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MillimeterDigital Cinema's Special K
Sep 1, 2003 12:00 PM, By Matthew Cowan
Is There an Economical Way to Bring 4K Processing Into the 2K Post World?
 Figure 1: Standard scanning resolution for Academy, Scope, and Super 35mm aperture in 2K and 4K. |
In the standardization discussions for digital cinema distribution, there is an ongoing debate over how much resolution is required for preparation, delivery, and display of theatrical images. How much resolution is necessary to deliver higher quality than film distribution currently offers? What is possible, what is practical, what is necessary, and what is affordable? Is 4K resolution required at all stages, from capture to display, to preserve adequate image quality?
The ultimate solution must satisfy a number of concerns. It must maintain the creative intent, provide better image quality than currently available, and it must present an economically compelling business plan.
To understand the debate, let's first examine the background of 2K, 4K, and resolution in general. For modern feature film production, with few exceptions, digital images are processed at 2K resolution (see Figure 1, next page).
A 2K workflow would normally mean that the original negative is scanned at 1828 pixels (for Academy or flat), is processed at that resolution for effects, and written to film at 1828 pixels. Similarly, a 4K workflow would operate at 3656 pixels. In each case, output may be for digital projection, which means it would be resized to suit the resolution of the digital projector — for example, 2048 pixels.
The 35mm film resolution debate
This is a contentious issue. Due to the transfer process, a film-based workflow loses information at each step in the chain. This information loss starts in-camera, where lens and film movement reduce resolution of the images captured. The film itself has resolution limitations, and each stage of replication further loses information.
 Figure 2: Note the aliasing on the fishing line and around the lamp that is present on the 2K scan, but not on the 4K. This indicates that 4K scans are preserving information from the original that is not available on the 2K scans. (Images courtesy of Cintel International Ltd.) |
Laboratory tests using carefully written test patterns on the film don't tell the whole story. From a practical approach, we can test the lens, camera, and film system by scanning an image and looking for aliasing, or jagged diagonal lines. If there are no such artifacts, then the sampling structure is more than twice the information content on the film. If there are “jaggies,” then the information content is close to (or greater than) the sample structure.
Figure 2 shows highly magnified 4K and 2K scans from the center of a full-frame 35mm image. Note the diagonal lines — very fine information on the lamp, fishing pole, and line. The 2K version of the shot shows alias artifacts, while the 4K scan shows smooth lines. This is an indication that the 2K sample structure is inadequate for sampling the image content, while the 4K is adequate. It is also possible to see these differences in the visual clarity and detail present in 4K and 2K scans.
Figure 3 (above), for instance, shows another image, shot on Kodak 5245, low-speed, micro-fine grain, daylight negative film. Compare the 2K and 4K scans, and you can see the additional details in the eyelashes and the increased sharpness in the eye visible in the 4K version. The structure of the grain on the film starts to become apparent in the 4K scan, though it is almost invisible in 2K. A 4K scan definitely carries more information than a 2K scan.
Scanning resolution, generation loss, MTF
We have seen that the original camera negative supports enough image information to (technically) justify a 4K scan. But what happens as this image travels through the processing chain?
Each time the image information is transferred to a different medium it suffers from generation loss. This occurs because film is not capable of fully capturing all the information from the scene at the highest resolution. In fact, through the generations from negative to I/P to I/N to print, the contrast of the finer details diminishes until those details finally disappear. Technically, this is described as modulation transfer function (MTF). Figure 4 (on page 38) shows a typical MTF curve for camera negative film.
 Figure 3: Close-up of 2K and 4K scans. Note that the 4K scan carries more information and detail than the 2K scan, especially in the eyelashes and the overall sharpness of the eye. Also note the increased grain visible in the 4K scan. (Images courtesy of Cintel International Ltd.) |
It is important to understand that MTF is multiplicative through process steps. When two processes are cascaded, the resulting MTF is the convolution (or multiplication) of the curves. Fundamentally, this means that each process will further degrade the modulation of fine detail, causing it to disappear sooner.
As an example of this, look at Figure 5. This is a comparison of 4K scans of original negative and an interpositive. The I/P has been contact-printed from the original negative. Note, in particular, that the scan of the I/P is noticeably softer. This is the result of generation loss. Additional loss is incurred at each step as the film is printed from I/P to I/N to release print.
A technical analysis can model the entire process from the scene to the screen and determine the actual amount of information that will reach the screen. The net effect of this cascading effect is that there is value in having one or more higher resolution steps in the processing chain.
Of particular interest in the MTF curve are the mid levels of detail between 10 and 20 lp/mm on film. This band carries the information that the human visual system is most sensitive to, and therefore, most interested in. If the contrast in this region is higher (as shown by the curve being higher), then the image will appear sharper, without actually having higher resolution (see Figure 4).
Generation loss and MTF degradation in the film postproduction chain result in projected film images with substantially lower performance than those same images captured on the original negative. There have been a number of studies done that support this conclusion, illustrating that the information contained on typical release prints is only some of what was contained on the original negative.
Diminishing Returns and Economics
We have seen that there is definitely more information carried in a 4K resolution image than a 2K image although, for many images, the difference is subtle.
 Figure 4: Typical MTF curve for Kodak 5218 film (brown curve), and the system MTF curve resulting from processing an image through a camera lens, then onward to intermediate stock, and eventually to release print. Note that the contrast of the mid-resolutions (20 lp/mm to 60 lp/mm) has significantly reduced from the system point of view from the original negative. (Chart created by Entertainment Technology Consultants.) |
Thus, while working at higher resolutions clearly offers better visual performance, it does so with diminishing economic returns for the extended effort. Since a 4K image contains four times as many pixels as a 2K image, with a proportional increase in scanning, rendering, and film output times, as well as four times as much storage required for the raw and final data, a significant cost multiplier exists when embarking on a 4K post process. This increased cost results in higher postproduction budgets, and longer schedule time for managing and rendering 4K data, instead of 2K.
The economics of 4K postproduction processing are likely to improve as processing power increases and disk storage costs drop in the future. Still, in the current film environment, almost all digital film work is being done at 2K, mainly due to the time and economic constraints associated with 4K work. In other words, the improved performance is not generally considered to be worth the cost by the industry's financial gurus.
How does this affect the digital cinema debate? Obviously, in today's production environment, the cost/performance trade-off has settled on 2K as the standard resolution for production of feature films. The current argument suggests that digital projectors are capable of maintaining 2K resolution right to the screen, and therefore, this is a superior approach to current release print performance.
 Figure 5: Scans of original negative and scans of corresponding I/P. Note the generation loss apparent in the print from negative to I/P. (Images courtesy of Cintel International Ltd.) |
Thus, the debate is about enhancing image quality for the future, not about maintaining the status quo.
The economic hurdles largely revolve around the cost of 4K projectors and developing 4K theater infrastructures. Certainly, developing a 4K projector is difficult. R&D investments to scale up from 2K to 4K are currently huge. And right now, these investments are supported by a fairly small potential market of worldwide movie screens. This class of performance is not very interesting to the multimillion unit consumer markets, and so they hope to amortize the R&D over much smaller volumes. This, in turn, renders the 4K projector economically unattractive.
Similarly for the 4K in-theater infrastructure of servers and high bandwidth data links between servers and projectors, the hardware requirements to support 4K are large, and the cost of supporting realtime 4K image bandwidth to the projector is prohibitive.
It is useful to look to the film world for guidance on the issue of economic trade-offs. It is well known, for instance,that 70mm produces a superior on-screen image to 35mm, but the economics and practical aspects of producing and projecting in 70mm have kept the format out of the mainstream.
Thus, the question is whether 2K is good enough, and if so, how to maximize its performance through appropriate postproduction techniques. Is there a compromise that will satisfy both the drive for higher quality, and the economic realities?
2K, 4K, Digital Cinema
This brings us back to the 2K vs. 4K argument. We have seen that there are visual merits in maintaining high resolution in one or more stages of the image-processing chain, even if the others are limiting.
 Figure 6: Comparison of 2K and 4K origination as displayed on a 2K projector (simulated). The 2K scan represents a 2K image directly projected from 2K data. The image on the far right represents a 4K image downconverted and projected at 2K. (Images courtesy of Cintel International Ltd.) |
It is instructive to compare the visual quality that would result from a 2K scan-process-display pipeline to a 4K pipeline. Generally, we would see some visual benefits from the 4K pipeline, as demonstrated in the images that accompany this article. Those benefits would be small, but apparent to the well-trained viewer in an optimal environment.
Economic issues therefore enter the argument. It is extremely unlikely that a 4K projector and server system will be economically available to the cinema market in the near term. 2K performance represents the best available for some time to come.
Does this mean that digital cinema cannot move forward? Not exactly.
Recent demonstrations of 2K resolution imagery from Texas Instruments and Kodak have shown resolution and image detail that is better than what 35mm film prints deliver to cinemas today. These 2K projectors, however, can be made to look even better.
These demos were done using postproduction workflows that scanned and processed at 1920 resolution with sub-sampled color. Is there a way to improve on this?
Because the image degradation is multiplicative step-by-step, if one or more steps can be performed at higher quality, visual benefits will result. Consider Figure 6: Both images are shown at 2K resolution, but one was scanned and processed at 4K, the other at 2K. Note the difference — while subtle, the 4K-scan-originated image clearly preserves more of the original information.
A Practical Solution
The solution to the question of how to develop cost-effective 2K/4K postprocessing systems involves complex trade-offs between absolute image quality, practical economics, and the determination of an industry-standardized acceptable level of performance.
Clearly, a 4K end-to-end system would be preferable to a 2K system, much in the same way that 70mm would be preferable to 35mm in film, if it were practical.
Unfortunately, this would render digital cinema cost-prohibitive. The current 2K prototype projectors being tested around the country show that 2K digital pictures projected on a large screen are outstanding and already capable of offering a better picture to paying audiences.
The 2K vs. 4K debate thus requires a practical solution: Mixing 4K and 2K processes will yield higher quality results than a 2K workflow alone. Scanning and postproduction processes done at 4K and converted to 2K for distribution to theaters will provide outstanding results on a high-quality 2K projector.
A compromise solution of 4K capture and processing, distributed at 2K resolution and playing back on a 2K projector, will yield superior visual images to a 2K end-to-end system, with the financial effects occurring in the mastering stage, not at every theatrical installation.
Matthew Cowan is a principal at Entertainment Technology Consultants, an organization specializing in the science and applications of digital cinema technology. He has more than 20 years experience in the development and application of new products in the media and display fields.
His background includes development of electronic projection systems, analysis of color reproduction issues in electronic displays, strategic technology sourcing, reviews of advanced electronic projection products, and detailed analysis of compression schemes for digital images. Cowan was instrumental in developing the current mastering processes used in digital cinema, which introduced the use of the digital mastering theater for color and dynamic range adjustment.
The author would like to thank Peter Swinson for discussions and insight into 4K scanning, and Cintel International Ltd. (www.cintel.co.uk) for kindly providing an excellent set of images for this article.
Continue the discussion on “Crosstalk” the Millimeter Forum.