to use them in compositing, to isolate elements and post process them if you need. They act as masks.
here are some usefull links:
Mia_Material_X multioutput those are all the passes you can get out of mia.
An overview on how to composite some of th em, done in maya but the concept is the same.
A nice tutorial from Simon Reeves aboout compositing, done in fxtree.
you can easily find a lot of info about compositing in passes by googling "render passes compositing" or anything similar.
There are also a lot of books, also the Softimage doc shows some basic examples.
The RAW passes are the output passes that contains "Raw" data so they have all the info you might need. For normal compositing you might not need those, but if you miss some particular data and you want to split or do something advanced or more custom you can find them handy.
Best way to learn this is practice, and test. Do a simple scene and output passes, and try to rebuild it in comp.
The link i gave you arent the same, so i dont understand where i posted it "twice".
One pdf shows you all the mia_material output passes available so you start to learn them visually.
The other is a link i've found that shows some random compositing.
The third link from simon reeves is just a normal compositing example.
Now to me it seems you are a little confused on compositing in general.
If to you it seems a waste of time to export ID passes or passes in general its because you dont need to do so. Or because you dont need to compose images.
The fact that there are passes doesnt mean that you always have to use them. When it makes sense to use you have them available.
I told you that ID passes are mainingly used as masks, and you consider this a waste of time.
Ok lets say you have an architectural render of a skycraper with 180 windows. Now you look at the render and you dont like how you rendered the transparency or glass of those windows, what do you do? re render everything?
Here you could just export an object ID pass where all your window have a constant color (saving your time to go and select them in photoshop manually). You open photoshop give the render layer this mask and voilà, you can use any effect you want in this particular area using the object ID pass on those windows.
This is the basic of photoshop, masks. How is it a waste of time?
Also you didnt apply enough into googling stuff because there are a lot of things regarding compositing and postwork.
here is one for example
As you can see the first is the raw render he ouptut from the 3d software, and after there is the final shot composited in photoshop. Guess what he used to isolate the elements and customized them in photoshop? Yep the ID Passes.
Seriously you dont know how to use google?
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