Color Mapping in Renderpasses?

[sebastian_k]

Hi!
While trying to work with the Main Passes I noticed that I have to enable the "CM" (Color Mapping) button in the channels panel in order to be able to composite RAW_Light and RAW_GI to the final image. If I keep that turned off it gives me an output that is probably mapped in some wrong way. However, I have to keep the diffuse channel with CM turned off for a correct result.
Enabling or disabling color-management in the Blender Render setting doesn't seem to have any impact on that.
Can someone give me some information about this behaviour?
Is the CM button specific to the Blender exporter?
Any other hints or advices for setting up renderpasses in the vb25 exporter?


Cheers!

Seb

[bdancer]

Re: Color Mapping in Renderpasses?

Is the CM button specific to the Blender exporter?

The same button in Maya too.
I guess just may be we need to add some default CM=On and some leave off, but I'm not a big compositing expert - hope your tests will help =)

[nizu]

Re: Color Mapping in Renderpasses?

Hi,
indeed knowing which channels need color mapping or not is complicate..
The 'CM' option (=use color mapping on this channel) is not blender specific. It's an 'universal' question.
Afaik , you should always leave CM off for all channels, except the main render. Only correct in compositing where necessary.
So VrayBlender defaults (which are vray standalone defaults) are good.

Then you can rebuild the render from channels , which then are all linear.
You'll get proper results multiplying raw GI or raw Direct with diffuse color.
Then blend the 2 results (by addition) and you get the render (minus reflect and refract of course)

Of course in this situation you need to look at all your passes with a gamma corrected viewer (1.0 data -> shown as 2.2 for your monitor)
You must also correct the final result when saving to disk (so you can see it in normal programs , w/o a corrected viewer)

But there are other ways .. you might want to work with a complete render (with lwf) and just composite some passes over it to tweak the looks.
Then anything is fine ..imho correct by eye , you're tweaking the render not rebuilding it from nodes anyway.

Or if you want to rebuild all passes but working always with corrected images and w/o a viewer...
The problem with that is knowing which channels need gamma corrrection and avoid overcorrecting them.
Also , the compositing math (multiply or add mix) will be wrong anyway , won't match the render (which uses correct lwf)

There are a few tutorials for compositing vray passes on the web , you can see there where gamma corrections are applied.
(i must look at them again.. i can't find now one with a full node setup rebuilding all passes..expecially reflect and refract)

Last thing , Blender color correction does a difference.. at least in getting a correct lwf render (as i said in post about gamma)
It should also do the 'viewer' thing , when 'on' blender nodes will work in linear space and display as 2.2 (preview nodes and saving ldr formats)

Ok.. these are a bunch of random things , if you have questions i'll try to answer , and i'll post when i have news since i'm trying to figure out the best practice myself..

[nizu]

Re: Color Mapping in Renderpasses?

..i'm doing some tests now , so here are a few more things to consider:

-- HDR or LDR files ? if you save your passes as hdr or exr , blender nodes will assume they are all linear , even if they aren't.
If you load any LDR format (jpg, png..) blender nodes will assume it's gamma 2.2 and won't correct it.
(it's a good standard to assume hdr files are linear ..but it's also normal to save a rgb render with 2.2 gamma as hdr for simple compositing tweaks)

---Blender will apply gamma correction to his nodes even with "color manage" off.
Notice this : "diffuse channel" pass from vray is linear , so it looks darker in vfb , darker in blender image/uv editor * , But it looks normal if seen through a viewer node.
This is good , it means we can leave blender color manage off , so it does proper lwf for render (as i posted) and it also does proper compositing after , w/o needing to turn on color manage.

(*= with color manage on , diffuse channel will look normal in blender image editor too , but that's not important, since nodes viewers and output are correct.
however blender CM 'on' creates trouble with viewports and lwf render -at least in my lwf setup-)

---tonemapping : i didn't mention reinhart or other tmos , i assume linear is used if you're trying to rebuild the exact render in nodes , it's the only way.

---Physical camera : i'm not sure how it's handled ..camera exposure might not be saved in the channels? that can also explain why reconstructed render doesn't match rgb render channel.

[bdancer]

Re: Color Mapping in Renderpasses?

nizu, great explanation =)

i assume linear is used if you're trying to rebuild the exact render in nodes , it's the only way.

This is the most vital part - just want to mention once again - use only "Linear" color mapping if you want to rebuild the exact render in nodes!