Can some one explain me the default camera z value: "(height/2.0) / tan(PI*60.0 / 360.0)"

I have no idea.
I think it's a value that depending from the height of the screen moves the camera outside of the screen towards you so that the whole scene (being at z=0) can be seen.

When I use camera I give fixed values to it, z=0 is screen,
z=-200 goes away from you into the screen
z=200 goes towards you out of the screen.

you can use this sketch (it's the entire sketch)

1. size(900, 500, P3D);
2. printCamera();

to see the values, see
http://www.processing.org/reference/printCamera_.html

for some fancy cam moves see
http://www.openprocessing.org/sketch/47022

Shows different types of Camera Behaviour / Movement:

• Circle and going up and down (aka 3D spiral),
• Circle with fixed height,
• with Mouse or
• on a Path.

Use 0,1,2,3 to change cam Behaviour.

how should i increment a radian expressed like (PI/4) or (2*PI)?

instead if using PI/4 I tend to use radians ( Mydegreevalue)

For example, 90° = PI/2 = 1.5707964.
http://www.processing.org/reference/radians_.html

but you can just use a float and increment it by 0.01.

Thanks, Chrisir. I was out this days, but back now. Gonna check the link you pointed me. I also use radians(degree) usually :) but was wondering if i could increment  radians directly. Thanks for that also.