    # The data is weighted according to barycentric coordinates
    bc0 = col(barycentric[pys, pxs, 0])
    bc1 = col(barycentric[pys, pxs, 1])
    bc2 = col(barycentric[pys, pxs, 2])
    for k in range(n_channels):
        dxs = xdiff[pys, pxs, k]
        dys = ydiff[pys, pxs, k]
        if f.shape[1] == 3:
            datas.append(np.hstack((col(dxs)*bc0,col(dys)*bc0,col(dxs)*bc1,col(dys)*bc1,col(dxs)*bc2,col(dys)*bc2)).ravel())
        else:
            datas.append(np.hstack((col(dxs)*bc0,col(dys)*bc0,col(dxs)*bc1,col(dys)*bc1)).ravel())

    data = np.concatenate(datas)

    ij = np.vstack((IS.ravel(), JS.ravel()))
    result = sp.csc_matrix((data, ij), shape=(image_width*image_height*n_channels, num_verts*2))

    return result
Construct a sparse jacobian that relates 2D projected vertex positions
    (in the columns) to pixel values (in the rows). This can be done
    in two steps.