How to create an isosurface along an arbitrarily oriented plane?
Sometimes one needs to create an isosurface that is arbitrarily oriented in space, i.e. not parallel or perpendicular to any coordinate axis. This is accomplished as follows: 1. First determine the direction cosines of the vector perpendicular to the wanted isosurface. If you know three points on the plane, then it is easy to determine the direction cosines of the vector perpendicular to the plane. Let the points be A B C. The vector cross product of vector AB and vector BC is a vector perpendicular to the plane containing A B and C. 2. Under Define>CustomFieldFunctions, create a customfieldfunction: custom_field_function_0 = ax+by+cz Here a, b, c are the direction cosines of the perpendicular vector that you have determined in step 1. x,y,z are Grid>Xcoordinate, etc. 3. Using the coordinates of one of the points A B or C, calculate the the value of ax+by+cz. 4. Make sure that the case is initialized. Solve>Initialize>Initialize 5. Go to Surface>IsoSurface Select IsoSurface of custom_field_function_0 Under 'value' type the value that you calculated in step 3. above. Click on Create/Apply The isosurface is created. 

