Question about linear workflow & gamma space

[scarriglio]

I am relatively new to VRay. The first thing I did after buying our first license of VRay was buy Christopher P Nichols' VRay Training DVDs (Exterior & Interior), and it help us get up and running within days! Thank you.

The main issue I have, after having some success with VRay on our last couple of projects, is the issue of linear workflow and gamma space.

I went over Christopher P Nichols' explanation on the DVDs, tried to find other ressources online, and I feel I understand the concept quite well now. However, when I use the technique of applying the gamma correction to the final image (whether burning it in or applying it post-prod.), the image always becomes washed out, as if there was a light grey film over the image. I even looked at a tutorial from the Chaos Group called "Rendering an Interior Scene" and at the end, after having output the final image, he applies the gamma correction and his image has that same washed out look.

The point of this rambling on is to get input on the topic. I feel that using the gamma correcting technique consistently gives us the same washed out look (it then requires a lot of post-production to make the image look good).

Meanwhile I see many many samples of beautiful images created with VRay that do not have that washed out look. The images are crisp and clean. I wonder if these images went through the same process or not.

Does anyone have any advice to share? Maybe we are missing a critical part of the process.

[alikashan]

http://www.cgarchitect.com/vb/17061-gamma-correction.html?highlight=gamma+correction

mayy b this help and just do a quick search on this topic on the form you will find plenty of stuff....

[Christopher Nichols]

Just a few things to keep in mind when you are looking at them, they look washed out. When I look at the image, it looks good, and it is not really a matter of choice on my end. The difference may be that I am looking at the image on my monitor and you are looking at it on yours. My monitor is professionally calibrated. SO... your monitor may not be calibrated correctly. And that is a key factor before you start dealing with color. I would do that first. The best (free) software monitor calibration I have found is this:

http://www.praxisoft.com/pages/support.downloads.html#wizi

 

If you want to spend a little coin and get yourself a pro hardware system... I like this one:

http://www.pantone.com/products/products.asp?idArea=2&showNav=123&idProduct=105

 

That is step one...

 

Next is to think about the shaders and colors you are using. In order for them to work for in linear space, you need to make sure that 3dsmax sees them as linear space. You can make 3dsmax automatically add the gamma to them, or you can paint in linear (ideal but a little more challenging). Next, make sure the colors you pick in the material editor are right. Remember that 50% gray is not 128 but 49 because of the gamma curve. What does this mean? This means that all your shaders will be naturally (and correctly) darker, because of this, they will bounce less light around, and because of that, the renders will render al lot faster, and be more contrasted, compared to the washed out look you complain about.

 

If you don't use linear space, Vray will not calculate your colors correctly, you compensate (unknowingly) by making your shaders brighter and your lights brighter. Because of that, your renders take longer, and your shadows are too saturated, which people "fix" by reducing the staturation in the GI... all of which leads to very very incorrect lighting. Looks ok, but all the math is wrong. Basically it is like you have really bad "vision" and you correct it by using three pairs of glasses, when all you need is one pair used correctly.

 

Color space is a complicated thing to understand, and "get." But ultimatly it makes your life easier, and leads to faster and more accurate renderings and less guess work.

[scarriglio]

My monitor is professionally calibrated. SO... your monitor may not be calibrated correctly. And that is a key factor before you start dealing with color.

That is a good point. We do calibrate our monitors to 2.2 Gamma, using Eye-One Display2 from GretagMacbeth. However, we are using LCD monitors,and I remember reading that LCDs should be calibrated in linear space instead of gamma. Is that typically an issue with LCDs?

 

Next is to think about the shaders and colors you are using. In order for them to work for in linear space, you need to make sure that 3dsmax sees them as linear space. You can make 3dsmax automatically add the gamma to them, or you can paint in linear (ideal but a little more challenging). Next, make sure the colors you pick in the material editor are right. Remember that 50% gray is not 128 but 49 because of the gamma curve. What does this mean? This means that all your shaders will be naturally (and correctly) darker, because of this, they will bounce less light around, and because of that, the renders will render al lot faster, and be more contrasted, compared to the washed out look you complain about.

I believe we are doing this. You explained it quite well on your DVDs, and I was able to understand it after watching it a few times.

How do you set-up 3dsmax to automatically add the gamma? We have been painting in linear using your method on the DVD of having a color only version of the material, switching gamma on & off in 3dsmax and going into Photshop and darkening the bitmaps accordingly, but it is very tedious and creates mainy extra versions of the bitmaps.

[Christopher Nichols]

That is a good point. We do calibrate our monitors to 2.2 Gamma, using Eye-One Display2 from GretagMacbeth. However, we are using LCD monitors,and I remember reading that LCDs should be calibrated in linear space instead of gamma. Is that typically an issue with LCDs?

 

True, but we also use an sRGB curve and also make sure our monitors are set to 6500k.

 

How do you set-up 3dsmax to automatically add the gamma?

 

Under Customize -> Preferences -> Gamma and LUT

Set the gamma to 2.2, select color selector and material editor, and set the Input to 2.2 and Output to 1.0

[scarriglio]

True, but we also use an sRGB curve and also make sure our monitors are set to 6500k.

What do you mean by "using an sRGB curve" though. Are you talking about the curve adjustment you can make in the VFB, the sRGB color setting in PShop, or something else?

I did re-calibrated my monitor to make sure it was set to 6500k. In fact it was set to 7000k. It did improve display a bit.

 

Under Customize -> Preferences -> Gamma and LUT

Set the gamma to 2.2, select color selector and material editor, and set the Input to 2.2 and Output to 1.0

I had the Input Gamma set to 1.0. What will changing it to 2.2 do exactly?

 

Thanks for your help so far. I feel I'm getting a bit closer to a good result!

[Christopher Nichols]

What do you mean by "using an sRGB curve" though. Are you talking about the curve adjustment you can make in the VFB, the sRGB color setting in PShop, or something else?

I did re-calibrated my monitor to make sure it was set to 6500k. In fact it was set to 7000k. It did improve display a bit.

 

 

I had the Input Gamma set to 1.0. What will changing it to 2.2 do exactly?

 

Thanks for your help so far. I feel I'm getting a bit closer to a good result!

 

OK great... already improving. yes... the curve thing that I did in VFB is a hack to simulate an sRGB curve pretty closely. in Vray 1.5 there is a button on the VFB that just turns it on for you. No need for that curve.

 

Next, Input set to 2.2... that means that it takes into account that all input images are painted 2.2 gamma, so there is no need to apply a 0.4545 gamma out of photoshop when saving. Max will do that for you when it reads it. Next is the color picker. You will notice with that 2.2 input, that the color picker have a big gamma curve on it. And while in the past you needed to pick 128 to get 50% gray, now 49 will be correct. That forces you to select colors that are much darker in value, so now things will get darker and bounce less light around, and finally be more contrasted. One last note... if you are using and HDRI, look at it in the material editor, if it looks washed out, it probably is in the wrong color space. Open it in Photoshop CS2, it should also be washed out there. Apply a 2.2 gamma correction on it (which will do an inverse gamma) and resave it.

 

Hope that helps.

[scarriglio]

Thanks. I will try out these settings and see how it goes.

One question about applying gamma correction to bitmaps I had previously darken & HDRIs. Can you do it out of PShop? Is it adjusting the levels or curves in a specific way? I cannot find any settings to adjust the gamma in PShop. I have PShop 7.0 & CS, not CS2.

[Christopher Nichols]

Thanks. I will try out these settings and see how it goes.

One question about applying gamma correction to bitmaps I had previously darken & HDRIs. Can you do it out of PShop? Is it adjusting the levels or curves in a specific way? I cannot find any settings to adjust the gamma in PShop. I have PShop 7.0 & CS, not CS2.

 

Yeah you need CS2... that is the only one that can do HDRIs

 

Try using HDRShop v1 (the free one). You can open it there and apply the corrrect gamma. Darkening it will not take care of the incorrect gamma (if there is one), as you need your mid tones pushed down, not the whole image.

 

http://gl.ict.usc.edu/HDRShop/