def txSift2Bundler(ptSift, imageDims):
    """
    Convert from David Lowe's (Matlab) image coordinates to Noah
    Snavely's image coordinates. Given (X,Y) in Sift coords and
    image dims (width, height), compute (x,y) in bundler coords.
    SIFT: the center of the upper-left pixel is (1,1), +X points
        right and +Y points down
    Bundler: the center of the image is (0,0), +X points right
        and +Y points up
    """
    sx,sy = ptSift
    w,h = imageDims
    bx = sx - 0.5 * (w+1);
    by = 0.5 * (h+1) - sy;
    return (bx, by)