lwf artifacts?

[gardeler]

After chopping through this forum for linear workflow tips and tricks for Vray i think I have figured out the right way of doing it the "true" way. (rendering to gamma 1.0 and using the sRGB button in the renderview, + the usual srgb stuff for textures.)

However, i do get the same kind of AA artifacts as described in the end of this article:
http://www.aversis.be/tutorials/vray...l_gamma_01.htm

the article concludes that if you want to render out your images as gamma 1.0 for comp, (like I want), you will need higher settings. If that is true, then it seems un-logical to use a gamma 1.0 work flow as it would mean longer rendertime? Would it make sense to render to gamma 2.2 so that the AA is working ok, and then de-gamma that before comp? Or is there a way to force the AA sampler to work like a 2.2 image on a 1.0 image?

I have not spent a lot of time trying to research render time impact and such, but on the first test I did the artifacts were clearly visible, and gone if I set it to gamma 2.2

Anybody out there that have dealt with this issue before? Or am I doing something wrong? And is this a Vray related issue, or is it happening in mental ray and renderman as well? I thought the render engine always rendered in 1.0 and it was only the viewing gamma that changed, but that does not correspond to what is happening here.

Thanks

Martin

[instinct]

I am not a maya user, but in max you'd render 1.0 and set your linear gamma in the colourmapping accordingly. By checking "Dont affect colors" you'd make the sampler sample linear without affecting the colors

Edit: http://www.spot3d.com/vray/help/maya...lormapping.htm

Regards,
Thorsten

[gardeler]

Thanks Thorsten! I did briefly try that, but got a little weird result. But that might have been in combination with something else as I was trying several things at the same time. I will try again!

Regards

Martin

[gardeler]

I did try it, and it seems like the button that would fix it, but I cant see any change...

Maybe some of my other settings are wrong? I have included some dumps of my color mapping and of the differences between post sRGB and render with gamma 2.2. Let me know if I screwed up some where...

Thanks

Martin

[gardeler]

btw. I have also tested "linear multiply" mode, but I get the same result.

I use SP1.5 in 2011 on 64bit if anybody would like to know.

Thanks

Martin

[seq]

Hi Martin,

perhaps I get you wrong, but for a LWF in VRay for Maya (not 100% correct, but working) we use the following settings:

Render to EXR (32 bit for the whole range)

Color Mapping Type: Linear Multiply
Dark & Bright Multiply: 1
Gamma: 2.2
Subpixel, Don't affect colors & linear workflow: On
Clamp output: 3

And use a physical camera (not a must, but much more like in real life).

Ciao,
Lars

[instinct]

You indeed need to set the Gamma to 2.2 to correct to linear

Regards,
Thorsten

[gardeler]

Hi Lars and Thorsten
Thank you so much for your quick replies! But now I'm even more confused.
As far as I see it in the tutorial, that would bake a 2.2 gamma curve into the rendered output?

Quote from the tutorial

"Color mapping

Color mapping provides a final way of burning in the gamma 2.2 curve. First we disable the srgb button again (the image will be very dark again).

Now in the Vray tab, open the 'color mapping' rollout. You can leave everything as is, you only have the change the gamma to 2.2.

Render the image again, and you'll see that it looks good now, without having to press the srgb button.

The big difference of this technique is that it applies the gamma 2.2 curve while rendering. The previous methods were all done after the rendering has finished (or as a preview only). This difference is pretty important, as it has serious consequences on image quality!"

This description seems logical to me, except for the part on "serious consequences on image quality".

I will go back and test again with your settings, I must be doing something wrong!

[gardeler]

Ok FINALLY! It works. And also as expected. You were both right, the gamma had to be set to 2.2 and if you don't checkbox the "don't affect colors" option, it will just bake the gamma into the image. (like the tutorial suggests) With it on, it uses it for sampling only and fixes the problems I was having with the AA, just as Thorsten was suggesting earlier. (not sure why I couldn't make it work the first time)

However, being a noob with vray, it seems a bit over complicated to achieve this? Imo it is not very logical having buttons called "linear work flow" if it is not the way that you would actually want the user to work when using a gamma 1 work flow? If what the button does is auto converting an old scene from gamma 2.2 > gamma 1, (which is a great feature btw), then maybe it should have a name that is closer to what it does. Just a thought.

Thanks again for helping me sorting this out!

Martin

[johnjoe]

Hi all,

I too have been looking more into the linear workflow for vray in maya and I am again unsure about the whole gamma setting in the color mapping options.

In the docs under Color Mapping parameters it states for Linear Workflow option:

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.

So my question.....If the above is not a replacement for proper linear workflow then....What is the "proper linear workflow" that you speak of?

Many thanks,

Joe

[gardeler]

Hi Joe
What I mean with "proper" is that you prepare textures, colors and other gamma sensitive things so they all work in a gamma 1 render. That way you will have full control over your render, and it will work perfectly with a compositing program, for instance Nuke. Nuke expects that a exr has a gamma of 1, and a tiff or tga has a baked in gamma of 2.2 and automatically applies a inverse gamma to them to get them down to 1.

Using the lwf button, I believe that vray, like nuke, tries to convert your scene as best as it can to gamma 1. that is pretty cool and easy. And I suppose it will do a good job too. My scare is that if something looks odd, it might be hard to determine where in all of this the problem might be.

So I guess to sum up:
lwf button: you setup you scene as you are used to, and you let vray take care of the convertion of 2.2 > 1.0 for you.

"proper": You are in charge of the setup your self, and makes sure that all textures and colors has the right settings. (more work, more control.)

[pechart]

My 2ct:

-I suggest don't using the "Linear Workflow" button but doing the corrections you need right in the file texture node with the corresponding vray attribute. That way you know what's going on.
-Don't do any color mapping (tone mapping) to the render, you'll do that in comp, if needed. That's one of the points WHY you render to a float format. The color mapping multiplications and stuff are there to bring your values to a value between 0 and 1. But with float you can store values above 1 without a problem. Those superbrights can be tonemapped in the compositing stage. So don't do that in the render.
-But: Set the gamma to 2.2, check "Don't affect colors". That's important, because now the thresholds for the sampling are adapted to a Gamma of 2.2. Otherwise you'll get a lot of noise in the dark areas of your images, where the sampling stops, because the threshold are to small. When you apply your sRGB curve later those areas get lit up and your noise gets visible.
-Problem with float values: the Antialiasing can get very bad. That's absolutely normal, as a value of 123 against a value of 0.5 will still make a value far beyond 1(=white) when antialiased. These are the bad edges appearing. Your digital camera does that, too, but it just filters/glows/bleeds those areas. So, either do what your camera does in comp and hide those areas with glows/flares or something or use the "Clamping" with something like 3. You'll loose your superbrights above this value, but most times that's acceptable.
-Render to a float format like vrimg or EXR, either in 32bit, or, most times accurate enough, in 16bit. Do your comp in linear space with some kind of a LUT applied to see the image in sRGB (but keep working linear) and only bake your sRGB curve to the final output (Quicktime, JPEG, etc). Nuke will do this pretty nice, After Effects can do it, too. (Not tested fully). I'm sure Fusion can.

Cheers!

[seq]

Thanks pechart for explaining my raw numbers with some sentences.

[pechart]

Just thought i'll write this down for the next people searching around.

[johnjoe]

Thank you pechart for the tips. This makes perfect sense. I had no idea about there being that Vray Attribute. This is much better.

Should be documented better.

Many thanks,

Joe