Preserving the Quality of the Image in the DI Process with ARRISCAN & ARRILASER

The Digital Intermediate (DI) process is one of today’s most exciting and fastest growing technologies in digital postproduction of motion picture films. Most individual parts of the process have been around for a long time — digital editing, scanning, digital effects, compositing, and film recording. The latest step in the progression is to apply the process to the entire movie.

In spite of all advantages, cinematographers have some concerns about the DI process. The motion picture workflow has been producing excellent quality for decades and there is some fear that this quality may degrade. The traditional method for post production of motion picture was relatively simple because there were not many variables to be controlled. Digital color correction, on the other hand, can manipulate the image to practically any degree. Some cinematographers may wonder if the DI process can maintain the original intent as well. This article provides some insight into the important factors that are relevant to retaining the image quality in the DI process.

Spatial Resolution

Scanning resolution is one key issue when it comes to scanner performance. Many tests and demonstrations have been done to show the ultimate resolution of film. Most scanning is currently carried out at 2K resolution, i.e. the image is sampled with 2048 pixels per line. For comparison, HDTV has 1920 pixels per line. 4K (4096 pixels per line) scans are sometimes performed for VFX and some productions, Spider Man 2 for example, have been done entirely in this format. Looking at the digital files in Figure 1, a 4K scan definitely contains more details than a 2K scan. To guarantee that all details from the scene are captured a 4K scan is necessary. The trade-offs are scanning time per frame, data transfer times, and processing costs.

Even more important is to ensure that no aliasing appears in the scans. In digital imaging, aliasing is a term used when patterns are created in an image that were not in the original. It can be avoided by scanning in a higher resolution (oversampling) and then resampling the image to a lower spatial resolution with a digital filter or scanning the image at a lower spatial resolution and using optical filtering. However, using an optical anti-aliasing filter tends to produce less sharp images. See figure 2.

The ARRISCAN scans with a native resolution of 3K or 6K and produces in this manner an oversampled image for 2K or 4K output resolution.

On the recording side spatial resolution is also an issue, since it must be guaranteed that the digital 2K or 4K files can be transferred onto film without any losses. The ARRILASER fulfills highest demands by recording out high quality 4K files in the unsurpassed speed of 2.9 s/frame.

Dynamic Range

Dynamic range is another key issue. The scanner needs to capture the entire density range of the negative. Even though this scan may contain information that will not be shown in the final image, it provides headroom to change the dynamics of a shot to fit creative requirements.

Together with dynamic range comes the question about quantization. A scanner has to translate the continuous range of densities in a negative into discrete digital numbers. The bit depth, or number of bits per channel, determines how many levels can be encoded. Usage of 8 bits results in 256 levels, a number generally considered to low for negative film scanning. One needs at least 256 levels per channel to produce a perfectly smooth tonal scale. Lower quantization breaks graduated colors into visible blocks. The effect is called banding or contouring and is illustrated in Figure 3.

Color processing may make banding visible where the tonal variation of the original image appears to be smooth. Therefore, scans with a higher bit depth than 8 bits are needed as source for the color correction. To achieve a good compromise between file size and dynamic range, the Cineon format has been introduced by Kodak in the early 1990s. Until today it is a commonly used file format for film scans. It uses 10 bits per channel, which equals 1024 levels, and linearly encodes the densities of the negative above base.

One code value in the cineon file represents 1/500 log density in the negative. That´s why it is usually called a logarithmic file format. The base density of the negative is encoded as 95 rather than zero, which is to ensure that the complete film grain is captured in the scan. Recording the Cineon image with an ARRILASER on an intermediate film results in a digital IN (inter negative), containing the full dynamic range of a negative shifted by 0.19 log D. This is equivalent to an optically copied IN (inter negative).

As about 500 16 bit code values represent the range between Code Value (CV) 800 to 900 in the cineon format, a 16 bit sensor is necessary to produce enough information for the desired resolution of 1/500 log D. See figure 4.

Using 14 bit A/D converters the ARRISCAN reaches the high dynamic range by combining a low and a high frame. The scanner takes two exposures per channel. The first one (Figure 5 left hand side) captures the negative densities up to 1.0 log D. The second one (Figure 5 right hand side) is done with 10 times more light and captures the densities above 1.0. Both images are combined into a true 16 bit file.

Low and high image used in the ARRISCAN: The upper right image is captured with ten times more light than the image on the left side. (If you are thinking the order is reversed, remember that a negative image is scanned.) The left image is shifted down by the equivalent of 1.0 log D, while the right image is shifted up. The resulting images in the second row are combined into one.

On the recording side the ARRILASER is the only film recorder that produces the same density range as an optically created Internegative, which is usually 2.0 log D above base. It is important that in the DI process the recording is done on intermediate material to avoid introducing of additional grain.

Color Management

A Cineon file presents a positive image but otherwise it keeps the characteristic of a negative. One could regard it as the digital version of an Interpositive. It looks ‘flat’, the blacks are too high, and the whites are too low since the additional tonal values below and above are linearly encoded as well. Like film negatives, cineon images are not meant to be judged by the human eye.

Nevertheless, a colorist has to use those files for color correction. This is usually performed on a CRT Display or a DLP projector. By applying a 1D-LUT (see figure 6) the grayscale characteristics of the image would be displayed correctly, but the colors of the film would look neither like the colors of a print from the OCN, nor like the colors of the print from the recorded IN. Therefore a 3D-LUT (see figure 7) is necessary to display digital images that match the tone scale and colors of print film.

1D – LUT: In a Look Up Table (LUT) each input value is processed without looking at the values of the other color channels. In the example the red input value “50” is transformed to “70” no matter what the values in the green and blue channels are.

3D- LUT: A 3D LUT defines for each input color triple an output triple, in the example the red input value “50” is transformed to three different output values depending on the green and blue values.

ARRI offers 3D LUTs for several grading and display systems.

Summary

A thorough knowledge of the DI process is necessary to maintain the maximum image quality and to adequately judge the necessary compromises between time, budget and quality. The ARRISCAN and the ARRILASER are able to deliver the highest quality for the DI process. Processing images of high quality throughout the complete chain gets more and more feasible because of ever increasing computing power.

This article is an excerpt of the DI Handbook written by Harald Brendel. To read the full length abstract, please visit our web side www.arri.de

Harald Brendel and Sibylle Maier