One one-hundredth of the range from blanking to reference white was historically referred to as an IRE unit, for the Institute of Radio Engineers, the predecessor of today’s IEEE. Now it is best to say units. The mapping from units to 8-bit digital video interface code is this:

V = 16+ 219 units

709 100

Taking black as code 16 makes interface design easy, but makes signal arithmetic design more difficult.

Processing coding

In signal processing, it is often convenient (and sometimes necessary) to use a coding that represents reference black at zero independent of coding range. To accommodate footroom, the number representation must allow negative numbers. In describing signal processing at an abstract level – or implementing signal processing in floating point arithmetic – it is simplest to use the range 0 to 1. The reference points 0 and 1 are taken to be reference black and reference white. (The range is also referred to as units, where there are 100 units from reference black to reference white.) To accommodate signals in the headroom region, the number representation must allow numbers greater than unity, and in the footroom region, less than zero.

In processing hardware, a sample is ordinarily represented as a fixed-point integer with a limited number of bits. It is usually most convenient to use two’s complement arithmetic. The bit depth required in processing is usually greater than that required at an interface. Black will ordinarily be coded at 0. Reference white will be coded to an appropriate value such as 219 in an 8-bit system or 876 in a 10-bit system.

In signal processing, even without the interface offset, it may be necessary to handle negative numbers. Two’s complement binary representation is common.

R’G’B’ or Y’CBCR components of 8 bits each suffice for distribution of consumer video. However, if a video signal must be processed many times, say for inclusion in a multiple-layer composited image, then roundoff errors are liable to accumulate. To avoid roundoff error, studio video data typically carries 10 bits each of Y’CBCR. Ten-bit studio interfaces have the reference levels of Figures 4.4 and 4.5 multiplied by 4: The extra two bits are appended as least-significant bits to provide increased precision. Within processing equipment, intermediate results may need to be maintained to 12, 14, or even 16 bits.

Figure 4.4 showed a quantizer for a unipolar signal such as luma. CB and CR signals are bipolar, ranging positive and negative. For CB and CR it is standard to

MIDTREAD

I use the subscript h to denote a hexadecimal (base 16) integer.

use a mid-tread quantizer, such as the one graphed in Figure 4.5, so that zero chroma has an exact representation. For processing, a signed representation is necessary; at a studio video interface, it is standard to scale 8-bit colour difference components to an excursion of 224, and add an offset of +128. (Note that chroma occupies five more 8-bit codes than luma.)

Two’s complement wrap-around

Modern computers use binary number representation. Signed integer arithmetic is implemented using two’s complement representation. When the result of an arithmetic operation such as addition or subtraction overflows the fixed bit depth available, two’s complement arithmetic wraps around. For example, in 16-bit two’s complement arithmetic, taking the largest positive number, 32,767 (in hexadecimal, 7fffh ) and adding one produces the smallest negative number, -32,768 (in hexadecimal, 8000h). It is an insidious problem with computer software implementation of video algorithms that wrap-around is allowed in integer arithmetic. In video signal processing, such wrap-around must be prevented, and saturating arithmetic must be used.

Heynderickx, Ingrid and Langendijk, Erno (2005), “Imagequality comparison of PDPs, LCDs, CRTs and LCoS projection displays,” in SID Symposium Digest 36 (1): 1502–1505.

User-accessible controls labelled contrast and brightness are found on nearly all electronic displays. These labels are indirectly and confusingly connected to the perceptual attributes brightness and contrast. In a CRT, adjusting brightness upwards from its optimum setting affects visual contrast much more than a comparable adjustment of the contrast control. Adjusting contrast affects visual brightness much more than

a comparable adjustment of the brightness control. brightness and contrast are therefore misleading labels. Today, contrast and brightness controls are implemented in literally billions of pieces of equipment. Hundreds of millions of people have a poor understanding of these controls, and have had it for half a century: Imaging system designers are faced with

a big problem.

This chapter describes the perceptual attributes brightness and contrast. I describe conventional contrast and brightness controls, I explain how those controls came to do what they do, and I conclude by making some recommendations to reduce the confusion.

Perceptual attributes

According to two well respected vision and display system researchers,

The four most important image quality attributes, at least for non-expert viewers when assessing image quality of high-end TVs, are brightness, contrast, color rendering and sharpness.

Drive historically referred to separate gain adjustments internally in the R, G, and B signal paths; screen or bias referred to independent internal R, G, and B offset adjustments. In home theatre calibration circles these are respectively

RGB-high and RGB-low.

Here we address the first two image attributes, brightness and contrast, which presumably the authors consider the most important. Heynderickx and her colleague are referring to brightness and contrast as perceptual attributes. There are like-named controls on display equipment; however, I argue that the controls don’t affect the perceptual attributes of a displayed image in the obvious manner. In the present chapter, including its title, we have to distinguish the names of the controls from the perceptual attributes. I typeset the names of the controls in small capitals – contrast and brightness – and typeset normally the visual attributes brightness and contrast.

Contrast refers to a measured or visual distinction between colours or grey shades. Contrast is usually quantified by the ratio of a higher-valued luminance (or reflectance) to a lower-valued luminance (or reflectance). The ratio can be computed between widely different luminances; for example, when evaluating

a display system we generally seek contrast ratio (the ratio of maximum to minimum luminance) of 100 or better, and perhaps up to 10,000. The ratio can be computed between similar luminances. Vision cannot distinguish two luminance levels when their contrast ratio falls below about 1.01 (“Weber’s Law”), and the ratio between two luminances near the threshold of human detection is sometimes called Weber contrast.

History of display signal processing

Television originated with analog vacuum tube circuits; CRTs are themselves vacuum tubes. Vacuum tubes and the associated analog components (primarily resistors and capacitors) were subject to drift owing to operating temperature variation and owing to age-induced component degradation. The main effects of drift were to alter the gain and offset of the video signal; so, gain and offset controls were provided. Drift was such a serious problem that the controls were located on the front panel; consumers were expected to use them.

User-adjustable contrast and brightness controls were implemented in vacuum tube television receivers of the early 1940s. Gain of the video amplifier circuitry was adjusted by a control that came to be called contrast. Control of offset (bias) was implemented at

Kallmann, Heinz.E. (1940), “The gradation of television pictures,” in Proc. IRE 28 (4): 170–174 (Apr.).

Oliver, B.M. (1950), “Tone rendition in television,” in Proc. IRE 38 (11): 1288–1300 (Nov.).

Fink, Donald G. (1952), Television

Engineering, Second Edition

(New York: McGraw-Hill)

the CRT itself, by a control called brightness. Gain control took effect earlier in the signal path than offset. Kallmann described a typical implementation:

… the so-called contrast control … is a voltage divider controlling signal amplitude … the back- ground-light control … adjusts bias on the cathoderay tube.

The scheme described by Kallmann prevailed for the whole CRT era. Contrast and brightness circuitry operated in the R’G’B’ domain – that is, operated on gamma-encoded signals. Historically, the CRT itself imposed the power function associated with display “gamma.” In CRTs, gamma wasn’t adjustable.

I have been unable to find any historical documents that discuss how the names contrast and brightness came about. Some early television receivers used the label brilliance for the gain control and some used background for the offset control. Some early television models had concentric contrast and volume controls, suggesting a single place for the user to alter the magnitude of the sound and the magnitude of the picture. One model had brightness on the front panel between vertical hold and focus!

Video scientists, engineers, and technicians have been skeptical about the names contrast and brightness for many, many decades. Sixty years ago, Oliver wrote:

… the gain (“contrast”) control certainly produces more nearly a pure brightness change than does the bias (“brightness”) control, so the knobs are, in

a sense, mislabeled.

The parentheses and quotes are in the original. Concerning brightness, Oliver stated:

… A good name for this knob might be “blacks,” or “background,” or “shadows.”

That these controls are misnamed was observed a few years later by the preeminent electronics engineer Donald Fink:

“Unfortunately, in television systems of the present day, ... the separate manipulation of the receiver

In some modern television receivers, the gain control is labelled picture instead of contrast.

brightness and contrast controls (both of which are misnamed, photometrically speaking) by the nontechnical viewer may readily undo the best efforts of the system designers and the operating technicians.”

Despite researchers of the stature of Oliver and Fink complaining many decades ago, the names stuck – unfortunately, in my opinion.

Over 70 years, video signal processing technology shifted, first in about 1965 to transistors used in analog mode, then in about 1975 to analog integrated circuits, and then in about 1985 to digital integrated circuits, whose complexity has increased dramatically over the last 25 years. Around 2000, display technology started to shift from CRTs to LCD and PDP technology. With all of these shifts, the need for adjustment diminished. Nonetheless, contrast and brightness have been carried forward (thoughtlessly, some would say) into successive generations of technology. Today, these controls are in use in around a billion CRT-based television receivers and another billion CRT displays in use with computers. The controls have been carried over (again, without much thought) into fixed-pixel displays; around a billion LCD displays are in use today, and virtually all have brightness and contrast controls implemented in the digital signal processing path.

In video processing equipment, gain and offset controls have historically been available; they operate comparably to the display controls, but the associated controls are usually labelled gain and black level.

LCD and plasma displays typically have contrast and brightness controls. Despite the professional users’ expectation that the controls would be implemented similarly to the like-named controls on a CRT display, and despite consumers’ expectations that such controls should function in a comparable manner to CRTs, the LCD controls often have quite different effect.

Contrast and brightness controls are widespread in image applications in computers. The effect of contrast and brightness controls in these domains is not necessarily comparable to the effect of like-named controls on display equipment. In particular, contrast in Photoshop behaves very differently than contrast in typical displays: Photoshop contrast controls gain, but it “pivots” the gain around a certain formulation of the