Multipass musings

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  06 June 2013
Multipass musings

After googling back and forth, I have the impression that compositing floating point images should be done with add/plus. I couldn't get this to work, expect for the following that creates a near perfect match with beauty pass (roughly 0.001 pixel value difference).

diffuse + indirect + reflection + specular
(render pass output to channels in a single 32bit exr file)

Any thoughts? I wonder, how can I add passes to this?
Are you warm, are you real, Mona Lisa?
  06 June 2013
What renderer (Vray, Mental Ray, Arnold, other) and what passes are you outputting?

The combination you've got there already makes sense, but hard to tell if your missing something or not.

The most common method is to plus/add together passes but some passes occasionally need to be subtracted, multiplied or even divided.
  06 June 2013
The passes was rendered with the mental ray renderer in maya.

I output a bunch of passes. The ones that added up were: 'indirect', 'reflection', 'diffuse' and 'specular'. Presumably, outputting an excessive amount of passes doesn't mess anything up.

The 'diffuse' and 'diffuse without shadows' pass looked the same to me.

I tried multiplying 'diffuse material color' with 'direct irradiance' (unless I remember wrong) to get what looked exactly like the 'diffuse' pass, but I got some artifacting where bright pixels from one pass met dark pixels on the other pass.

Before ending up removing the lens shader from the camera (simple or photographic), I tried rendering out 32bit passes with the lens shaders, but the passes had to be adjusted and I never seemed to get them to match the beauty pass. I read later on that supposedly one would want to remove the lens shader when rendring out floating point render passes.
Are you warm, are you real, Mona Lisa?
  07 July 2013
Everything you've said seems to make sense and indicates you're doing it the right way. Whats the issue?

(Hi Will, how's Van? Say hi to Maggie for me)
  07 July 2013
There is no issue as such other than my attempt to say something meaningful about multipass compositing. I never knew how to put renderpasses together before and so I have some revelations now. No problem to solve here unless you feel compelled to pose one.

I was watching a v-ray comp video and learned to fix the odd thing I saw earlier when trying to multiply 'diffuse' pass with 'direct irradiance'. So that was about the divide operation mentioned by earlyworm above.

'direct irradiance' can apparently be "fixed" this way: 'diffuse' divided by 'diffuse material color'

So then 'diffuse' can be split into 'diffuse material color' multiplied with 'direct irradiance', so you could tweak the texture color and the shading

'diffuse material' x "fixed" 'direct irradiance' = 'diffuse'
+ 'indirect'
+ 'reflection'
+ 'refraction'
+ 'specular'
Are you warm, are you real, Mona Lisa?
  07 July 2013
Ah ok, I thought there was something that wasn't going together properly

Yep by and large you can just plus most of the passes together if you're dealing with the basic ones (diffuse, reflection, spec, indirect) Occasionally you might have to subtract a shadow pass to adjust the intensity of the shadows, but that depends on the renderer...

When you start dealing with the raw lighting passes then it can get a little more confusing as you are separating out the diffuse colour information (a texture or solid colour) from the lighting information. This of course gives you the lovely possibility of when you need to do 10 different versions of a pack shot you can swap the texture out in comp without having to re-render it 10 times.

Each renderer handles the process in slightly different ways but when you start understanding the ideology behind it they're all the same really. There's definitely a few epiphany moments to be had along the way.

Last edited by Hezza : 07 July 2013 at 02:46 PM.
  07 July 2013
Um yeah, not sure I can add too much here. I was also under the impression that something wasn't quite working for you.

(Hi Martin, things are well. Will say hi to M for you.)
  07 July 2013
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