LCD: liquid crystal display

PDP: plasma display panel

This section describes Photoshop brightness and contrast controls for versions up to and including CS2, and for later versions when the “Use Legacy” option is enabled. The default brightness and contrast controls for versions CS3 and above behave differently.

Brightness and contrast controls in LCDs

In LCD displays, brightness typically alters the luminance of the backlight; its function is comparable to the contrast control of a CRT display. LCD displays produce luminance that is a nonlinear function of drive voltage. In early LCDs, contrast adjusted an electrical bias voltage at the panel. In modern LCDs, contrast adjusts gain in the signal path. There is no good reason for LCDs to have separate R, G, and B bias controls (RGB-low).

Brightness and contrast controls in PDPs

In PDP displays, maximum luminance is fixed by the electronic design of the panel; brightness and contrast are implemented by digital signal processing. PDP displays produce luminance that is a linear function of drive level. DDL 0 produces the smallest possible luminance from the display, so reference black video code should produce DDL 0 – there is no good reason to have it otherwise. There is no good reason for PDPs to have separate R, G, and B bias controls (RGB-low).

Brightness and contrast controls in desktop graphics

Adobe’s Photoshop software established the de facto effect of brightness and contrast controls in desktop graphics. Photoshop’s brightness control is similar to the brightness control of video; however, Photoshop’s contrast differs dramatically from that of video.

The transfer functions of Photoshop’s controls are sketched in Figures 5.10 and 5.11 (opposite). R’, G’, and B’ component values in Photoshop are presented to the user as values between 0 and 255. Brightness and contrast controls have sliders with a range of ±100.

Brightness effects an offset between -100 and +100 on the R’, G’, and B’ components. Any result outside the range 0 to 255 clips to the nearest extreme value, 0 or 255. Photoshop’s brightness control is comparable to that of video, but its range (roughly ±40% of full scale) is greater than the typical video range (of about ±20%).

Photoshop’s (legacy) contrast control follows the application of brightness; it applies a gain factor. Instead of leaving reference black (code zero) fixed, as

Figure 5.10 The brightness control in Photoshop applies an offset of -100 to +100 to R’G’B’ components ranging from 0 to 255. If a result falls outside the range 0 to 255, it saturates; headroom and footroom are absent. The function is evidently linear, but depending upon the image coding standard in use, the input and output values are generally nonlinearly related to luminance (or tristimulus values).

Figure 5.11 The contrast control in Photoshop applies a gain factor between zero (for contrast setting of -100) and infinity (for contrast setting of +100) to image data, but “pivoted” around a weighted average pixel level (APL) of the image data, instead of “pivoting” around zero (as is the case for gain and contrast controls in video). Each component result saturates if it falls outside the range 0 to 255.

Figure 5.12 Photoshop contrast control’s gain factor depends upon the contrast setting according to this function.

0.66 = 1.45

2.2

a video contrast control does, Photoshop “pivots” the gain adjustment around a weighted average of the image data formed as 0.299 R‘+0.587 G‘+0.114 B‘. (For image data having the “gamma correction” of video, the weighted average corresponds to BT.601 luma, or average pixel level, APL.) The transfer function for various settings of contrast adjustment, for a weighted image average of 127.5, is graphed in Figure 5.11.

The gain available from Photoshop’s contrast control ranges from zero to infinity, far wider than the typical range of 0.5 to 2 of studio gain. The function that relates Photoshop’s contrast to gain is graphed in Figure 5.12. From the -100 setting to the 0 setting, gain ranges linearly from zero through unity. From the 0 setting to the +100 setting, gain ranges nonlinearly from unity to infinity, following the reciprocal curve described by Equation 5.4:

In desktop graphics applications such as Photoshop, image data is usually coded in a perceptually uniform manner, comparable to video R’G’B’. On a PC, R’G’B’ components are by default proportional to the

1/2.2-power of reproduced luminance (or tristimulus) values. On Macintosh computers prior to Mac OS X 10.6, QuickDraw R’G’B’ components were by default proportional to the 0.66-power of displayed luminance (or tristimulus). Modern Macintosh computers conform to the sRGB standard. However, on both PC and Macintosh computers, the user, system software, or application software can set the transfer function to nonstandard functions – perhaps so far as effecting linear-light coding – as will be described in Gamma, on page 315.