Why 2048 (More or Less) Equals 1080 in Digital Video
When industries converge, there is often a collision of terminologies. Different traditions have their own ways of describing the same thing. In the latter half of the 1980s, personal computing converged with the typesetting industry. While the latter talked about points and picas, the former saw everything in terms of pixels.
Now computing is converging with entertainment media and, again, we find a heritage of looking at the same thing in different ways.
Consider the resolution of digital video.
Anyone who has recently purchased a Blu-ray player or a large-screen television is no doubt familiar with the numbers used to characterize high-definition television — 720p, 1080i, and 1080p — even if the exact meaning of these figures may remain elusive. And most people know that, for a given resolution, ‘p’ (progressive scan) is better better than ‘i’ (interlaced).
What may be less clear is: 720 or 1080 what? What do these numbers measure? As many of you — the technically savvy readers of this blog — are no doubt aware, these figures refer to the number of horizontal scan lines in the image — in other words, its vertical resolution.
If you’ve been following developments in digital cinema, you also know that movie theaters may use what is termed a “1K” or a “2K” digital projector. My recent post on the IMAX Conundrum, for example, mentioned that IMAX Digital uses dual 2K Christie projectors. But what is the “2K” (that is, 2,048) measuring? 2,048 what? Is it the same vertical resolution described above?
No. In this case, the number indicates the number of pixels in each scan line — the image’s horizontal resolution.
Why are consumer displays indicated by their vertical resolution and professional projectors by their horizontal resolution?
Television’s focus on vertical resolution may come from the legacy of traditional TV signals which, although analog, contain a fixed number of horizontal scan lines: 525 for NTSC and 625 for PAL. The new digital formats continue this tradition of thinking of image quality in terms of the number of scan lines — the vertical resolution. Why cinema projectors took the opposite tack of specifying the horizontal resolution is anyone’s guess.
The upshot is that a projector with a resolution of, say, 2048 pixels horizontally and 1080 pixels vertically could be described as either 2K (‘2048’) or 1080p.
Differences in aspect ratios add to the confusion. A 1080p digital television may have a resolution of 1920 by 1080 pixels, while a 2K cinema projector may be 2048 by 1080. Sony has a 4K projector with a native resolution of 4,096 by 2,160 pixels. The RED ONE digital camera is also termed a 4K device, even though it has a native resolution of 4520 x 2540 pixels.
Computer monitors and video display cards have long avoided this confusion by simply indicating both the horizontal and the vertical resolution. Thus early VGA displays were characterized as “640 x 480,” that is, a horizontal resolution of 640 pixels and vertical resolution of 480 pixels. An XGA display is 1024 by 768. And so on.
The computer geeks have it right in this case. The convenience of having a single number to indicate resolution — 2K, 1080p — is outweighed by the clarity of giving both dimensions. Let’s avoid the confusion and state video resolution as two numbers — the horizontal by the vertical resolution.