(sec:run-aspect:visualizing-results)=
# Visualizing results

Among the postprocessors that can be selected in the input parameter file (see
{ref}`sec:run-aspect:overview` and {ref}`parameters:Postprocess/Visualization`) are
some that can produce files in a format that can later be used to generate a
graphical visualization of the solution variables $\mathbf u, p$ and $T$ at
select time steps, or of quantities derived from these variables (for the
latter, see {ref}`sec:extending:vis-postprocessors`).

By default, the files that are generated are in VTU format, i.e., the
XML-based, compressed format defined by the VTK library, see
<http://vtk.org/>. This file format has become a broadly
accepted pseudo-standard that many visualization program support, including
two of the visualization programs used most widely in computational science:
VisIt (see <https://visit.llnl.gov/>) and ParaView (see
<http://www.paraview.org/>). The VTU format has a number of advantages beyond
being widely distributed:

-   It allows for compression, keeping files relatively small even for sizable
    computations.

-   It is a structured XML format, allowing other programs to read it without
    too much trouble.

-   It has a degree of support for parallel computations where every processor
    would only write that part of the data to a file that this processor in
    fact owns, avoiding the need to communicate all data to a single processor
    that then generates a single file. This requires a master file for each
    time step that then contains a reference to the individual files that
    together make up the output of a single time step. Unfortunately, there
    doesn't appear to be a standard for these master records; however,
    both ParaView and VisIt have defined a format that each of these programs
    understand and that requires placing a file with ending `.pvtu` or
    `.visit` into the same directory as the output files from each processor.
    {ref}`sec:run-aspect:overview` gives an example of what can be found in the output
    directory.

:::{note}
You can select other formats for output than VTU, see the run-time parameters in {ref}`parameters:Postprocess/Visualization`. However, none of the numerous formats currently implemented in deal.II other than
the VTK/VTU formats allow for splitting up data over multiple files in case of parallel computations, thus making subsequent visualization of the entire volume impossible. Furthermore, given
the amount of data ASPECT can produce, the compression that is part of the VTU format is
an important part of keeping data manageable.
:::

:::{toctree}
visit.md
stat-data.md
large-data-issues.md
:::