Histogram of oriented gradients: a technique for the study of molecular
  cloud formation
  We introduce the histogram of oriented gradients (HOG), a tool developed for
machine vision that we propose as a new metric for the systematic
characterization of observations of atomic and molecular gas and the study of
molecular cloud formation models. In essence, the HOG technique takes as input
extended spectral-line observations from two tracers and provides an estimate
of their spatial correlation across velocity channels.
  We characterize HOG using synthetic observations of HI and $^{13}$CO(J=1-0)
emission from numerical simulations of MHD turbulence leading to the formation
of molecular gas after the collision of two atomic clouds. We find a
significant spatial correlation between the two tracers in velocity channels
where $v_{HI}\approx v_{^{13}CO}$, independent of the orientation of the
collision with respect to the line of sight.
  We use HOG to investigate the spatial correlation of the HI, from the THOR
survey, and the $^{13}$CO(J=1-0) emission, from the GRS, toward the portion of
the Galactic plane 33.75$\lt l\lt$35.25$^{o}$ and $|b|\lt$1.25$^{o}$. We find a
significant spatial correlation between the tracers in extended portions of the
studied region. Although some of the regions with high spatial correlation are
associated with HI self-absorption features, suggesting that it is produced by
the cold atomic gas, the correlation is not exclusive to this kind of region.
The HOG results also indicate significant differences between individual
regions: some show spatial correlation in channels around $v_{HI}\approx
v_{^{13}CO}$ while others present this correlation in velocity channels
separated by a few km/s. We associate these velocity offsets to the effect of
feedback and to the presence of physical conditions that are not included in
the atomic-cloud-collision simulations, such as more general magnetic field
configurations, shear, and global gas infall.
