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The Digital negative

Assuming that you've read the page on "DI - Preparation" you know that after the film is edited, information like EDLs, tape transfers or Quicktimes are sent to the DI facility. With this, they recreate the original edit.

The DI facility takes all this information and needs to create a "digital negative" to grade and create a "look" for the film. This digital negative consists of individual shots in the film at a high resolution. Each shot is made up of a frame sequence - a set of numbered files, one per frame. These files are stored on a hard disk. A 2.5 hr film can have over 2,00,000 frames spread over hundreds of folders.

The file format of each scanned frame .dpx or DPX file. DPX stands for Digital Picture Exchange. This format stores data in a manner easy for a grading system to read very rapidly. Remember, the DI system needs to open and close 24 files each second.

The DPX file format stores not just the picture, but the time code and the Keycode of that frame and other parameters. This is the basis of the Digital Intermediate process and this property of DPX is what makes the DI process even possible.

You do the math.

There are 24 frames i a second. And 60 seconds in a minute, 60 minutes in an hour.
So one minute has 24x60=1440 frames and one hour has 1440x60= 86,400 frames.

Scanning

How does the film which is in the form of a negative or OCN get into the form of a DPX sequence, or simply, a 'digital negative'?

To convert the actual movie film to a 'digital negative' series of files, one needs to scan the film negative. This is done in a "scanner" or a 'datacine'. So, the second step in the DI process is scanning.

The scanner is a rather large device which can run film in up to 2000 feet rolls, light it up with a precisely calibrated light and convert each frame into a digital picture. It does this with as much as 16-bit sampling depth. Simply put, it can convert all the individual pixels in each frame into digital pixels with a range of 2 raised to the power of 16 or 65,536 levels of brightness. Think of this as 16 stops.

And it does this for all three primary colours red, blue, and green so it can resolve many billions of colours. In effect you end up with each film negative frame as a digital picture which is faithful to the original.

How faithful? Is the digital neg the same as the original neg? Are there losses? All this is still hotly debated and will very likely be till the end of time.

If you really want to 'do DI' on your film, you better start by believing that it is, and there are no losses really. And that this process will actually enhance the look of your film and be worth the money you're spending.

And to help this belief, I strongly recommend taking out a piece of your negative, going to the DI place you are considering, making them pass this through their circuit, and taking out a neg and a print and watching it in a theatre. Most likely you'll be amazed at how good it is.

2k or 4k

Once people get into DI, the next thing that's hotly discussed in '2k or 4k'? Before getting into a final answer, let's see what these numbers mean.

The scanner that scans the negative, 'images' it into a digital picture or a DPX file as earlier said. This imaging happens at a resolution commonly called 2k. Or at a higher resolution called 4k. So 2k and 4k are resolutions.

2k means each 35mm frame is 2048 pixels wide by 1556 pixels tall. Which puts it at about 3 million pixels total.
4k is 4096 pixels wide by 3112 pixels tall. Which puts it at about 12 million pixels total.
2048 is about 2 thousand or 2k while 4096 is about 4 thousand or 4k. 'k' for kilo meaning thousand.

The exact resolution of DI scanning is as follows.

4k CScope - actual frame 3656x3112 pixels. Scope 4k frame 3656x1556
4k Super35 - actual frame 4096x3112 pixels. S35 4k frame 4096x1774
2k CScope - actual frame 1828x1556 pixels. Scope 2k frame 1828x778
2k Super35 - actual frame 2048x1556 pixels. S35 2k frame 2048x872

So the scanner takes your negative, scans it and spits out digital pictures in a form called DPX. One DPX file per frame.

These DPX files which are about 12 MB per frame (at 2k) are then stored on a hard disk. And each such frame contains a timecode, and a keycode. And they're all organised into folders and neatly classified by camera roll or lab roll so further work on them is not too difficult. If scanned at 4k the files are 50 Mb per frame.

Scanners are typically rather slow. Some work at 2 sec per frame, some do 2 frames per sec, and datacines work at 24 frames/sec. This speed of scanning doesn't affect your sound sync in any way, since scanners scan each and every frame and put out exactly as many frames as there are in the film. You just need to run them at the proper speed to sync with your sound. Think about this carefully and it makes sense.

But back to 2k vs 4k. Exactly how many 'k' is needed to faithfully represent the original negative? Depending on who you ask, 2k is 'good enough'.

So far, very few Hindi or other Indian film has been 'made in 4k'. Even in Hollywood, only a handful of the highest budget films get done in 4k.

4k, has more pixels than 2k. There is no two ways about that. Whether it actually appears sharper is subjective with some people unable to spot the difference. While being better than 2k for sure, is too challenging for systems to do reasonably. And with file sizes being larger, systems will work slower at 4k, almost a quarter as slow. So if a 2k DI would take a month, leave aside 3-4 months for a 4k DI.

3 million pixels is also 3 megapixels. And most mobiles now have 2 Megapixel cameras, and many of us have digital cameras that are 4, 5 or even 6 or more megapixels.

And we've all seen a 3 Megapixel camera image on our computer screen. So suddenly 2k doesn't seem like a huge resolution after all.

Lets see other examples.

A normal TV set shows 720x576 pixels. or 0.4 megapixels. Most computer monitors 15-17 inch show 1024x768 pixels or 0.8 megapixels.

Larger monitors show 1600x1200 pixels which is 2 megapixels. And the Apple Cinema display 23in shows 1920x1200 or 2.3 megapixels.

Log and Lin

Another 'angle' to this scanning is log and lin. Short for logarithmic and linear. Almost all scans these days are log scans. Although the scanner itself works and images the nagative in a linear way, it converts and stores it as 'log'.

The scanning process takes place with a bit depth of 16 bits per pixel. To give a sense of what this means, most video work is done at 10-bit (smoke, flame, even FCP work at 10-bit) DigiBeta is 10-bit. Most displays however are 8-bit. DVD and Beta can be considered to be 8-bit. But film has much more latitude so scanning happens at 16-bit.

Back to 'log' scans. This log-lin thing is one of these eternal mysteries even for those involved in film DI. I plan on writing a simple explanation on this, and am trying to make is simple. Till then, All I can offer is an analogy. Not precise but a parallel.

Think of film negative. 

What is before us and visible to the eyes is... well... 'positive'. All of reality around us is 'correctly coloured' and positive. But when shot on film, it becomes negative. Colours are reversed. But we still consider something correctly shot and recorded if its exposed on film. Even if its negative. Because that negative when again exposed to another film, becomes positive and even 'real' once again. We know that.

Think of log to lin and lin to log as a positive to negative and negative to positive process. Not exactly, of course, but a parallel. (Don't they call this a 'metaphor' or something like that?)

Anyway, the real reason for log is that film's response to light and exposure is not really 'linear' but 'logarithmic'. As you increase exposure, the exposed image on film gets denser or 'blacker' (its negative, remember) But this increase in density is not exactly proportional to increase in exposure. As exposure increases, density increases, but this increase gets slower as exposure increases.

Another analogy for this 'not proportional' behaviour of film. If you drive a car you will observe that as you push the accelerator the car goes faster. More push, more speed. But in the beginning of the accelerator's movement the increase is more and as you push it harder it seems to 'level out' as you reach the floor. So does your car's speed. So its not exactly proportional. Its not linear. Not proportional. It is logarithmic.

Back to film. Film's response to exposure is like that. Its not linear. Logarithmic. So when we scan film in a film scanner, we create a non-linear or logartihmic image. It looks cloudy or non-contrasty. That same image when (mathematically) linearized, looks right. Just like when negative is exposed to another negative we get a picture that's right.

Another reason for working log is that log images are 10bit meaning they take up less disk space. A linear image of the same latitude would take up 14-16 bits. Meaning more hard disk space, more data to transfer.

So log is just a mathematical representation of 'reality'. We have to make it linear for humans to enjoy and appreciate it. But the computer needs it to be log. Log is not right and linear is not wrong. Log is right and even linear is right. But both are needed for specific reasons.

Scanners

Scanners for DI use a variety of technologies but do the same basic job - to take an original camera negative, refer to an EDL or a cut list, scan only the frames needed, and store them neatly on a hard disk.

To do this, all scanners use the same basic principle. They shine a light through the film. And cast an image on an image sensor like a CCD or CMOS chip. Some use a CRT imager. These imagers 'digitize' the film image and converts it into data.

Then there is a computer system that takes this data and is saves it to hard disks as dpx or cineon or even tif files.

What separates one scanner from another is resolution of scans. type of lighting, type of imager, and speed of scanning.

Northlight

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