.. _CMGUI-contours:

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Visualizing element fields using contours (isosurfaces and isolines)
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.. _example a7: http://cmiss.bioeng.auckland.ac.nz/development/examples/a/a7/index.html
.. _example a2: http://cmiss.bioeng.auckland.ac.nz/development/examples/a/a2/index.html

*Contours* are graphical representations used to visualize 3D or 2D objects that connect every point where the value of a certain field is the same. This idea is analogous to the contour lines on a map, where a line connects every point of a certain height. In 3D this means that a surface connects every point of a certain value.  An example of this is in `example a7`_ where an iso-surface is used to show the surface where every point is at 100 degrees Celsius. For a simpler example, `example a2`_ shows the creation of an iso-surface at x=0.5 within the simple cube mesh.

Note: prior to Cmgui v3.0, contours were called isosurfaces.

Apart from the coordinate field, contours require the selection of the 'iso-scalar field' for which you want to visualise contours of constant value, and the iso-values you wish to see. Iso-values can be specied either by a space-separated list of explicit values, or as a sequence consisting of a number of values and the first and last value, with sampling evenly spaced between them.

As with :ref:`surfaces <CMGUI-surfaces>`, the detail level of iso-surfaces is determined by the selected *tessellation* setting in *settings editor* area of the :ref:`scene editor <CMGUI-scene-editor-window>` (Figure 2).  Iso-surfaces are drawn to connect points on the edges of the sub-element divisions where the iso-scalar matches the chosen value.  With an *tessellation* setting of 1, an iso-surface will only connect points on the line edges of the element.

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.. figure:: /Cmgui/images/tessellation_iso_surfaces.png
   :align: center

   **Figure 2: How tessellation affects iso-surface detail:**  The *tessellation* setting of the iso-surface sets the number of divisions in each xi direction within each element.  Here, an iso-surface is used to connect every point within the element cube where "temperature" is 20.  The "temperature" field is shown on the surface of the element cube using a spectrum in panel A.  In B, the iso-surface is shown in wireframe (green) and a single face of the cube is divided up according to the *tessellation* setting.  You can see that the iso-surface links the discretization divisions at points where the iso-scalar is at the chosen value.  These intersections are indicated by yellow circles. Panel C shows how different *tessellation* settings affect this iso-surface; settings of 2, 3, 4, and 8 minimum divisions are shown.