«PROFESSIONAL USER GUIDE © 2007 Adobe Systems Incorporated. All rights reserved. Copyright Adobe® After Effects® CS3 User Guide for Windows® and ...»
The human visual system does not respond linearly to light. In other words, our perception of how bright a light is does not double when twice as many photons hit our eyes. Similarly, a CRT monitor’s display elements do not emit light that is twice as bright when a voltage twice as great is applied. The relationship of light intensity to signal intensity for a display device is expressed by a power function. The exponent of this power function is called gamma.
The relationship of light intensity to signal intensity for an input device is the inverse of the relationship for an output device, though the gamma values may differ for input and output devices to accommodate the difference between scene lighting and lighting of the viewing environment.
Raising any number to the power of 1 gives the original number as a result. A gamma of 1.0 is used to express the behavior of light in the natural world, outside the context of our nonlinear perceptual systems. A system with gamma of 1.0 is sometimes said to operate in linear light, whereas a system encoded with a gamma other than 1.0 to match the human visual system is said to be perceptual.
Note: Moving the midtone slider (such as the Levels effect’s Gamma control) in a color-correction histogram has the same result as modifying gamma, changing the tone response curve without moving the white point. Modifying the curve in the Curves effect also modifies tone response, but not necessarily with a gamma curve.
If you have enabled color management (by specifying a working color space), you can perform all color operations in linear light by linearizing the working color space. A linearized color space uses the same primaries and white point as the nonlinear version; the tone response curve is just made straight.
If you have not enabled color management, you can still perform blending operations using a gamma of 1.0.
AFTER EFFECTS CS3 240 User Guide The gamma value for an entire system—from capture, through production, to display in the viewing environment— is the product of the gamma values used for each of the phases in the system. This product is not always 1.0, as it would be if the operations performed for encoding exactly matched (inverted) the operations performed for decoding. One reason for a system gamma other than 1.0 is that there is often a difference between the lighting conditions in which a scene is captured and the lighting conditions in which it is viewed. (Consider that you usually watch a movie in a dim environment, but the scenes aren’t normally shot in a dim environment.) For example, the device gamma for an HD camera is approximately 1/1.9, and the device gamma for an HD display is approximately 2.2. Multiplying these values gives a system gamma of approximately 1.15, which is appropriate for the somewhat dim television viewing conditions of a typical living room. The system gamma for motion picture production is much higher (approximately 1.5–2.5) to accommodate the darker viewing environment of a movie theater. The gamma for the film negative is approximately 1/1.7, and the gamma for the projection film is approximately 3–4.
QuickTime and gamma in non-color-managed projects After Effects 7.0 and earlier used QuickTime codecs to decode several kinds of media, and the gamma adjustments performed by QuickTime on Windows were different from the gamma adjustments performed on Mac OS. The gamma adjustments performed by After Effects CS3 differ from the gamma adjustments performed by these QuickTime codecs. Gamma adjustments performed by After Effects CS3 on Windows are the same as gamma adjustments performed by After Effects CS3 on Mac OS. Also, by not using QuickTime codecs, After Effects preserves over-range values in 32-bpc projects.
Select Match Legacy After Effects QuickTime Gamma Adjustments in the Project Settings dialog box to accomplish
any of the following:
• Avoid color shifts when working with projects created in After Effects 7.0 or earlier
• Match the colors in a project created in After Effects 7.0 or earlier
• Ensure that colors in Composition panel match colors in QuickTime player The Match Legacy After Effects QuickTime Gamma Adjustments option is selected by default for projects created in After Effects 7.0 or earlier. You should create new projects without this option selected.
Note: On Mac OS running on a PowerPC processor, QuickTime codecs are used for some formats (including DV, 2vuy, and v210), whether or not you select Match Legacy After Effects QuickTime Gamma Adjustments. However, selecting this option does modify the gamma adjustments used so that they match the behavior of After Effects 7.0 and earlier.
Important: Install the most recent updates for After Effects CS3. Versions 8.0.0 and 8.0.1 of After Effects CS3 do not correctly export data for some QuickTime RGB codecs.
For current and detailed information on issues related to QuickTime Player and gamma adjustments, go to the Adobe website at www.adobe.com/go/learn_ae_quicktimegamma.
Mark Christiansen has a blog post about QuickTime and gamma:
Color management Why you should use color management To see a video tutorial on color management in After Effects, go to the Adobe website at www.adobe.com/go/vid0260.
For step-by-step instructions on using color management to create movies for Flash, HDTV, motion-picture film, and other common media, go to the Adobe website at www.adobe.com/go/learn_ae_colormanagementpaper.
Color management provides many benefits, including the following:
• The colors in imported images appear as the creators of the images intended.
• You have more control over how colors are blended within your project, for everything from motion blur to antialiasing.
• The movies that you create will look as you intend when viewed on devices other than your computer monitor.
If you don’t enable color management for your project, then the colors in your composition are dependent on the color characteristics of your monitor: the colors that you see are the colors that your monitor displays based on RGB numbers in your footage items. Because different color spaces use the same RGB numbers to represent different colors, the colors that you see and composite may not be the colors that the creator of the footage intended. In fact, the colors may be very far from the intended colors.
By setting a working color space for the project (which enables color management), you do two things:
• You define a common color space for compositing and other color operations.
• You control the appearance of colors in your composition.
If a footage item has an embedded color profile (for example, the footage item is a Photoshop PSD file), then the colors intended by the person who created the image can be accurately reproduced in your composition. The color profile contains the information that determines how to convert the RGB numbers in the image file into a deviceindependent color space; the monitor’s color profile can then be used to determine which RGB numbers in your monitor’s color space represent the colors intended for the footage item. This becomes even more important as you import footage items with many different color profiles, from many different sources.
The color conversion process takes no effort on your part. The colors simply appear on your monitor just like they appeared when the image was created. Your monitor may, of course, have a limited gamut compared to the color space that you choose for the working space, and colors can be clipped when displayed on the monitor. However, you still have the full range of color data in your project, and the colors are not clipped internally.
When you are ready to output your composition, you can use color management to transform your colors into the space appropriate for your output media. At this stage, you are preserving the appearance of colors as you intend them to look.
Be sure to read the helpful text in the Interpret Footage, Project Settings, and Output Module Settings dialog boxes.
This text helps you to understand the color conversions that will be done as you interpret footage, composite, and output rendered movies.
Color management and color profiles Color information is communicated using numbers. Because different devices use different methods to record and display color, the same numbers can be interpreted differently and appear to us as different colors. A color management system keeps track of all of these different ways of interpreting color and translates between them so that images can look the same regardless of the device used to display them.
In general, a color profile is a description of a device-specific color space in terms of the transformations required to convert its color information to a device-independent color space.
In the specific case of working within After Effects, ICC color profiles are used to convert to and from the working
color space in the following general workflow:
1 An input color profile is used to convert each footage item from its color space into the working color space. A footage item may contain an embedded input color profile, or you can assign the input color profile in the Interpret Footage dialog box or interpretation rules file.
2 After Effects performs all of its color operations in the working color space. You assign a working color space in the Project Settings dialog box.
3 Colors are converted from the working color space to the color space of your computer monitor through the monitor profile. This means that your composition will look identical on two different monitors, as long as the monitors have been properly profiled. This conversion does not change the data within the composition. You can choose whether to convert colors for your monitor using the View Use Display Color Management menu command.
4 Optionally, After Effects uses a simulation profile to show you on your computer monitor how the composition will look in its final output form on a different device. You control output simulation for each view through the View Simulate Output menu.
5 An output color profile for each output module is used to convert the rendered composition from the working color space to the color space of the output medium. You choose an output color profile in the Output Module Settings dialog box.
The file format for color profiles is standardized by the ICC (International Color Consortium), and the files that contain them usually end with the.icc filename extension. After Effects comes with a large number of color profiles for color spaces for common (and some not so common) input and output types.
For information on color profiles, see the International Color Consortium website at www.color.org.
After Effects loads color profiles from multiple locations, including the following:
• Mac OS: Library/ColorSync/Profiles
• Windows: WINDOWS\system32\spool\drivers\color When you create or install new profiles, put them in these folders. You can create a custom ICC profile using Adobe Photoshop.
When you choose a profile—for input, output, or simulation—you will not see the motion-picture film profiles unless your footage is Cineon footage or you select Show All Available profiles. If your footage is Cineon footage, you will see only the motion-picture film profiles, unless you select Show All Available Profiles.The DPX Theater Preview and DPX Standard Camera profiles provided by After Effects 7 for use with the Proof Colors command have been replaced by the Kodak 2383 and Kodak 5218 profiles used with the Simulate Output command. Proof Colors has been replaced by Output Simulation. (See “Simulate how colors will appear on a different output device” on page 248.) If you open an After Effects 7 project that uses DPX Scene and DPX Theater color profiles in the Color AFTER EFFECTS CS3 243 User Guide Profile Converter effect, After Effects CS3 will not automatically update these profiles to the new equivalent profiles (Kodak 5218/7218 Printing Density and Kodak 2383 Theater Preview). Instead, the profiles will be listed as Embedded. You can convert your project by manually assigning the new profiles in After Effects CS3. However, if the same profiles were assigned to the footage or selected in Proof Colors in After Effects 7, they will be automatically updated to the new profiles in After Effects CS3.
Note: The NTSC (1953) color profile corresponds to obsolete television equipment and should not be used. For standarddefinition NTSC television, use one of the SDTV NTSC color profiles.
Make sure that your work environment provides a consistent light level and color temperature. For example, the color characteristics of sunlight change throughout the day, which can alter the way colors appear on your screen, so keep shades closed or work in a windowless room.
All color profiles used in a project are saved in the project, so you do not need to manually transfer color profiles from one system to another to open the project on another system.
See also“Interpret footage items” on page 51
Calibrate and profile your monitor When you calibrate your monitor, you’re adjusting it so that it conforms to a known specification. After your monitor is calibrated, the profiling utility lets you save a color profile. The profile describes the color behavior of the monitor—what colors can be reproduced on the monitor and how the color values in an image must be converted so that colors are displayed accurately.
Note: Monitor performance changes and declines over time; recalibrate and profile your monitor every month or so. If you find it difficult or impossible to calibrate your monitor to a standard, it may be too old and faded.
1 Make sure that your monitor has been turned on for at least half an hour. This gives it sufficient time to warm up and produce more consistent output.
2 Make sure that your monitor is displaying millions of colors (24 bits per pixel) or higher.