There was an interesting article in The Wall Street Journal, on page B1 of the
11 August 2003 issue, titled "Engineering Blue Skys".
In short, some movies are shot on silver-based film ("the medium still
considered to provide the most reliable quality"), converted into a huge
digital file, manipulated to taste, and converted back to silver-based film for
distribution.
The advantage is that one can easily make corrections like making the river
water blue (not brown), and the sky contrast better with the adjusted river, as
well as adding special effects. The current disadvantage is a substantial loss
of resolution, to the point that the films don't look right on IMAX, the claim
being that the resolution is OK for ordinary theaters. (IMAX films were
traditionally shot on 70mm film.) Nor do color corrections always work right,
so it appears that they don't have quite enough headroom.
Anyway, the master film is digitized at 4100 (h) by 3000 (v) pixels per movie
frame, and ends up at 2048 by 1556 pixels per frame after processing. The bit
depth was not discussed, but each of these pixels will contain all three
colors, unlike the way the digital still cameras are specified, and it must be
at least 8 bits per color, with 10 or 12 bits being more likely, and 14 bits
being possible. The intent is to have enough extra pixels and bit depth to
allow significant processing without the resulting artifacts becoming
noticable. A movie frame is about one half the area of a 35mm still camera
frame. (I don't recall the exact dimensions, but somebody posted it to the
list a while ago.) A movie has 24 frames per second.
Master: 4100*3000= 12e6 three-color pixels per movie frame. (If this were a
still camera, it would be advertised as 24 megapixel, and trumpets would sound.)
At 24 frames/second, a two-hour movie would require (12e6 pixels/frame) (3
bytes/pixel) (2*60*60*24 frames/movie)= 6.22e12 bytes/movie. This is 6,220
gigabytes, or 6.22 terabytes, uncompressed, to achieve minimal color fidelity.
A more likely size is 10 terabytes, and use of compression is unlikely in a
master, because of the artifacts all compression schemes introduce. Nor was
use of compression mentioned in the article.
Distribution print: 2048*1556= 3.187e6 three-color pixels per movie frame.
The quality is reported as adequate for some kinds of movies. One assumes that
opinion varies on that point. (If this were a still camera, it would be
advertised as 6.4 megapixel.)
Joe Gwinn
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