Displaying and Rendering Gamma Correction
In very simplistic terms, gamma is the measure of the contrast -- brightness of the midtone values -- produced by a device (like a computer monitor) or in a photographic image. Displaying and rendering a gamma correction is useful when you want to adjust the appearance of an image to compensate for the differences between display systems.
The greatest effect of gamma on the representation of colors is a change in overall brightness. A higher gamma value yields an overall brighter image. A change in gamma can also affect the hue of a particular color representation.
A render region shows a side-by-side comparison of a scene with a display-only gamma correction.
The same scene comparison now shows the wooden floor image clip with a gamma correction removed from its rendered output.
To successfully work with images in Softimage and accurately set the gamma correction needed for those images, you need to know the following:
• What color profile did you use to originally create your images?
• Is there already gamma in the image from previous processing (in other words, have the pixels in the image already been gamma corrected)?
• Can you identify this gamma value?
• What is your system’s gamma settings (or those of the target system where the images will be used)?
For more information about gamma correction in general, see Gamma, Linear Color Space and HDR [Softimage Wiki].
Gamma Correction Options and Tools
Softimage offers many ways of displaying and rendering gamma-corrected images. These options and tools are described in context throughout the documentation, but they are summarized below for your convenience.
Global Gamma Correction Display Preference
You can define a color management display preference that stores a global lookup table built from values specified in a LUT file or from RGB gamma values that you set on thetab of your [ ].
Viewing Global Gamma Correction
You can select which tools and controls will display the gamma correction you specified for the global preference. On the Apply To... checkbox options to enable a display-only gamma correction in render regions, shaderballs, render previews, and in most of the color control widgets throughout Softimage.tab of your [ ], set the
In addition, the display of gamma correction can be directly toggled (on and off) for all color sliders (including the gradient slider), color chips, and the various color views within the color editors. Note that turning the gamma correction on or off for a particular color widget is only turning it on or off locally for this widget. The global color management display preference setting remains as is.
See[ ] and [ ] for more information.
Gamma Correcting a Render Pass
You can render out gamma-corrected images for a pass by enabling the Apply Display Gamma Correction checkbox in the [ ] for that pass. The render pass is output using the gamma values set for the global color management display preference. The images’ pixels are modified accordingly on output.
Gamma, Color Profiles, and Image Clips
Softimage converts all image clips into linear floating point data. To accurately convert and render out each image clip, you can select the color profile that corresponds to the color space in which your original image was created. This option indicates to mental ray how to convert (if necessary) the image clip from its current color space to the linear floating point color space, including whether or not any gamma correction data is stored. For information about specifying a color profile and rendering user defined gamma correction for individual image clips, see theoptions on the Adjust tab of the [ ].
As a convenience, you can set a color profile preference that declares to mental ray the color space that you typically use to create your input images, so that by default, all newly created image clips will be converted accordingly. See theoptions in the [ ].
Gamma Correcting HDR Images
If you are working with HDR files saved in the .hdr or .exr formats, you can adjust their exposure and correct their display gamma from several places in Softimage. Seefor more information.
Gamma Correcting in the Fx Viewer
In the Fx viewer, choose View > Viewer Preferences and enable a Gamma LUT to perform a gamma correction, or the Cineon LUT to properly display images stored in logarithmic color space. See the[ ].
Gamma Correcting in the Fx Tree
In the Fx tree, you can load a number of operators that allow you to gamma correct your images:
• Color Adjust operators let you color correct clips. You can do gamma corrections using, , , , and . See the gamma example in [ ].
•[ ] is one of the Composite operators that offers comprehensive facilities for matte manipulation and color correction, including gamma correction.
•[ ] is one of the Image Operators that can perform display-only gamma correction on High Dynamic Range images.
The[ ] lets you set various preferences for Cineon/DPX image file conversion when working in the Fx tree. This includes adjusting both film and display gamma correction settings.
Gamma Correcting in the Render Tree
In the render tree, you can load and connect shaders that allow you to gamma correct your images such as theand shaders [ ].
Gamma Correcting Surface Color Maps
On the Surface Map Settings tab of the[ ], you can perform a basic gamma correction for a surface color map that you created using a rendermap property or a rendervertex property. See [ ] for more information.
Gamma Correcting with Standalones
The imf_info standalone displays information about all specified images, in a spreadsheet-like layout. Information includes the image width and height, number of color components, bit depth, gamma, line ordering, image type, and image format.
The imf_diff standalone compares two image files. The files are compared, and a comparison summary is printed. If an output image is specified, a difference image with a histogram is written to the output image. You can also perform a gamma correction on the output image.
See[ ] for more information.
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