What Is a Render Pass and Why Do I Need Them?
There are a number of reasons you may want separate render passes from a 3dsmax scene:
a. You want to break out the elements into background/midground/foreground to control Depth of Field.
b. You want to render your scene from multiple cameras, to multiple outputs.
c. You want to break your scene into separate image sequences so you can composite the shot in a 2d compositing package to control integration with live plates to a level not possible within 3dsmax itself.
While RPManager will help with any of the above scenarios, it has been designed from the perspective of point c. - breaking a scene down to elements that enable as much control as you want and need in a 2d compositing package.
What is a Render Pass in the context of this help? A render pass is effectively the collection of settings that define what is needed to produce a single image sequence - ie camera to render from, output path to render to, what is visible and what properties all these elements have.
Separate Renders From My Scene - Why Do I Need Them?
Chances are that if you are at all excited about what this script can do for you, you will already know why you need them - control, and, ultimately, reduced render times.
Control: you want to produce photoreal work. Lets say you are using subsurface scattering in your 3rd party renderer, but find the render times are pretty high, and you are still tweaking lots of parameters.
So you decide to break out your translucency, and rather than let the renderer composite the final image, you will do that yourself. You render the translucency once, your diffuse pass once, your reflections once. You comp it in your 2d package, and then you can grade each element separately to adjust your final image. Need more or less translucency? Drop a gain tool and tweak it in almost realtime.
This sort of level of control is why rendering in passes may well be worth the effort.
Reduced render times:
How can this save you time when you have to do 10 renders, and then spend time setting up the composite? Having your scene broken down into elements can save a huge amount of time when initially setting up the look you want, and especially when needing to make changes late in a project; if the reflection component of a shot is found to be too strong and you have the reflection as a separate pass, changing the intensity of the reflection in 2d and re-rendering the output is a trivial and usually extremely quick process; re-rendering an entire shot in 3d is often a matter of hours.