【转】Using Gamma 2.2

This is a detailed description of the work with Gamma 2.2. If you are only interested in exact instructions of how to use Gamma please look at "Gamma 2.2 Tip" in the Tips'n'Tricks section.

 

Many artists, especially those who faced the rendering of the interiors noticed that with physically correct formulation of the lights there is an overall dark-fetched result in illumination. That is noticeable in the corners and on the shadow side of objects especially.

Everyone tried to deal with this issue in different ways. Beginners immediately tried to correct this by simply increasing the brightness of lights.

This approach brings some results increasing the overall illumination. However, it also led to no less unlikely overbrights made by these light sources. This does not alter the situation with unrealistic image. One bug of darkness (in the hard-to-reach for light places) is replaced by another bug of overbrights (near the light sources).

Someone used more complex ways to "solve" the problem by adding extra lights, and making them not visible to the camera so dark places were simply illuminated. However, with this way no physical accuracy and realism of the image could not be considered. Along with lighting of the dark places, shadows disappeared and was the impression that the scene objects are flying in the air.

All of the above ways of dealing with implausible darkness rather more literate than subtle :)

The heart of the problem of dark renders is in that the image and monitor gamma values are different.

Gamma is the degree of nonlinearity of the color gradient from dark to bright values. In mathematical point of view the linear gamma value is 1.0 and this is why software such as Max, V-Ray by default performs calculations in the gamma 1.0. But the gamma 1.0 value is consistent only with the «perfect» monitor, which has a linear dependence of the display from white to black. Because there is no such monitors, the actual gamma of devices is nonlinear.

The gamma value for the video standard NTSC is 2.2. For computer displays the gamma value is typically between 1.5 and 2.0. But for convenience the nonlinearity of the color gradient on all screens is considered as 2.2.

When the monitor with gamma 2.2 shows an image with a gamma 1.0 we get dark gamma 1.0 colors instead of needed gamma 2.2 bright ones. So the middle-range colors (Zone 2) become dark when viewing the gamma 1.0 image with the gamma 2.2 output device. However, in range of dark tones (Zone 1), gamma 1.0 and 2.2 representation is quite similar, what lets to display the shadows and black colors properly.

In the areas of light tones (Zone 3) there also are the great similarities. Therefore, bright gamma 1.0 image is also quite correctly displayed on the gamma 2.2 monitor.

And so, in order to get at the appropriate output in Gamma 2.2 the source image gamma should be modified. Surely this can be done in Photoshop, simply by adjusting gamma there. But each time to change the image settings, saving them to your hard disk, and editing in raster editor can hardly be called convenient. Because of that we will not consider this option, and in addition this method provides an even more significant deficiencies. Modern renderers, such as V-Ray, calculate the image adaptively, so the calculation accuracy depends on many parameters, including the light brightness in the area. Therefore, in the shadow areas image is calculated less accurately and become noisy. And in the bright and visible areas the calculation passes with more accuracy and with minimal artifacts. This allows faster renders due to the saving time on a slightly visible areas. By raising the output image gamma in Photoshop you change the brightness of those parts, which renderer considered as a less significant and reduced the quality of a calculation. Thus, all unwanted artifacts become prominent, and the picture will look awful, but more bright than before :) In addition the gamma of textures also will change and they will look pale and discolored.

The only correct way out of this situation is the changing the gamma value in which the renderer is working. That way you will get an acceptable brightness in the midtones and there will no obvious artifacts, as when changing gamma in raster editor.

We will show you how this is done in V-Ray renderer and 3ds Max.

To change the gamma in which the renderer will work with it's enough to find drop-down tab named "V-Rray: Color mapping" in the "V-Ray" tab on the "Render Scene:" (F10) window, and set the value "Gamma:" to 2.2.

A feature of V-Ray renderer is that the color mapping gamma correction works in the V-Ray Frame buffer only, so if you want to see the results of your manipulation with gamma it is necessary to turn on frame buffer on "V-Ray: Frame buffer" in the "V-Ray" tab.

After this the resulting rendered image will be with needed gamma 2.2, with a normal lit midtones. Yet another disadvantage is that the textures which are used in the scene will be lighter and will look discolored and faded.

Almost all textures we use have a normal appearance on monitor. That is because of they are already adjusted by the monitor and have a range of 2.2 initially. In order the renderer to configure a gamma 2.2, and to not to set image gamma at 2,2 × 2,2 value, textures must be in the gamma 1.0. Then, after their correction by renderer their gamma will become 2.2.

You can make all textures darker, by setting their gamma from 2.2  to 1.0 in raster editor, counting on further lightening by renderer. However, that approach would be very tedious and will require time and patience to ensure that every texture in the scene are in 1.0 gamma, and secondly it will make impossible the viewing the textures in the normal gamma because they will be shaded at that time. 
To avoid this, just force them to adjust the 3ds Max input. Fortunately this 3d editor has enough settings as for the gamma. Gamma settings available from 3ds Max main menu:

Customize <=> Preferences ...<=> Gamma and LUT

The main 3ds Max gamma settings are in the "Gamma and LUT" tab. In particular, we need input texture correction setting named "Input Gamma". We should not delude ourselves that there is a default value of 1.0. This is not the adjustment value, this is an input texture gamma value. By default it is considered that all the textures is composed in a gamma 1.0, but in reality as previously mentioned they are at gamma 2.2. And that what we must specify the 2.2 instead of 1.0 default value.

Don't forget to enable "Enable Gamma / LUT Correction" checkbox to access the gamma settings.

Images made with that gamma settings look much better and more correct than those that were obtained by using the settings described a little earlier. They have correct midtones, there are no overbrights near the lights and no artifacts in slightly lit areas. In that way textures will also be saturated and bright.

It seems that's all, but finally we would like to tell about one more thing in the work with gamma. Since the renderer operates in a unusual gamma we got to set 3ds Max display mode to gamma 2.2 for "Material Editor" and "Color Selector" colors to become correct. Otherwise there may be a confusion, as the apparent tuning of materials will be produced in gamma 1.0, but actually inside the program it will be transformed into gamma 2.2.

To set the correct display of materials in the "Material Editor", you should use the settings in the "Gamma and LUT" tab. For this the 2.2 value of Gamma in "Display" segment and checkboxes "Affect Color Selectors" and "Affect Material Editor" in a "Materials and Colors" segment must be set.

Gamma 2.2 has become the standard for many 3d professionals working with 3ds Max and V-Ray. We hope that this lesson will help you to correctly configure your workflow in 3d!

NOTE

For the theoretical correctness the expressions "in gamma" or "at gamma" should be read as "for gamma". So, the expression "image in gamma 2.2" or "image at gamma 2.2" means nothing but "image for presentation by a system with a decoding value of γ equal 2.2".

Check out detailed Gamma 2.2 correction theory, clearly explained by maxattivo.

【转】http://renderstuff.com/using-gamma-2-2-cg-tutorial/