I am using S3D regularly, and developed a USB user interface that allows me to adjust S3D parameters such as focal plane using a contour express dial.

I have stopped using scale factor by disabling the keyboard scroll, and switching to z translation.

I am finding that I need to adjust the focal plane a lot.​  Usually it is between the clipping planes, but when I do z translations it normally gets between the camera and the near clipping plane, or out past the far clipping plane.

For S3D animations I can work around it.  For interactive use, people are having major eye strain. 

I would like to modify the chimera code to automatically adjust the focal plane as the clipping planes change.

Can you point me to the area of code where I could make the change?