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#1 Ashim

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Posted 14 October 2006 - 06:23 AM

Hello,

When a fim is scanned for DI in the Cineon or DPX format is the films density recorded in a
linear mode or a log mode.

And why does the scanned image appears dark even after gamma corr.?


Thanks
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#2 Dirk DeJonghe

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Posted 14 October 2006 - 07:48 AM

Cineon is LOG by definition, DPX is usually LOG but there are many varieties of DPX.
Most DI work is done with LOG images, you need a special viewer LUT to see the images correctly.

Think of LOG images as digital negatives. Nobody is projecting a negative and expecting it to be 'normal'.

There is a log of good information on Steve Shaw's website Digital Praxis
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#3 Ashim

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Posted 14 October 2006 - 08:27 AM

And isnt the LUT based on the stock used for the shoot.
What if I use different stocks for the shoot???

And once the images have been color corrected we do a reverse scan on a new negative?
Cant we do it directly on the positive?
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#4 David Mullen ASC

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Posted 14 October 2006 - 12:13 PM

And isnt the LUT based on the stock used for the shoot.
What if I use different stocks for the shoot???

And once the images have been color corrected we do a reverse scan on a new negative?
Cant we do it directly on the positive?


LUT's (Look-Up Tables) are vague terms that describe processing of the signal to alter it to suit a certain destination, usually a type of display device, but also when the signal or data is going to a certain type of film recorder recording to a certain type of film stock.

Generally though different color negative stocks are similar enough that a Cineon Log scan of them would use the same LUT to view them correctly on the same type of monitor, let's say.

The reason that most people do not record a data file to color print stock is that is it so expensive to do film recording that it's cheaper to record to a negative and thus be able to strike as many prints as you need at normal printing costs. For a feature, for example, it may cost $50,000 to laser record out to film, so you don't want to go to a print and have it cost you $50,000 for each print you need, when you could go to a negative and have it cost you $2000 for each print after that using normal methods of printing.

Some people will record out to an IP (interpositive) and then make as many dupe negatives as they need for making release prints. Only trouble with that is you need to strike the dupe negative off of this IP just to be able to then make a print to project and check to see that the IP came out OK.
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#5 Michael Most

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Posted 14 October 2006 - 02:59 PM

LUT's (Look-Up Tables) are vague terms that describe processing of the signal to alter it to suit a certain destination, usually a type of display device, but also when the signal or data is going to a certain type of film recorder recording to a certain type of film stock.

Generally though different color negative stocks are similar enough that a Cineon Log scan of them would use the same LUT to view them correctly on the same type of monitor, let's say.


This is generally true, except that it's not really a vague term. A lookup table defines a series of pre-calculated input and output values, eliminating the need for mathematical calculations. For every possible input value (or, in the case of 3D LUTs, combinations of input values), the table has an output value. This is possible because in digital imaging, there are a finite number of input values - in 10 bit work, for instance, the only possible values are 0 through 1023, by definition. They are generally used for converting from one color space to another, but they can also be used for pre-calculated color correction, or, in some cases, to simulate the color gamut of one type of output device - say, print film - on a completely different type of output device - say, a digital projector. Lookup tables as used in DI work are primarily based on the characteristics of three devices: the monitor being used (usually a projector, but it can be a monitor of various types), the film recorder being used, and the print stock being used. The characteristics of the negative stock being recorded to are less significant in terms of color reproduction when compared to the three devices already mentioned, primarily because the stock used in any given pipeline usually doesn't change - it's either an intermediate stock (usually 5242 these days) or a camera stock (most commonly 5245) - although when you create a lookup table, it is inevitably influenced by that negative stock. The print stock can be more significant, because different print stocks can be and are used in different situations. For general viewing, both on a set and for dailies, generalized LUT's can be created that are built for, say, a "generic" CRT or LCD display. These will generally be "1D" LUTs, intended for viewing only. For DI work, it is much more important to correctly simulate the characteristics of print film with great accuracy for the entire color gamut. In this case, "3D" LUTs are used, which take into account all possible color combinations and yield an electronically projected image that is virtually identical to the resulting film print. For this to work, the viewing conditions must be properly calibrated and maintained, and the devices used are profiled and new LUT's generated on a regular basis - something that is usually out of the question for "do it yourself" situations.
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#6 Ashim

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Posted 15 October 2006 - 11:31 AM

David, Michael:

Thanks a ton. The information you provided has proven to be a great help to me.

Thanks again,

Ashim
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#7 Matt Goldberg

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Posted 16 October 2006 - 02:37 PM

And isnt the LUT based on the stock used for the shoot.
What if I use different stocks for the shoot???

And once the images have been color corrected we do a reverse scan on a new negative?
Cant we do it directly on the positive?


You should be able to record to positive via a Lasergraphics film recorder (at least). This may have credible differences against striking from a dupe negative or intermediate. You may want to test this out-- often a post house will do it complementary, normally less than 1 min. of footage.
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#8 Dirk DeJonghe

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Posted 16 October 2006 - 03:05 PM

In fact we have done several feature films where only 1 or 2 prints were required. We used the Lasergraphics P3 to expose directly to color positive stock. At our request Lasergraphics added black mask capability. Sound track is exposed on standard contact printer.
We call this a direct digital blow up.

It takes about 1 hour per minute of film, the latent image fading must be taken into account on long runs.
Blacks are less deep than from a negative/intermediate negative. Sharpness is very good, grain is nonexistent, picture stability is excellent as it is pinregistered.

For one or two prints maximum this is a good economical way to work if you can live with the slightly milky blacks.
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