Gamma shift refers to an undesired alteration of effective decoding gamma that results from inadvertent application of Macintosh-related gamma correction upon import or export of video involving a Macintosh computer. Gamma shift usually involves inadvertent application of a 1.45-power function or its inverse,

a 0.69-power function.

Gamma in computer graphics

Computer-generated imagery (CGI) software systems generally perform calculations for lighting, shading, depth-cueing, and antialiasing using approximations to tristimulus values, so as to model the physical mixing of light. Values stored in the framebuffer are processed by hardware lookup tables on the fly on their way to the display. If linear-light values are stored in the framebuffer, the LUTs can accomplish gamma-correction. The power function at the CRT acts on the gammacorrected signal voltages to display the correct luminance values at the face of the screen. Software systems usually provide a default gamma value and some method to change the default.

The BT.709 function is suitable for originating image data at high light levels (2000 lx or more) intended for viewing at about 100 nt in a dim surround. For other origination or viewing environments, see the comments on page 118.

The framebuffer’s LUTs enable software to perform tricks to manipulate the appearance of the image data without changing the image data itself. To allow the user to make use of features such as accurate colour reproduction, applications should access lookup tables in the structured ways that are provided by the graphics system, and not by direct manipulation of the LUTs.

Gamma in pseudocolour

In Pseudocolour, on page 70, I described how the colour lookup table (CLUT) in a pseudocolour system contains values that are directly mapped to voltage at the display. It is conventional for a pseudocolour application program to provide, to a graphics system, R’G’B’ colour values that are already gamma corrected for

a typical monitor and typical viewing conditions.

A pseudocolour image stored in a file is accompanied

by a colourmap whose R’G’B’ values are intended to be subject to an EOCF approximating a 2.4-power function at display.

Limitations of 8-bit linear coding

As mentioned in Gamma in computer graphics, on page 332, computer graphics systems that render synthetic imagery usually perform computations in the linear-light – or loosely, “intensity” – domain. Low-end graphics accelerators historically performed Gouraud shading in the linear-light domain, and stored 8-bit components in the framebuffer. In The “code 100” problem and nonlinear image coding, on page 31,

I explained that linear-light representation cannot achieve high-quality images with just 8 bits per component; such images typically exhibit contouring. The visibility of contouring is enhanced by a perceptual effect called Mach bands; consequently, the contouring artifact is sometimes called banding.

High-end systems for computer-generated imagery (CGI) typically operate in the linear-light

(“gamma = 1.0”) domain using more than 8 bits per component (often floating point). Some systems perform gamma correction in software, then write gamma-corrected values into a limited-depth framebuffer. Other systems have “deep colour” framebuffers (having components with more than 8 bits, and often 16 bits); a unity ramp is loaded into the LUT of the framebuffer. This arrangement maximizes perceptual performance, and produces rendered imagery without the quantization artifacts of 8-bit linear-light coding.

Linear and nonlinear coding in CGI

Computer graphic standards often make no explicit mention of transfer function. Often, linear-light coding is implicit. However, in the JPEG standard there is no mention of transfer function but nonlinear (video-like) coding is implicit: Unacceptable results are obtained when JPEG is applied to linear-light data. All of these standards deal with RGB quantities; you might consider their RGB values to be comparable, but they’re not!

Luma and

This chapter describes colour coding systems that are used to convey image data derived from additive RGB primaries. I outline nonlinear R’G’B’, explain the formation of luma, denoted Y’, as a weighted sum of these nonlinear signals, and introduce the colour difference (chroma) components [B’-Y’, R’-Y’], [CB, CR], and

[PB, PR].

The design of a video coding system is necessarily rooted in detailed knowledge of human colour perception. However, once this knowledge is embodied in

a coding system, what remains is physics, mathematics, and signal processing. This chapter concerns only the latter domains.

Colour acuity

A monochrome video system ideally senses relative luminance, described on page 256. Luminance is then transformed by the gamma correction circuitry of the camera, as described in Gamma in video, on page 318, into a signal that takes into account the properties of lightness perception. At the receiver, the display – historically, the CRT itself – imposes the required inverse transfer function.

A colour image is sensed in three components, red, green, and blue, as described in Additive reproduction (RGB), on page 288. To minimize the visibility of noise or quantization, the RGB components should be coded nonlinearly.

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Figure 28.1 RGB and R’G’B’ cubes. RGB components form the coordinates of a three-dimensional colour space; coordinate values between 0 and 1 define the unit cube. Linear coding, sketched at the top, has poor perceptual performance when 8 or even 10 bits are used for each component. In video, RGB components are subject to gamma correction to impose perceptual uniformity.