Picture rendering

Giorgianni, Edward J., and Thomas

E. Madden (2008), Digital Color

Management: Encoding Solutions,

Second Edition (Chichester, U.K.:

Wiley).

I use the term white to refer to diffuse white, which I will explain on page 117. A diffuse white reflector has a luminance of up to

30,000 cd·m-2 in daylight, and perhaps 100 cd·m-2 at twilight.

Examine the flowers in a garden at noon on a bright, sunny day. Look at the same garden half an hour after sunset. Physically, the spectra of the flowers have not changed, except by scaling to lower luminance levels. However, the flowers are markedly less colourful after sunset: Colourfulness decreases as luminance decreases. This is the Hunt effect, first described (in 1952) by the famous colour scientist R.W.G. Hunt. Reproduced images are usually viewed at a small fraction, perhaps 1⁄100 or 1⁄1000, of the luminance at which they were captured. If reproduced luminance were made proportional to scene luminance, the reproduced image would appear less colourful, and lower in contrast, than the original scene.

To reproduce contrast and colourfulness comparable to the original scene, we must alter the characteristics of the image. An engineer or physicist might strive to achieve mathematical linearity in an imaging system; however, the required alterations cause reproduced luminance to depart from linearity. The dilemma is this: We can achieve mathematical linearity, or we can achieve correct appearance, but we cannot simultaneously do both! Successful commercial imaging systems sacrifice mathematics to achieve the correct perceptual result.

If “white” in the viewing environment has luminance significantly less than “white” in the environment in which it was captured, the tone scale of an image must be altered. An additional reason for correction is the surround effect, which I will now explain.

Figure 11.1 Surround effect.

The three squares surrounded by light grey are identical to the three squares surrounded by black; however, each of the black-surround squares is apparently lighter than its counterpart. Also, the contrast of the black-surround series appears lower than that of the white-surround series.

DeMarsh, LeRoy E., and Edward J. Giorgianni (1989), “Color science for imaging systems,” in Physics Today 42 (9): 44–52 (Sep.).

I use the term glare (or viewing glare, or veiling glare) to refer to uncontrolled, unwanted light that reflects from the display surface, either diffusely or specularly. Image-related scattered light is called flare. Some people use these terms interchangeably, thus failing to distinguish unmodulated and modulated light.

Unfortunately, simultaneous contrast has another meaning, where it is a contraction of simultaneous contrast ratio, preferrably called intra-image contrast ratio. See Contrast ratio, on page 29.

Surround effect

Human vision adapts to an extremely wide range of viewing conditions, as I will detail in Adaptation, on page 247. One of the mechanisms involved in adaptation increases our sensitivity to small brightness variations when the area of interest is surrounded by bright elements. Intuitively, light from a bright surround can be thought of as spilling or scattering into all areas of our vision, including the area of interest, reducing its apparent contrast. Loosely speaking, the visual system compensates for this effect by “stretching” its contrast range to increase the visibility of dark elements in the presence of a bright surround. Conversely, when the region of interest is surrounded by relative darkness, the contrast range of the vision system decreases: Our ability to discern dark elements in the scene decreases. The effect is demonstrated in Figure 11.1 above, from DeMarsh and Giorgianni. The surround effect stems from the perceptual phenomenon called the simultaneous contrast effect, also known as lateral inhibition. The surround effect has implications for the display of images in dark areas, such as projection of movies in

a cinema, projection of 35 mm slides, or viewing of television in your living room. If an image were reproduced with the correct relative luminance, then when viewed in a dark or dim surround, it would appear lacking in contrast.

Image reproduction is not simply concerned with physics, mathematics, chemistry, and electronics: Perceptual considerations play an essential role.

Intra-image contrast ratio is the ratio of luminances of the lightest and darkest elements of an image. For details, see Contrast ratio, on page 29.

Tone scale alteration

Tone scale alteration is necessary mainly for the two reasons that I have described: The luminance of a reproduction is typically dramatically lower than the luminance of the original scene, and the surround of a reproduced image is rarely comparable to the surround of the original scene. Two additional reasons contribute to the requirement for tone scale alteration: limitation of contrast ratio, and specular highlights.

A typical original scene has a ratio of luminance levels of more than 1000:1. However, contrast ratio in the captured image is limited by optical flare in the camera. Contrast ratio at a display is typically limited by physical factors and by display flare to perhaps 1000:1. Diffuse white refers to the luminance of a diffusely

reflecting white surface in a scene. Paper reflects diffusely, and white paper reflects about 90% of incident light, so a white card approximates diffuse white. However, most scenes contain shiny objects that reflect directionally. When viewed in certain directions, these objects reflect specular highlights having luminances perhaps ten times that of diffuse white. At the reproduction device, we can seldom afford to reproduce diffuse white at merely 10% of the maximum luminance of the display, solely to exactly reproduce the luminance levels of the highlights! Nor is there any need to reproduce highlights exactly: A convincing image can be formed with highlight luminance greatly reduced from its true value. To make effective use of luminance ranges that are typically available in image display systems, highlights must be compressed.

Incorporation of rendering

The correction that I have mentioned can be achieved by subjecting luminance – or, in the case of a colour system, tristimulus values – to an end-to-end power function having an exponent between about 1.1 and 1.5. The exponent depends primarily upon the ratio of scene luminance to reproduction luminance; it depends to some degree upon the display physics and the viewing environment. Nearly all image reproduction systems require some tone scale alteration.

In Constant luminance, on page 107, I outlined nonlinear coding in video. Continuing the sequence of

Figure 11.2 Imposition of picture rendering at decoder, hypothetical

sketches from Figure 10.9, on page 111, Figure 11.2 shows that correction for typical television viewing could be effected by including, in the decoder, a power function having an exponent of about 1.2:

Fairchild, Mark D. (2005), Color

Appearance Models, Second

Edition (Chichester, U.K.: Wiley).

James, Thomas H., ed. (1977), The Theory of the Photographic Process, Fourth Edition (Rochester, N.Y.:

Eastman Kodak). See Ch. 19

(p. 537), Preferred tone reproduction.

Concatenating the 0.5-power at encoding and the 2.4-power at decoding produces the end-to-end 1.2-power required for television display in a dim surround. To recover scene tristimulus values, the encoding transfer function should simply be inverted; the decoding function then approximates a 2.0-power function, as sketched at the bottom right of Figure 11.3. The effective power function exponent at a CRT

varies depending upon the setting of the brightness control. In a dark viewing environment – such as

a home theater – the display’s brightness setting could be so low as to push the decoder’s effective exponent up to about 2.6; the end-to-end power will be about 1.3. In a bright surround – such as a computer in a desktop environment – brightness will be increased; this will reduce the effective exponent to about 2.2, and thereby reduce the end-to-end exponent to about 1.1.

The encoding exponent, decoding exponent, and end-to-end power function for cinema, television, and office CRT viewing are shown in Table 11.1.

In film systems, the necessary correction was historically designed into the transfer function of the camera