  Time-averaged series of granulation images are analysed using COLIBRI, a
purpose-adapted version of a code originally developed to detect straight or
curvilinear features in aerospace images. The algorithm of image processing
utilises a nonparametric statistical criterion that identifies a straight-line
segment as a linear feature (lineament) if the photospheric brightness at a
certain distance from this line is on both sides stochastically lower or higher
than at the line itself. Curvilinear features can be detected as chains of
lineaments, using a criterion modified in some way. Once the input parameters
used by the algorithm are properly adjusted, the algorithm highlights
``ridges'' and ``trenches'' in the relief of the brightness field, drawing
white and dark lanes. The most remarkable property of the trenching patterns is
a nearly-universally-present parallelism of ridges and trenches. Since the
material upflows are brighter than the downflows, the alternating parallel
light and dark lanes should reflect the presence of roll convection in the
subphotospheric layers. If the numerous images processed by us are
representative, the patterns revealed suggest a widespread occurrence of roll
convection in the outer solar convection zone. In particular, the roll systems
could form the fine structure of larger-scale, supergranular and/or
mesogranular convection flows. Granules appear to be overheated blobs of
material that could develop in convection rolls due to some instabilities of
roll motion.
