View Full Version : Layer Passes

11 November 2010, 04:12 PM
Hello guys,

Thank you for all the help so far. I am really having a hard time trying to get these layers to look correct. I kind of half way get there and then shadow passes become a pain, or my character becomes transparent etc. Losing my hair pulling it out here lol.

I have put together a list of all the passes that we have been assigned in my compositing class and if anyone would be able to give me a better understanding as what the correct operations should be I would be very grateful. Any help or advice at all would be superb.

Many, many thanks in advance.

Have a great day :beer:

This is the list of elements we have been given to play with:

11 November 2010, 04:22 PM
I should have also added that I have attempted to place all elements into a single multichannel.exr file to read from. This in itself maybe causing a couple of issues as I am not quite sure what to copy when it comes to the sequence containing multiple alpha channel information. For example, the 'MAIN CHARACTER SHADOW' element is a particular problematic area as it only contains alpha channel information and nothing in RGB.

I am going to go an sit in a dark room with a large mug of coffee for 5 minutes to let my brain cool lol.

Cheers again.

11 November 2010, 12:05 AM
Multi channel exr can be problematic.

Most places are still using 4 channel exr. For the most part the problems outweigh the benefits.

You tutor or teacher should be teaching you about how renderers work and not talking about how to combine passes. 99% of all commercial renderers only perform 2 operations when finally calculating combined light and shader contributions. They are multiply and add.

In order to ensure the highest degree of accuracy between the so called BEAUTY and any re-combined multi-pass setup you will want to AVOID any multiplication in your compositing application.

why ?

Because all you are doing is re-creating the internal mathematics of this final stage of a a renderer. EXCEPT in compositing you have to do it on a pixel level. Renderers will supersample and use things like anti-aliasing filters so that they are getting results on a larger version of the image, what essentially will become a 'sub pixel' level, and then resolving them to a pixel level.

What this means is that performing a multiply on a pixel level will not yield the same result as the actual internal rendered result.

Many people, me included, wrote a lot of stuff about recombining renders in a compositing application that was “almost but not quite” what a renderer did internally. This gives you decent results but not really the same.

So too many people used to work gamma corrected. Most people now realize the benefit of working in a Linear-To-Light fashion. In this style of working you really should only be working with addition and subtraction of passes and only in a floating point format.

why ?

Because renderers calculate internally in a Linear-To-Light format and use floating point calculations to determine pixel values. At the end of the day its just about keeping things the same through both Lighting/Rendering and Compositing stages so as to ensure the best result.

In many large studios passes are added only and operations like multiplying diffuse or ambient contributions with Ambient Occlusion or Reflection Occlusion are performed in the Render ONLY.

But doesn’t this mean you can’t adjust these things ?

No. What it means is that you can Subtract out things like AO tweak them and then Add them back in. I work at a large studio and this is how we do things.

11 November 2010, 11:49 PM
Thank you very, very much for the in depth explanation aneks, I appreciate you taking the time to go into such detail. I've been following along with the Digital Tutors set of Nuke vids as well as what I have been learning at the NCCA (as an old git mature student no less!).

My Nuke classes at Uni are really well run and the lecturer really delivers a great, well structured and incredibly informative class within a 2 hour slot each week. However, the Digital Tutors online vids I have also been following are good, but they lack a deep explanation of why things are being done in the tutorial and seem to be 'follow what I am doing' rather than actually teaching.

Therefore your post has really helped me to strengthen ideas our superb compositing lecturer (can you tell that my compositing class is my favourite? lol) has delivered to us this week.

Anyways, enough of my rambling, cheers again and enjoy your evening.

11 November 2010, 12:02 AM
You are very welcome. Part of what I do in my job is train the compositing staff and occasionally do things like consult industry.

From what I have seen, NCCA students from Bournemouth seem to be amongst the best graduates we get in terms of knowledge and grasp of the state of the Art.

if your lecturer is named Ian then I know him quite well and think he is a very good sort.

best of luck with your studies I am sure I will see you in Soho soon enough.

11 November 2010, 12:24 AM
Hello again, I had Ian last year for a few programming sessions, a top bloke and very animated in his classes! I used to get out of breath just listening to him lol.

I've taken the BACAA track (BA Computer Animation Arts) rather than the more techy side of BACVA (BA Computer Visualisation Arts) and my track will be learning advanced compositing techniques from next term. Having only just started in the wonderful world of compositing (I know that sounds sarcastic when you read it out loud but I do mean it sincerely) I think I can safely say that my class has been given the best possible start with the superb lecturer we have.

I worked for a little while in the games industry before going back to Uni (I miss those days when I used to actually earn money) but before starting this course I never really thought of a career as a Compositor. I've really found myself getting more and more into Nuke as the few weeks we have had so far have flown by. Last time I got this excited about something CG related was when I first got to grips with ZBrush. With that said, what would you say makes a good compositor? And what are the prospects like out there for graduates in this particular field? I know I'm only in the second year right now but, being a mature student of 35, who knows how long I've got left before those late nights trying to keep up with the whipper snappers will do me in?

Cheers again for the help. All the best.

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