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

A render region shows a side-by-side comparison of a scene with a display-only gamma correction.

B

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 the Color Management tab of your Display Preferences [Preference Reference].

Viewing Global Gamma Correction

You can select which tools and controls will display the gamma correction you specified for the global preference. On the Color Management tab of your Display Preferences [Preference Reference], set 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.

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 Defining Color Properties [Interface and Tools] and Creating Gradients [Texturing] 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 Render Pass Property Editor [Properties Reference] 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 the Color Profile options on the Adjust tab of the Image Clip Property Editor [Properties Reference].

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 the Color Profile options in the Rendering Preferences [Preference Reference].

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. See Adjusting HDR Image Exposure and Display Gamma for 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 FX Viewer Settings Dialog Box [Properties Reference].

 

The global gamma correction display preference is not used when viewing in The Fx Viewer [Compositing and Effects].

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 Color Correct, HSV Adjust, Lin2Log, Log2Lin, Component Parser and Pixel Parser. See the gamma example in Using Parser Expressions [Fx Operator Reference].

Keyer [Fx Operator Reference] is one of the Composite operators that offers comprehensive facilities for matte manipulation and color correction, including gamma correction.

File Input [Fx Operator Reference] is one of the Image Operators that can perform display-only gamma correction on High Dynamic Range images.

The Compositing Preferences Property Editor [Preference Reference] 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.

 

The global gamma correction display preference is not used when viewing in The FxTree [Compositing and Effects].

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 the Color Correction and Simple Tone Mapping (mia) shaders [Shader Reference].

Gamma Correcting Surface Color Maps

On the Surface Map Settings tab of the RenderMap Property Editor [Properties Reference], you can perform a basic gamma correction for a surface color map that you created using a rendermap property or a rendervertex property. See RenderMap Property Editor [Properties Reference] 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 Getting Image File Information [Standalones] for more information.



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