Lookup-table (LUT) Correction

Lookup-table (LUT) correction provides compatibility with the same feature used in other Autodesk Media & Entertainment products such as Combustion and the Vfx suite: Inferno, Flint, Smoke, and so on. This feature allows studios to implement a consistent way of displaying colors, assuming that their monitors are calibrated to the same reference. Thus 3D artists can produce results that are closer to what the compositor expects by eliminating a variable in the equation: the way colors are displayed on screen.

Also, please note that the LUT system of the systems suite is complex and offers a variety of controls and flavors of lookup tables. This feature integrates the View LUTs only, such as that found in Combustion, where only the displayed images are modified.

Gamma Correction

Gamma correction compensates for the differences in color display on different output devices, so that images look the same when viewed on different monitors, or are used as bitmaps or in printed matter.

The calculation of “gamma” is output_intensity = input_intensity ^(1/gamma) . That is, the result is the original value raised to the power of the inverse of the gamma value. One result of this calculation is that a gamma value of 1.0 does not adjust the image at all.

Gamma = 1.0: no correction

I stands for Intensity.

Another result of the gamma calculation is that black is not affected by the adjustment, and neither is white (or any fully saturated color). Gamma adjustment affects only the midtones of an image.

Left: gamma = 1.8

Right: gamma = 2.2, the standard

The horizontal axis represents input (the original value) and the vertical axis represents output (the gamma-corrected value).

Computer monitors don’t display color in a linear way (as in the first illustration). Also, the brightness of a monitor tends to make an image seem brighter than its intensity values really specify. Gamma correction fixes this problem, and can ensure consistency between different applications or different monitors. When you set gamma, find a value that makes middle gray on your own monitor match a true middle (50 percent) gray.

Changing gamma value to match a monitor's middle gray

The standard gamma value is 2.2. This is the theoretically correct value, making the linear color space stored in the bitmap and used by the renderer appear to be linear on screen.

However, because the response of photographic film isn’t linear either, some users find that this theoretically correct value looks too bright and washed out. A common compromise is to render to a gamma of 1.7 or 1.8, making things look more photographic; that is, as if the image had been shot on photographic film and then developed.

We recommend that you always turn on gamma correction when you share bitmaps between different computers, between different applications on the same computer, or create renderings to use in printed matter.

Two common mistakes have to do with gamma correction:

• Not applying gamma correction when it should be used. This results in renderings or bitmaps that are too dark.
• Inadvertantly applying gamma correction twice. This results in renderings or bitmaps that are too bright.

See the Procedures section below for more detailed instructions.

Gamma and LUT Settings Mismatch Dialog

Gamma And LUT correction settings are saved with the market defaults for the current custom UI. See Custom UI and Defaults Switcher. If you open or merge a scene whose gamma or LUT settings differ from the settings active for 3ds Max, you see a dialog that gives you the choice of using the current settings, or adopting those of the file you are loading. For example:

Procedures

To set the gamma for renderings you will use as bitmaps or in printed matter:

1. Choose Customize Preferences.

2. Turn on Enable Gamma/LUT Correction.

3. In the Display group at the left-hand side of the panel, choose Gamma.

4. Use the Gamma spinner to adjust the Gamma value until the gray square in the center of the display shows no contrast with the surrounding border.

   

   Above: A bad gamma value

   Below: A good gamma value

   The value you choose can vary, depending on your system's monitor.

5. Once you have a good gamma value, change Output Gamma in the Bitmap Files group on the right-hand side of the tab to the same value as Display gamma.
6. Click OK.

To set the gamma for incoming bitmaps generated (or edited) by other applications:

1. Find out whether the other application has its own gamma correction.

2. If it does, choose Customize Preferences Gamma And LUT. Turn on Enable Gamma/LUT Correction, and choose an Input Gamma value that matches the applications.

   If the file type (for example, Targa) has an embedded gamma value, you might need to set Input Gamma to the inverse (1/gamma) of the file’s embedded gamma, to avoid applying gamma correction twice.

3. If it does not, choose Customize Preferences Gamma And LUT. Turn on Enable Gamma/LUT Correction, then in the Bitmap Files group, change the Input Gamma to a suitable value.

   

   Input gamma set to a value suitable for a different application

   It might take some experimentation to find the gamma value that best adjusts the bitmaps created by the other application. The bitmap should appear the way it does in the other application: if it appears too light or too dark in 3ds Max, try a different Input Gamma value.

   ImportantIf you use bitmaps from multiple applications, and these different applications have different ideal Input Gamma values, then turn on gamma correction, but leave Input Gamma set to 1.0. Instead, for each bitmap you read, use the individual Select Bitmap Image File dialog to set the gamma value appropriate to the program that created the bitmap.

Example: To use gamma correction with Adobe Photoshop:

1. In Photoshop, make sure Edit Color Settings is set to North America General Purpose Defaults, with Working Space RGB set to sRGB IEC61966-2.1.

   This is the default color correction in Photoshop.

2. Also in the Photoshop Color Settings dialog, change Color Management Policies RGB to Convert To Working RGB.
3. In Photoshop, click OK and then save the bitmap file.
4. In 3ds Max, choose Customize Preferences Gamma And LUT.
5. On the Gamma And LUT tab, turn on Enable Gamma/LUT Correction. In the Display group, choose Gamma, then set the gamma value to 2.2.
6. In the Materials And Colors group, turn on both Affect Color Selectors and Affect Material Editor.
7. In the Bitmap Files group, set both Input Gamma and Output Gamma to 2.2.
8. Click OK to close the Preferences dialog.

   Now when you use a bitmap saved by Photoshop, the colors in 3ds Max should preserve the bitmap’s original colors.

To use gamma with video hardware:

Video devices such as video tape recorders usually have their own hardware gamma-correction circuitry.

• If your video device has its own hardware gamma correction, do not turn on gamma correction in 3ds Max.
• If your video device does not perform gamma correction automatically, follow the preceding steps for setting Input Gamma and Output Gamma, using values approprate to your video device. Typically these will be Input Gamma = 0.45 and Output Gamma = 2.2.
  

  Typical input and output gamma settings for video that has no hardware gamma correction

Interface

Enable Gamma/LUT Correction

Makes available the controls for adjusting gamma or LUT correction. Turn off to disable gamma/LUT correction. Default=off.