D-Cinema; a quick guide to pixels.

As promised this is a round-up of my understanding of the options for cinemas choosing digital projection. This does not relate to any cinema in particular, and does not provide any information not already in the public domain. This is intended to be a guide for the “lay person”.
Guide to numbers of pixels in selected image formats.
Each rectangle has an area proportional to the number of pixels contained within, the shape of the rectangle is the correct ratio of height to width, the aspect ratio.
To give a reference to something most people will be familiar with, I give the actual design resolution of television. I’m using the PAL specification, as I live in the UK where only PAL is broadcast.
The middle row of my diagram gives two similar resolutions. On the left is the maximum resolution broadcast from Sky TV, and FreeSat. If you’ve bought Sky+ HD, this should be what you’re watching depending on your TV. On the right is entry-level D-Cinema resolution, the so-called “2k” format as it has just over 2000 pixels in the horizontal direction.
Finally, at the bottom is the dimensions for “4k” format. It is considered to be the minimum resolution for reproducing the resolution of 35mm film. Most feature films are edited digitally using that resolution, in various editing and production systems. Digital Cinema is gaining popularity, but (currently) most cinemas play 35mm film and these “digital intermediate” films are recorded on to film at “4k” resolution before distribution.

Further reading:
Wikipedia on D-Cinema.
Wikipedia on broadcast TV.

Further notes, not necessarily part of “the quick guide” but may be interesting for the more enquiring reader:

About the PAL resolution: In common speech PAL is referred to as “625 line”. Which it is. However, the cathode ray tube TV set (what we had before these fancy plasma and LCDs arrived) had a phenomena called ‘bloom’ which meant that the picture changed size slightly depending on how bright it was. This lead to designers pushing the outer parts of the image off the sides of the visible area of the screen. This is called ‘over-scan’. It was a neat solution to the problem. The resolution quoted is the maximum you could expect most TVs to display without too much cropping caused by over-scan. Also some lines are used for over features of broadcast TV, such as Ceefax. These lines are in the over-scan area.

About Sky+ HD. I understand the broadcast is in interlaced format, and usually called ‘1080i’. In the same way that PAL can be described as ‘576i’, on successive scans (or fields as they are called) the odd-numbered lines are shown, then the even-numbered lines. D-Cinema (2k) doesn’t do interlacing. The image is formed in a progressive manner, and might be considered to be slightly-better to the ‘1080p’ format produced by Blu-ray players and certain game consoles.

Also, the definition of “wide-screen” varies between the home and the cinema. In the home it is usually known as 16:9 (giving an aspect ratio of 1.77). In the cinema it refers to a aspect ratio of 1.85. For films produced in the wide-screen format (1.85), showing these on TV ought to require very small black bands at the top and bottom of your TV. These black bands tend to be lost on the over-scan, along with a little bit from the left and right of the original image. I’m not sure if this applies to a domestic blu-ray player, as it ought to be able to achieve a 1:1 mapping of the original image pixels to the pixels on your big shiny new LCD TV.

There doesn’t seem to be a digital version of the Cinemascope (or Panavision) anamorphic process in the D-Cinema world. For some films you won’t get anywhere near the vertical resolution suggested. I estimate it will be about 870 pixels for a “2k” system, and 1740 pixels for “4k”. Decent for “4k” and a bit lame for “2k”.

22/4/2010: A quick edit to fix the broken link to the image sizes comparison chart.

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