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Re: Information, Compression, and storage.





Steven Gladstone asked:
>I don't understand. Are you starting from the lower resolution and then
>Compressing? Or are you starting at the higher resolution 1440 by 960,
>and then compressing that?

For the purposes of this thought experiment, money is no object, so we'll
do three telecine passes using an imaginary machine that gives us exactly
the format we want as its native mode every time.  First, we do 720 x 480,
and put it on the tape uncompressed.  In the second pass, we do 1440 x 960,
and put it through 4:1 compression to yield exactly the same number of bits
on the second tape as the first.  The third really very imaginary pass is
72,000 x 48,000 with 10,000:1 compression, yielding again exactly the same
number of bits on the third tape.

The first tape should look OK, in fact it should look just like D-1.  But
the second tape will look better than the first, because it came out of the
telecine with twice as much resolution, and lost virtually nothing we can
see through the very mild 4:1 compression.  The third tape, however, should
look like total crap.  Even though it starts with enough resolution to see
the shapes of individual film grains, it gets way too much compression to
deliver anything but compression artifacts to the final display.

So my point is that compression is like green.  You can, as a colorist,
put way too much green in a picture and make it look bad.  You can also put
nowhere near enough green in the picture and make it look bad.  Somewhere
in between is the amount of green that makes the picture look the best it
can for whatever purpose the picture is being made.  Likewise, somewhere
between uncompressed and massively compressed there is a best amount of
compression to use for a particular purpose.

If the purpose is just to get the picture to Joe Sixpack's TV set so he can
watch it, you can do well with about 75:1 compression.  If you're going to
do some compositing and tape to tape color correction, the right place is
probably going to be in the single digits.



-- J.S.






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