User-accessible controls labelled contrast and bright-ness 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 equip-ment. 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
Heynderickx, Ingrid and Langendijk, Erno (2005), “Image-quality comparison of PDPs, LCDs, CRTs and LCoS projection displays,”
in SID Symposium Digest 36 (1):
1502–1505.
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.
Here we address the first two image attributes, bright-ness 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 reflec-tance). 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 oper-ating 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 ser-ious 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
Drive historically referred to sepa-rate gain adjustments internally in the R, G, and B signal paths; screen or bias referred to independent internal R, G, and B offset adjust-ments. In home theatre calibration circles these are respectively RGB-high and RGB-low.
the CRT itself, by a control called brightness. Gain control took effect earlier in the signal path than offset.
Kallmann described a typical implementation:
Kallmann, Heinz. E. (1940), “The gradation of television pictures,” in Proc. IRE 28 (4): 170–174 (Apr.).
… the so-called contrast control … is a voltage divider controlling signal amplitude … the back-ground-light control … adjusts bias on the cathode-ray 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 televi-sion 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 bright-ness for many, many decades. Sixty years ago, Oliver wrote:
Oliver, B. M. (1950), “Tone rendition in television,” in Proc.
IRE 38 (11): 1288–1300 (Nov.).
… 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
Fink, Donald G. (1952), Television Engineering, Second Edition (New York: McGraw-Hill)
brightness and contrast controls (both of which are misnamed, photometrically speaking) by the nontech-nical viewer may readily undo the best efforts of the system designers and the operating technicians.”
In some modern television receivers, the gain control is labelled picture instead of contrast.
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 televi-sion 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
average pixel level instead of pivoting at zero as is usual in video equipment and in displays. For other computer imaging applications, there are no standards; it’s often difficult or impossible to tell exactly how a particular application implements these controls.
Digital driving levels
The term pixel value is ambiguous because pixel values accessible to application software can be altered by the graphics subsystem on the way to the display – for example, they can be altered by the lookup table in agraphics adapter. DDL numbers are not necessarily those passed to the display panel column drivers and “glass”: Modern display equipment ordinarily incorporates signal processing – often including lookup tables (LUTs) – to compen-sate for the native display panel response.
The term digital driving level (DDL) refers to a pixel component data value (typically produced by a PC graphics subsystem, or by a consumer electronics signal source such as a Blu-ray player) that crosses an inter-face (typically DVI, HDMI, or DisplayPort) and drives a display device. A DDL is interpreted as an integer value from 0 to 2k- 1 (where k is the bit depth at the interface, typically 8, but potentially 10 or 12).
Computer interfaces such as DVI carry 8-bit DDLs where DDL 0 is reference black and DDL 255 is refer-ence white.
Video interface standards such as HD-SDI in the studio and HDMI in consumer equipment allow foot-room below reference black and headfoot-room above refer-ence white. HD-SDI is standardized with 10-bit values;
interface code 64 corresponds to reference black and interface code 940 corresponds to reference white. In consumer use, eight-bit HDMI is commonly used; face code 16 corresponds to reference black and inter-face code 235 corresponds to reference white.
To simplify the rest of the discussion I’ll refer to pixel values in terms of normalized DDLs (NDDLs) where reference black at the interface corresponds to NDDL 0 and reference white at the interface corresponds to NDDL 1. Pixel values in video are permitted to have modest excursions outside the reference 0 to 1 range, -15/219 to 235/219 (about -0.07 to +1.09). The NDDL range 0 to 1 corresponds to what an HD engineer might call IRE levels 0 through 100.
Relationship between signal and lightness
The sRGB standard requires an electro-optical conver-sion function (EOCF) comprising a 2.2-power function.
Documentation associated with sRGB makes clear that the fundamental EOCF – the mapping from pixel value to display luminance – is supposed to be a pure 2.2-power function, followed by the addition of
a veiling glare term. The sRGB standard also docu-ments an OECF that is intended to describe a mapping from display luminance to pixel value, suitable to simu-late a camera where the inverse power function’s infi-nite slope at black would be a problem. The OECF has a linear segment near black, and has an power function segment with an exponent of 1/2.4. The OECF should not be inverted for use as an EOCF; the linear slope near black is not appropriate for an EOCF.
Poynton, Charles (2009), “Percep-tual uniformity in digital imaging,”
in Proc. Gjøvik Color Imaging Symposium (GCIS 2009): 102–109.
The sRGB 2.2-power function, the de facto 2.4-power function of today’s studio reference displays, and the 2.6-power function of digital cinema all almost perfectly invert L*, as depicted in Figure 5.1. The 2.4-curve, which typifies video and HD practice, has a highly linear rela-tionship with L* for NDDL 0.2 and above (that is, for 8-bit interface codes above 59). An NDDL of 0.2 yields an L* value of 16. The line from [0.2, 16] through refer-ence white has slope of 105; extending that line back towards the x-axis yields an x-intercept of 0.0475, and back further, ay-intercept of almost exactly -5.
Algorithm
The effect of conventional contrast and brightness controls in video is approximated by the following
0.2
0 0.4 0.6 0.8 1.0
20
0 40 60 80 100
Pixel value, normalized DDL
Lightness,L*
2.22.4 2.6
Figure 5.1 Relationship between pixel value and L*. The x-axis plots normalized pixel (or video signal) value. The corresponding L* values are plotted for display power functions (“gamma”) of 2.2 (typical of computer imaging, e.g., sRGB), 2.4 (typical of studio video), and 2.6 (standard for digital cinema). This graph, and those to follow, apply to three channels when R = G = B, or to individual channels scaled appropriately (where the other two channels are zero). Adapted from Poynton’s “Perceptual uniformity in digital imaging.”
equation, where x represents NDDL (one of R’, G’, or B’) scaled to the range 0 to 1.
Contrast alters the m parameter – Oliver and Fink (cited on page 49) would have called it gain– over a range approximately 0.5 to 2. Brightness alters the b parameter – Oliver and Fink would have called it offset or bias– over a range approximately ±0.2.
Figures 5.2 and 5.3 show the effect on the display tristimuli of changing gain and offset respectively.
The result y of equation 5.1 is clipped (historically by the action of the CRT itself, or now by signal processing) so as not to fall below zero. At a sufficiently high signal
y=m x b+ Eq 5.1
0 0.2
0 0.4 0.6 0.8 1.0
0.2 0.4 0.6 0.8 1.0
Pixel value, V
Tristimulus value, T
Figure 5.2 Effect of gain control (“contrast”) for nominal offset (“ bright-ness”) setting (b = 0). Gain values 0.8, 1.0, and 1.25 are shown; these are values of m in Equation 5.1. The y-axis is tristimulus value, a linear-light quantity that is not directly perceptu-ally meaningful.
0 0.2
0 0.4 0.6 0.8 1.0
0.2 0.4 0.6 0.8 1.0
Pixel value, V
Tristimulus value, T
Figure 5.3 Effect of offset control (“brightness”) for nominal gain (“contrast”) setting (m = 1). Offset values -0.2, 1.0, and +0.2 are shown; these are values of b in Equation 5.1.
value, probably not too much over 1.09, saturation is likely to set in. The user adjusting gain or offset should be careful that his or her settings don’t induce clipping or saturation.
The x and y signals in equation 5.1 are in the gamma-corrected (R’G’B’) domain. The result is then raised to a modest power γ (gamma, ranging from about 2.0 to 2.6) to produce a displayed tristimulus value (R, G, or B, on the y-axes of Figures 5.2 and 5.3).
Historical analog gain control circuitry implemented contrast as a “one-quadrant” multiplier on R’G’B’
video signals clamped at blanking level (0). In PAL video, black and blanking levels were identically 0;
consequently, adjusting contrast in a PAL receiver left black of a properly coded signal where it was supposed to be (having minimal interaction with brightness). In NTSC encoding, +7.5-unit “setup” was inserted, causing black level of a properly encoded signal to lie at 0.075 on the 0 to 1 scale. However, clamping was still at blanking level (0). With gain “hinged” at zero, adjusting gain from 0.5 to 2 would alter black level from 0.0375 to 0.15, thereby causing contrast and brightness to interact.
There is no standard or convention for the range of m and b, for the relationship of m and b values to the controls, or for numerical control values presented to the user. Today’s video studio reference displays (“BVMs”) are adjustable allowing m to range between about 0.5 and 2, and b to range about ±0.2; however in studio practice it is common for both controls to lack numerical control values. In today’s consumer equip-ment, contrast is typically presented to the user as a value (here denoted C) from 0 through 100 – shown for example on an on-screen display (OSD) – and brightness as a value (here, B) from 0 through 100.
Suitable mappings from those control values to parame-ters m ranging 0.5 and 2 and b ranging ±0.2 are these:
Brightness values might alternatively be presented in the range -50 through +50, in which case the second mapping would be b=B/250.
m b
C
= = B
− −
2
50
50 50
; 250 Eq 5.2
Figure 5.4 graphs the lightness produced for various settings of the gain parameter m, with the display char-acteristic considered a function of that parameter. The range of values in the graph corresponds to acontrast range of 0 to 100 under the mapping of Equation 5.2.
Figure 5.5 graphs the lightness produced for various settings of the offset parameter b, with the display char-acteristic considered a function of that parameter. The range of values in the graph corresponds to abrightness range of 0 to 100 under the mapping of Equation 5.2.
0.2
Pixel value, normalized DDL
Lightness,L*
Figure 5.4 Effect of gain control (“contrast”) on lightness, for nominal bias setting (b = 0), with gain control viewed as setting the display function. Gain values correspond to values of m in
Pixel value, normalized DDL
Lightness,L*
Figure 5.5 Effect of offset control (“brightness”) for nominal gain setting (m = 1), with bias control viewed as setting the display function. Bias values correspond to values of b in Equation 5.1.
Black level setting
To set brightness (or black level) in the studio, display a pattern containing pluge (levels -0.02, 0, +0.02) on a test image having average relative luminance of about 0.01 (1%). Set black level high, then reduce it until the -0.02 and 0 pluge levels become just barely indistin-guishable. You’re finished.
If you have no pluge pattern, display a picture that is predominantly or entirely black. Set black level to its minimum, then increase its level until the display barely shows a hint of dark grey, then back off a smidge.
Historically, black level setting was somewhat dependent upon ambient light. However, modern displays have such low faceplate reflectance that ambient light contributes very little unwanted lumi-nance, and the black level setting is no longer very sensitive to ambient light. Modern display equipment is very stable; frequent adjustment is unnecessary.
Here I use standard digital studio video levels, not computing levels.
In the end, eight-bit codes 0 through 16 are
expected to be indistinguishable. Code 16 (NDDL 0) is supposed to produce luminance that is visually indistin-guishable from that of the negative-going bar of pluge (8-bit interface code 12, NDDL -0.02): The positive-going bar of pluge (8-bit interface code 20,
NDDL +0.02) is expected to be visible.
See Relative luminance, on page 258, and Display white reference, on page 310. Astonishingly, no current studio standard specifies the lumi-nance of reference white. I suggest 100 nt.
Once black level is set correctly, contrast can be set to whatever level is appropriate for comfortable viewing, provided that clipping is avoided. In the studio, the contrast control can be used to achieve the desired luminance of reference white, typically around 100 cd·m– 2. (Historically, Europe used a somewhat lower reference white luminance, perhaps 80 cd·m– 2.) Effect of contrast and brightness on contrast and brightness
To explore the visual effect of contrast and brightness controls, consider an ideal, properly adjusted 8-bit HD studio display.
Decreasing brightness leads to a darker image. Ignoring “shadow detail,” a naïve viewer may find the resulting picture superior!
Decreasing brightness from its optimum setting causes clipping of video content lying just above refer-ence black. Clipping doesn’t impair contrast ratio per se, but stripping out image content “in the shadows”
pluge: Picture line-up generator, see page 421. ITU-R BT-814.2 standard-izes a suitable test pattern. My proce-dure involves only the negative pluge bar, and is independent of its excur-sion. Traditional standards such as SMPTE RP 167 call for setting “so that the darker [negative] patch of the pluge just merges with the refer-ence black level, but the brighter [positive] patch is clearly distinguish-able.” In my view the “but” phrase is not properly part of the optimiza-tion; instead, it provides a cross-check after the fact.
produces obvious artifacts, so we won’t explore decreasing brightness.
Let’s compute the effect of contrast on contrast ratio. Assume a typical studio contrast ratio of 3333 (100 nt white, 0.03 nt black). Decreasing contrast by 20% reduces the white video signal to 0.8, yielding a relative luminance of 0.585. Increasing contrast by 20% increases the white signal to 1.25, yielding a relative luminance of 1.71. Starting with a contrast ratio of 3333, adjusting contrast ±20% decreases contrast ratio to about 1950 or increases it to about 5700.
Let’s compute the effect of contrast on “bright-ness,” as estimated by L*. Adjusting contrast ±20%
yields L* ranging from 81 to 118.
To compute the effect of increasing brightness on contrast ratio, increasing brightness by 20% takes the y-intercept of the 2.4-gamma curve of Figure 5.1 from -5 to +3. Reference black code now produces relative luminance of about 0.00332; reference white produces relative luminance of about 1.08. Increasing brightness thus causes contrast ratio to drop from 3333 to
1/0.00332, that is, to 325.
Finally, increasing brightness by +20% causes the
Finally, increasing brightness by +20% causes the