Digital Movie Projection: Can It Live Up To The Hype?

hobb writes "OK, so Roger Ebert's not a technical genius, but he's written an interesting piece on the future of digital movie projection (theatres, not home.) Read his essay here. Digital for home systems is great, but will 1280x1024 be good enough for theatres? That's about 10mm dot pitch, folks... "

Comments

Angular resoultion: some numbers

[Paul Johnson]

What counts is not the dot pitch, but the angular resolution of the system as seen by the audience.

What I know of this comes from still photography, but its also at 35mm (i.e. a negative 24x36mm), so I can say something intelligent.

If you do the sums for a 35mm still, it is considered "sharp" if a single point on the object maps to a cirlce of diameter less than 0.004 inches on the negative (known as the "circle of confusion"). That corresponds to a digital resolution of around 3000x2000. Of course you can go finer. But that is roughly the best performance you can expect from a 35mm film.

Now, whether this makes any difference depends on whether you can see such a small object. The question is: given two small dots in the scene, can you see whether there is one dot or two in the projected image? The point at which the two dots merge into one is the resolution, and the angle subtended by the two dots is the angular resolution. I'll dodge the difference between angle for the camera and angle for the viewer: projection systems are designed so that the middle seats get the right perspective.

The angular resolution of a good human eye is 1/60th of a degree (1 arc minute). So an ideal cinema screen would need to match that with around 60 pixels per degree. Right now I'm wearing spectacles, and without moving my head they put a frame on my vision about 80 degrees wide. I haven't measured a cinema screen from the centre seat, but I'd expect something nearer 40 degrees. 40 degrees times 60 pixels per degree gives 2400 pixels. Which is not too far off what 35mm film gives (at its theoretical best).

So current XVGA systems are not up to the job of replacing film, but give us a 3000x2000 pixel screen and it will look better. And Moore's Law suggests that we will be able to do that fairly soon.

Of course there are other issues. As others have noted you have the problems of physical wear and dirt getting onto film, and the costs of printing, versus the 100% reproducability of digital and the costs of piping all that data around. But you can bet that the studios have looked at these numbers and figured that the lifecycle costs look interesting. And no doubt someone has told them of Moore's Law too.

I remember the same argument in the early days of digital audio. The first CD players sounded harsh in the high treble thanks to the steep filters required. Analogue purists declared that digital would never replace analogue. But where is analogue now? A niche split between rich die-hards and poor elderly people who can't afford to replace their existing LPs. Physical analogue film will go the same way.

Paul.