pluge: Picture line-up generator, see page 421. ITU-R BT-814.2 standardizes a suitable test pattern. My proce dure involves only the negative pluge bar, and is independent of its excursion. Traditional standards such as SMPTE RP 167 call for setting “so that the darker [negative] patch of the pluge just merges with the reference black level, but the brighter [positive] patch is clearly distinguishable.” In my view the “but” phrase is not properly part of the optimization; instead, it provides a crosscheck after the fact.

Here I use standard digital studio video levels, not computing levels.

See Relative luminance, on page 258, and Display white reference, on page 310. Astonishingly, no current studio standard specifies the luminance of reference white. I suggest 100 nt.

Decreasing brightness leads to a darker image. Ignoring “shadow

detail,” a naïve viewer may find the resulting picture superior!

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 indistinguishable. 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 luminance, and the black level setting is no longer very sensitive to ambient light. Modern display equipment is very stable; frequent adjustment is unnecessary.

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 indistinguishable from that of the negative-going bar of pluge (8-bit interface code 12, NDDL -0.02): The positivegoing bar of pluge (8-bit interface code 20,

NDDL +0.02) is expected to be visible.

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 from its optimum setting causes clipping of video content lying just above reference black. Clipping doesn’t impair contrast ratio per se, but stripping out image content “in the shadows”

Concerning user adjustment of “poor sources,” consider Poynton’s Fourth Law: Once a program is approved and packaged, errors in mastering are indistinguishable from expressions of creative intent.

For consumer equipment, the black levels of modern source material are quite stable, and user adjustment to compensate for poor sources is no longer required. The diffuse ambient reflectance of modern displays is so low – around 0.01 – that ambient illuminance has

a minor effect on contrast ratio. User adjustment to compensate for ambient light is now rarely necessary. Manufacturers should therefore consider relegating black level to an internal or service adjustment.

In a display, black level is normally used to compensate for the display, not the input signal, and so it should be effected downstream of the gain (contrast) control.

In processing equipment, it is sometimes necessary to correct errors in black level in an input signal while maintaining unity gain: A black level control should be implemented prior to the application of gain (and should not be called brightness). Figures 5.8 and 5.9 plot the transfer functions of contrast and brightness controls in the video signal path, disregarding the typical 2.4-power function of the display.

Figure 5.8 The brightness

(or black level) control in video applies an offset, roughly ±20% of full scale, to R’G’B’ components. Though this function is evidently a straight line, the input and output video signals are normally in the gamma-corrected (perceptual) domain; the values are not proportional to intensity. At the minimum and maximum settings, I show clipping to the BT.601 footroom of -15⁄219 and headroom of 238⁄219. (Light power cannot go negative, but electrical and digital signals can.)

Figure 5.9 The contrast

(or video level) control in video applies a gain factor between roughly 0.5 and 2.0 to R’G’B’ components. The output signal clips if the result would fall outside the range allowed for the coding in use. Here

I show clipping to the BT.601 headroom limit.