Since 1996, color on the Web has been locked into a narrow-gamut, low dynamic-range colorspace called sRGB. Originally derived from a standard for broadcast High Definition Television (HDTV) and intended to be a lowest common denominator for interoperability, it has since become an increasing burden to creative expression and a key differentiator between the web, on the one hand, and native applications and entertainment devices which have kept up with, indeed driven, advances in technology.
Display technologies have vastly improved since the bulky, cathode-ray tube displays of the 1990s. The most obvious change is display resolution, but the range of displayable colors (the gamut), accompanied by a reduction in reflected glare and an increase in dynamic range, are also notable.
Wide Color Gamut
Colors more vivid than a typical display are common in nature. The varied orange hues of a spectacular sunset, the irridescent blues and greens of a butterfly or a bird's wing, the colors in a firework display, even the colors seen in ordinary grass, lie outside the sRGB gamut.
At first, displays that could show brigher, more saturated colors than typical displays were costly, high-end devices used by graphic designers and professional photographers. Over the last five years however the prices have come down and the quality has gone up. An emerging standard in consumer electronics devices such as tablets, TVs, phones, and laptops is display P3, derived from a standard for Digital Cinema projectors (DCI P3). It can display 50% more colors than sRGB.
Meanwhile the movie and TV industries have moved beyond P3 to an even wider-gamut colorspace, ITU Rec BT.2020, which can display an astonishing 150% more colors than sRGB. Professional displays used in color grading movies can display 90% or more of the 2020 gamut; history shows that similar capabilities will become available in the consumer market soon.
The Web needs to provide access to these colors - not just for video or images, but for everyday Web content.
High Dynamic Range
The human eye can perceive a vast range of brightnesses, from dimly-seen shapes under moonlight to the glare of sunlight reflected from metallic surfaces. This is termed the dynamic range and is measured by the luminance of the brightest displayable white, divided by the luminance of the deepest black. sRGB, with a peak luminance of 80 cd/m², has a viewing flare of 5% (4 cd/m²) giving a total dynamic range of 20x.
Display P3, which is often used with a peak white luminance of around 200 cd/m² and a black luminance of 0.80 cd/m², has a total dynamic range of 250. The luminance limits are set by power consumption and heating, if the entire display is set to the maximum brightness white; and also user comfort, as the white is a typical background for text and represents a "paper white" or "media white". In the broadcast industry, this is termed standard dynamic range (SDR). It falls far short of what the human eye can perceive.
In nature, very bright objects occupy a very small fraction of the visual field. Also, we can see detail in an almost dark room. By taking advantage of these two aspects - scene by scene lightness changes and localized highlights, a display can produce a wider dynamic range by turning down the backlight in dark scenes and turning it up in bright ones; in addition, if each small portion of the screen has its own backlight, small highlights much brighter than a paper white can be produced, for a small area and for a small time. This is called High Dynamic Range (HDR).
As an example, the broadcast standard ITU BT.2100, with the PQ electro-optical transfer function, will display a paper white at around 200 cd/m². But the deepest black is 0.001 cd/m²; and the peak, short-term, small-area white is 10,000 cd/m² giving a total dynamic range of ten million. While this is a theoretical peak, reference monitors with peak luminance of 1,000 cd/m² to 4,000 cd/m² are in widespread use for movie and TV production. Consumer devices with peak luminances of 500 to 1200 cd/m² are becoming common.
HDR is in widespread daily use for the delivery of streaming movies, broadcast television, on gaming consoles, and even for the recording and playback of HDR movies on high-end modile phones. It is also starting to be used for still images. The Web, meanwhile, is currently stuck with Standard Dynamic Range.
It is all very well to complain that consumer entertainment technology is far in advance of the Web, but what can be done about it? Are entirely new standards needed, or can existing ones be smoothly extended, adding capabilities already common in the broadcast industry?
In addition, we need to compare like with like. The experience of watching a blockbuster movie in a darkened room with a home theatre setup, or watching a live sports broadcast inside under afternoon daylight, or catching news items on a phone while commuting in broad daylight, are necessarily different. Just as we are used to reponsive Web designs that adjust to different display reolutions, content for the Web needs to adaptable for different gamuts, different peak luminances, and a very wide range of viewing conditions.