5. Geometry Plotting Specification – plots.xml

Basic plotting capabilities are available in OpenMC by creating a plots.xml file and subsequently running with the --plot command-line flag. The root element of the plots.xml is simply <plots> and any number output plots can be defined with <plot> sub-elements. Two plot types are currently implemented in openMC:

  • slice 2D pixel plot along one of the major axes. Produces a PNG image file.

  • voxel 3D voxel data dump. Produces an HDF5 file containing voxel xyz position and cell or material id.

5.1. <plot> Element

Each plot is specified by a combination of the following attributes or sub-elements:


The unique id of the plot.

Default: None - Required entry


Filename for the output plot file.

Default: “plot”


Keyword for plot coloring. This can be either “cell” or “material”, which colors regions by cells and materials, respectively. For voxel plots, this determines which id (cell or material) is associated with each position.

Default: “cell”


Universe depth to plot at (optional). This parameter controls how many universe levels deep to pull cell and material ids from when setting plot colors. If a given location does not have as many levels as specified, colors will be taken from the lowest level at that location. For example, if level is set to zero colors will be taken from top-level (universe zero) cells only. However, if level is set to 1 colors will be taken from cells in universes that fill top-level fill-cells, and from top-level cells that contain materials.

Default: Whatever the deepest universe is in the model


Specifies the (x,y,z) coordinate of the center of the plot. Should be three floats separated by spaces.

Default: None - Required entry


Specifies the width of the plot along each of the basis directions. Should be two or three floats separated by spaces for 2D plots and 3D plots, respectively.

Default: None - Required entry


Keyword for type of plot to be produced. Currently only “slice” and “voxel” plots are implemented. The “slice” plot type creates 2D pixel maps saved in the PNG file format. The “voxel” plot type produces a binary datafile containing voxel grid positioning and the cell or material (specified by the color tag) at the center of each voxel. Voxel plot files can be processed into VTK files using the openmc-voxel-to-vtk script provided with OpenMC and subsequently viewed with a 3D viewer such as VISIT or Paraview. See the Voxel Plot File Format for information about the datafile structure.


High-resolution voxel files produced by OpenMC can be quite large, but the equivalent VTK files will be significantly smaller.

Default: “slice”

<plot> elements of type “slice” and “voxel” must contain the pixels attribute or sub-element:


Specifies the number of pixels or voxels to be used along each of the basis directions for “slice” and “voxel” plots, respectively. Should be two or three integers separated by spaces.


If the aspect ratio defined in pixels does not match the aspect ratio defined in width the plot may appear stretched or squeezed.


Geometry features along a basis direction smaller than width/pixels along that basis direction may not appear in the plot.

Default: None - Required entry for “slice” and “voxel” plots

<plot> elements of type “slice” can also contain the following attributes or sub-elements. These are not used in “voxel” plots:


Keyword specifying the plane of the plot for “slice” type plots. Can be one of: “xy”, “xz”, “yz”.

Default: “xy”


Specifies the RGB color of the regions where no OpenMC cell can be found. Should be three integers separated by spaces.

Default: 0 0 0 (black)


Any number of this optional tag may be included in each <plot> element, which can override the default random colors for cells or materials. Each color element must contain id and rgb sub-elements.


Specifies the cell or material unique id for the color specification.


Specifies the custom color for the cell or material. Should be 3 integers separated by spaces.

As an example, if your plot is colored by material and you want material 23 to be blue, the corresponding color element would look like:

<color id="23" rgb="0 0 255" />

Default: None


The special mask sub-element allows for the selective plotting of only user-specified cells or materials. Only one mask element is allowed per plot element, and it must contain as attributes or sub-elements a background masking color and a list of cells or materials to plot:


List of unique id numbers of the cells or materials to plot. Should be any number of integers separated by spaces.


Color to apply to all cells or materials not in the components list of cells or materials to plot. This overrides any color color specifications.

Default: 255 255 255 (white)


The meshlines sub-element allows for plotting the boundaries of a regular mesh on top of a plot. Only one meshlines element is allowed per plot element, and it must contain as attributes or sub-elements a mesh type and a linewidth. Optionally, a color may be specified for the overlay:


The type of the mesh to be plotted. Valid options are “tally”, “entropy”, “ufs”, and “cmfd”. If plotting “tally” meshes, the id of the mesh to plot must be specified with the id sub-element.


A single integer id number for the mesh specified on tallies.xml that should be plotted. This element is only required for meshtype="tally".


A single integer number of pixels of linewidth to specify for the mesh boundaries. Specifying this as 0 indicates that lines will be 1 pixel thick, specifying 1 indicates 3 pixels thick, specifying 2 indicates 5 pixels thick, etc.


Specifies the custom color for the meshlines boundaries. Should be 3 integers separated by whitespace. This element is optional.

Default: 0 0 0 (black)

Default: None