Rotation in 3D Project, Active or Passive?

In “3D Project” (Image->Stacks->3D Project), when we rotate the object by Y-axis, is the object itself rotating, or is the point of view rotating?

This has important consequences. Out research group would like to use the Mean Value projection method and calculate certain intensities on the projected 2D image. If the point of view is rotating, imagine our object of interest lives in a 3D cube (image width x image height x total z-stack height), the Mean Value would depend on the length of the segment of ray from the viewing point normal to the projection plane living inside this cube. Therefore, if this ray happens to be the diagonal of the cube, the Mean Value would be underestimated as the ray segment is made artificially long by a factor of sqrt(3).

Also, is it possible to have “Sum Projection” for 3D Project? If so I don’t even have to care about the whole business of active vs passive rotation… Which plugin does this job?

See the 3D Project section of the ImageJ user guide for a detailed description of the algorithm. If that is still not enough, you can study the source code to see what it is doing internally.