Contrast Loss in LINEAR WORKFLOW

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  09 September 2012
Contrast Loss in LINEAR WORKFLOW

While doing a test for Vray in Maya for linear workflow, I noticed that there is a lot of loss in the contrast when you add a sRGB LUT to the viewer.

When try to bring back the contrast in NUKE (with a grade node), the result seems unsatisfactory. Even the AO pass seems to brighten up excessively.
Wanted to know few workarounds for this issue. I mean to get the contrast back.
One thing I figured out was to copy the same image and desaturate it and overlay it to the original, seems to get back a little contrast. Or to use an AO pass and mult it over.

Here is the TEST image I am working on:-

(The swatch is corrected to LINEAR WORK FLOW)
  09 September 2012
The linear image looks correct for the matte surface you have. If you're trying to achieve a darker feel, trying working on the ratio between specular and diffuse reflection. Also make sure you're using either 16 bit or 32 bit images before you're converting the LUT. Also make sure to ungamma any textures or swatches with a .455 gamma.

  09 September 2012
hey kanooshka,

I felt that my matte is wrong as well.
Here are my setting for Vray

I read that in V-Ray (un-like mental ray) if you select affect swatches, it takes care of the gamma part of adding .454 (i can be wrong of course, because I am still learning V-Ray).

I think I will try Reflection like you said and see if things darken a bit more.

Thnx a lot for the help !!
  09 September 2012
color mapping

Hey there, here's the workflow I use:

In color mapping, type is 'linear multiply', and set the gamma slider to 2.2

Check affect background, subpixel mapping, and don't affect colors. do NOT check linear worklow.

In your vray frame buffer you can toggle on the sRGB, this will let you see it in the correct gamma.

I'd render to a 16 bit exr.

In vray you don't have to worry about the .454 tricks anymore, having the settings under color mapping will take care of all of that for you.
  09 September 2012

One other thing, a good way to keep all your textures kosher, I'd open them in nuke, regardless of their format, and save them as an .exr. Being an exr, maya/vray will take them down to .454, and you can even see this in your texture swatches. exr in, exr out, it'll simplify your life
  09 September 2012
danny - the right one is "physically" correct. And the good thing is, you can always adjust it later to get the effects you want. Linear images (exr, hdr) stores a lot more information than, say, jpg or png. So you can darken or brighten them much more.
  09 September 2012
Hey dannyboy374, thnx for the tip.

CG TALK is awesome for this, I get tips that I don't find in tutorials !! will do that .exr conversion in NUKE...saves time. However Since I started texturing in Mari, I can output exr directly from it, so no worries.

Anyways, When I un-check the LINEAR WORKFLOW button, I get extremely "blown out" results, on a simple "matte grey" v-ray Shader.

I did post a thread about LINEAR WORKFLOW switch on Chaos Group, but didn't get any help so far.

Here's the link:-
  09 September 2012
I know why we check Affect bg, Don't affect colors, affect swatches but why do we check sub-pixel mapping?

I tried switching off and on, the LINEAR workflow button and got these result

And the "affect swatches" switch, seems to add 2.2 to the shader if checked on and keeps in LINEAR if un-checked...!! I can be wrong but here is an image, just to be more clear to what I mean
However it doesn't seem to make any difference in the final render, I rendered with it on and off, but the render remained same. the render is affected only when I turn the L.W.F switch on & off.

Here is a little HI-REZ image with settings . (the images are directly screen grabbed from Maya) -

THIS is what is given on V-Ray for Maya's manual in Spot3D:-

Linear workflow - when this option is checked V-Ray will automatically apply the inverse of the Gamma correction that you have set in the Gamma field to all VRayMtl materials in your scene. "Note that this option is intended to be used only for quickly converting old scenes which are not set up with proper linear workflow in mind. This option is not a replacement for proper linear workflow."

Last edited by govindk : 09 September 2012 at 07:32 PM.
  09 September 2012
Turn off the linear workflow option, set the gamma back to 1, render to EXR and then turn on the sRGB button in the VRay render buffer.

In Nuke, leave the colourspace setting on the read node as linear, set the viewer lut to sRGB. If you compare the Vray render buffer and the Nuke viewer both should match.

You don't say what your output format is. But assuming you just want a JPG image sequence, all you need to do is set the colourspace on the write node to sRGB and hit render.

I avoid gamma nodes in Maya and pre-linearize my textures in Nuke.

I hope that helps. Best to keep things simple when working in linear.
  09 September 2012
My out format is going to be exr...!! Used to that from Mental Ray Linear Workflow

I would love to keep it simple, however, different tutorials says different things.
I wanted one that would match, so I can stick to it.

Some say switch on LWF switch with 2.2 gamma (v-ray will automatically inverse everything to .454)..!!

Then I read Spot3D and came to know that LWF switch is merely there for legacy purposes.

Expereinced artist like Robert Nederhorst, check's the LWF switch with Gamma 2.2 and seems to work fine. (in his Archetype tutorial in Maya from Gnomon).

I understood what Linear Workflow is, but how to do it inside V-Ray properly is what I am trying to figure out.


I made a mistake, R.Nederhorst on Archetype does not use LINEAR WORKFLOW button, got confused with his V-Ray Primer documented in which he has turned on LINEAR WORKFLOW button

Last edited by govindk : 09 September 2012 at 05:54 AM.
  09 September 2012
Well, legacy or not, if you know what it's doing, then it's in your hand to exploit it I leave mine off and add Vray gamma attribute to textures myself.

My usual settings are

Adaptive DMC
gamma 2.2
tick - don't affect colours.

You can also tick "affect backgrounds" if you like, but the other 2 I usually leave them off. The reason to do it this way, if you noticed, vray doesn't clear the noises in darker area much. It pays much more attention to the brighter area. Now, if you look at "Don't affect colours" you'll also see (adaptation only.) That means, Vray will use the brighter image only to compute noises. You can really notice the difference if you have soft shadows in your render.
  09 September 2012
I use the settings you must mentioned, however I used to use it with LWF switch on.

Now I don't and I apply a 0.454 gamma node to my color swatches (of no textures), and it seemed fine.

  09 September 2012
You don't need gamma node if you're using Vray.

Simply select your file node, open attribute editor (ctrl+A if it isn't already opened)
choose Attributes > Vray > tick "Texture input gamma"
  09 September 2012
Originally Posted by Panupat: You don't need gamma node if you're using Vray.

Simply select your file node, open attribute editor (ctrl+A if it isn't already opened)
choose Attributes > Vray > tick "Texture input gamma"

Even for plain swatches?!
  09 September 2012
Setting the gamma to 2.2 in the Colour Mapping options* is only for when your outputting to a non-linear format such as JPG. If your rendering to EXR then you want to leave it at 1.

*Exception given to having the "don't affect colours" option on - which doesn't apply any colour mapping to the final image. But I don't use that option and I don't know enough about what it's doing to comment on it.

If you leave the gamma at 2.2 and bring it into Nuke as a linear EXR then all you are doing is doubling up your colour transforms (which is bad for the image) and your confusing what should be a very simple workflow.

Here is a simple setup between Vray and Nuke using a constant shader set to 0.18 (mid-grey). Both the Vray and Nuke viewers are set to an sRGB lut and I'm rendering to an linear EXR. It's simple, easy to understand and is technically correct. If you pre-linearize or add the extra attributes to your file nodes - as Panupat suggested - in order to linearize the texture during rendering then you don't need any gamma nodes (or mention of gamma) in your scene whatsoever.

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