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Now You See Them Now You Don’t: Colorimetry and Electronic Cinema

Electronic cinema is a hot topic for 1999, drawing lots of interest frommajor studios, theater owners, and large-screen display manufacturers. Theemergence of high-definition television is adding more fuel to the fire, asare digital playback formats and satellite program distribution.

Until recently, the biggest stumbling block to electronic cinema had beenthe projectors themselves. It was not technically feasible to project largewide-screen images with brightness, contrast, and color saturation thateven came close to 16mm film, let alone 35mm. But that has all changed inthe past six years with the introduction of imaging engines based ontransmitted light (liquid-crystal displays), reflected light (TexasInstruments’ Digital Light Processing), and hybrid light-shutter(Hughes-JVC’s Image Light Amplifier) technology.

None of these technologies are ready to replace film immediately, butmanufacturers are making impressive demonstrations of film-like projectionquality on a daily basis. Texas Instruments has been showcasing standard4:3, 16:9, and anamorphic 2.35:1 electronic cinema to several studios usingthree 1280 x 1024-pixel digital micro-mirror devices (DMDs).

Hughes-JVC Technology, manufacturer of the super-bright ILA-12Kcinema-grade projector, recently announced a joint partnership withQUALCOMM in a new company called CineComm. According to Alan Brawn,director of worldwide marketing for Hughes-JVC, the two companies formedCineComm “to replace a century-old tradition of celluloid with anend-to-end digital system.”

There are many issues to be resolved before e-cinema can take its place inthe entertainment mainstream, such as security (how do you protect againstunauthorized copying of bitstream-delivered featured films?) and imagequality (what bit rate and compression scheme should you use for programdelivery?). But perhaps no issue will be more important to filmmakers thancolor.

The Vision ThingAs audiences, we crave film projection because it delivers pictures asclose to reality as we are likely to get in an “artificial” perceptualenvironment. With high resolution, random grain, wide contrast, and aseemingly infinite palette of colors, 35mm motion picture film has set avery high standard for electronic projection systems.

With the adoption of high-brightness short-arc xenon lamps and efficientoptical paths, the brightness of many high-end electronic projectors is nowon par with conventional film projectors. Improvements in switching speeds,aperture ratios, polarization, and optical coatings have taken electronicprojectors from the washed-out, flat images of five years ago to rich,contrasting blacks and whites.

Fixed-resolution displays such as DMDs and LCDs have now reached over 1000vertical pixels and are flirting with 1200, while resolution-independentimaging systems like the ILA and the liquid crystal light valve (LCLV) arecapable of more than 1500 vertical lines. In terms of brightness, contrast,and resolution, e-cinema projection systems are looking like seriouscontenders.

Color is the last variable to contend with. Color is both qualitative andquantitative, and it evokes many different responses from viewers. Artistsdebate it, printers go crazy trying to match it, art directors argue withpost houses about it, companies spend thousands of dollars trying tomeasure it, and cinematographers and lighting directors devote theircareers to manipulating it.

Color is a contentious issue for everyone from camera operators to transferhouses. Such debate is usually due to misunderstandings about how to createcolors, define color palettes (or “spaces”), reproduce and view colors, anddistinguish the differences between additive and subtractive color. To geta handle on the color quality requirements for electronic cinema, we shouldtake a closer look at the nature of color.

Color ABCsThere are two processes for the creation of colors. The first, known as”additive” color, is simply the combination of red, green, and blue (RGB)primaries to achieve a full palette of color shadings. The addition ofequal amounts of red, green, and blue results in white light, which we canverify by using a prism to refract these colors back out. Increasing theintensity of each primary color also increases brightness (luminance).

The second color system-“subtractive” color-is the basis for pigments,dyes, and inks. In this system, the addition of all three primaries-cyan,magenta, and yellow (CMY)-results in black, and the absence of all threecolors results in a white image. Increasing the density of each colordecreases brightness in a subtractive system.

Incident light from sources like the sun and projection lamps is consideredadditive. However, when light strikes any surface, portions of its colorspectra are absorbed or reflected. The reflected light gives us the colorof that surface, while the absorbed wavelengths are subtracted. If thesurface is an equal-energy reflector, the resulting surface color shouldhave no color impurities. If not, we may see a reddish or bluish tint tothe reflected image. (The sun is an equal-energy light source containing afairly “flat” spectral response from red to blue. Artificial lights likemetal-halide or tungsten-halogen studio lamps are not and are usuallydeficient in certain colors.)

I Need My SpaceThe total number of colors that exist in any system defines a color space,also known as a “color gamut.” There are several color spaces or gamuts,and each resembles a rough triangle shape. The coordinates for each pointof this triangle can be mathematically expressed as x and y coordinates,although the actual appearance of a color space is more like a 3D “anthill”with the third dimension (z-coordinate) being luminance.

The maximum shades of gray possible (luminance) and the three primarycolors used to mix and derive other colors determine the number of colorsdefined within any color space. Because the sun is capable of generating anintense amount of both visible and invisible (infrared and ultraviolet)light, its natural color palette-or color space-is by far the widest. Infact, there are millions of natural colors we will never see since our eyesare incapable of responding to them.

What complicates matters further is the fact that our eyes do not see allcolors with equal sensitivity. We are most sensitive to yellowish-greenlight (59 percent), followed by red (30 percent), and then blue (11percent). This human color response was first plotted in 1931 by theInternational Commission of Lighting (CIE in French), but it also forms thebasis of our present-day analog NTSC color television system and theCCIR-601 digital color specification. The 1931 CIE chart was the first toplot specific mathematical values, thereby defining a real, measurablecolor space.

Disappointed to learn that there are colors you can’t see? Guess what-thereare colors you can see that a television monitor can not reproduce. Thephosphors used in TV sets (as well as studio monitors) have their owncoordinates on the CIE diagram to ensure consistency from one monitor toanother. So, we must further truncate our color boundaries in order to staywithin a “phosphor-friendly” range. In North America, it is known as theSMPTE ‘C’ color space.

Now we can create any colors we wish for our broadcast electronic images,as long as they are derived from color values contained entirely withinthese boundaries. This may sound a bit restrictive, but withoutstandardization there would be no way to ensure a consistent color matchfrom one monitor to another. Because the SMPTE ‘C’ space is much smallerthan the visible color space, it is virtually impossible to display thesaturated colors created by rainbows, prisms, or “pure” coherent lightsources like lasers.

As television evolves into a digital form, there is a color space to gowith it. The CCIR-601 YUV standards determine the current boundaries. Thissystem has its roots in the CIE color diagram, but its colors aredetermined by digital sampling of luminance and two chrominance signals ina 4:2:2 ratio instead of the relative levels of analog voltages. The colorspace for HDTV-a pure digital format-is also very close to the CCIR-601 andSMPTE ‘C’ coordinates.

It makes sense to use the CCIR-601 color palette for all aspects ofelectronic media production and display regardless of whether one displaysthe final product on television, transfers it to disk or tape, or views itin a theater. The evidence so far is that film-to-601 data transfers-andeven 601 data-to-film projects-compare favorably to “pure” film-basedcolorimetry, assuming the data-to-film transfers are done with tightcontrols on color consistency.

For example, at selected screenings of “Saving Private Ryan,” audiences sawcinema commercials for Reebok and Sony intermixed with the comingattractions previews. The spots originated on 35mm stock and were posted asD1 video for television. Tape House Digital Film in New York took this D1material, eliminated the 3:2 pulldown and reduced the frame rate to 24fps,and then repositioned the material to 1.85:1 aspect. After checking for anyfield “glitches,” Tape House recorded the data to film at 2K resolutionusing proprietary pixel-scaling software and conversion from CCIR-601 YUVto RGB color space. The result? Image quality compared favorably betweenthe previews and these film-to-digital video-to-data-to-film spots.

Drop that MouseBy definition, the colors that exceed the 75-percent saturation limits ofNTSC are known as “illegal” colors, although you will not get arrested fortrying to use them. Print designers and art directors are often frustratedby the inability to reproduce specific CMYK print (subtractive) colors inthe CCIR-601/SMPTE ‘C’ phosphor (additive) space and often ask for colorsaturation levels to be boosted to get their special hue.

A creative colorist can “jigger” a monitor or play other tricks with colorto get a “buy” from a client, but it is a sure bet those colors will neversee the light of day when viewed on a conventional video monitor. Thereason? “Jiggered” luminance levels will fall outside the SMPTE ‘C’ space.If you need more saturation in printed color, you add more ink. However, ifyou increase luminance (and thus saturation) of a video color, you will runout of head room in the encoded, base-band NTSC video signal.

Some examples of “illegal” colors in the NTSC system that also fall outsidethe SMPTE ‘C’ color space include turquoise, lime green, indigo, cherryred, and pumpkin orange. (Not coincidentally, all of these shades arelocated near the very edges of the visible color space.) You may be able toreproduce one or more of these colors on a standard RGB monitor, which uses255 steps to dial in saturation levels and color matching systems likePantone. But you would need to transfer the data back to film or use adichronic RGB projection system to view them.

The shift to digital media production is producing many skilled coloristswho like to experiment within the precise coordinates of the CCIR-601 colorspace. By carefully tightrope walking along the edge of the SMPTE ‘C’curves, the resulting images have an almost “illegal” look with saturatedreds and oranges, glowing pastel colors, and even something that resemblesthe seemingly unattainable turquoise shading.

In the Thick of ItTo get a better perspective of color spaces and digital media production, Ispent a few hours at Tape House’s Advanced Imaging Center (AIC) in New YorkCity. AIC is a busy film-to-digital transfer and HDTV edit/post facility.With its sister digital-to-film operation Tape House Digital Film, AICcreates many spots, trailers, and other projects for both broadcast andtheatrical release. AIC installed the first Philips Spirit DataCine in theworld and is active in data conversion from film to both HDTV and DTV.

AIC’s vice president and Spirit DataCine director John Dowdell is wellknown for his transfers and coloring and has produced work for Miramax andMerchant-Ivory Productions, as well as Ken Burns’ Baseball, Lewis & Clark,and Frank Lloyd Wright miniseries. His transfer suite revolves aroundPandora’s Pogle color correction system, and the bulk of his work involvesconverting 35mm negative to 1920 x 1080 files.

“When we work with digital data transfers, we standardize on the 601 colorspace,” says Dowdell. “There’s absolute consistency and predictability tostaying within that color gamut that facilitates moving from film to data,doing composites, layering effects, and even transferring back to film.”The Spirit is capable of 4:4:4 sampling, recording luminance information at1920 pixels of resolution per line, and full-depth RGB color.

I viewed a mix of both 35mm and HD footage from a D5 machine using a smallSony VPL-W400Q 16:9 dichronic LCD projector and saw plenty of saturatedcolors.

“The Spirit records a much wider gamut of colors than can be seen withinthe SMPTE ‘C’ color space,” says Dowdell. “You’d need an RGB display toview them, but they’re in the data. When files are transferred back tofilm, all of those colors are retained.”

This means that a client may not be able to see a desired color unless anLCD or DLP projector were available.

“Pushing the limit on color saturation is a common request in TV spots likecosmetics,” Dowdell comments. “The Spirit has enough pixel sampling depthto bring out more subtle shades of a color like those seen in lipstick ads.The Spirit’s color matrix results in the correct luminance values for eachcolor. We can achieve excellent color saturation and absolute coloraccuracy.”

A few blocks away at Tape House Digital Film, David Kuttner, director ofresearch and development, works with a wider color gamut than SMPTE ‘C’-hisarea of concern is transferring data files back to film using Solitairerecorders, and he is currently looking at other high-performance filmrecording systems.

“We use dozens of color look-up tables for our Solitaire film recorders,and we’re constantly modifying them to preserve all the colors seen in 601data files,” says Kuttner. “Our calcits monitor the intensity of each colorplane. We’re also constantly checking film gamma to match the SMPTE ‘C’phosphors. It’s a never-ending process that involves lots of little tweaks.”

Kuttner agreed that achieving a “film look” will be the toughest challengefor any electronic cinema system. “Film has a greater color depth thanvideo and tracks color logarithmically like the human eye. Any electronicprojection system will need to support 36-bit color with 12 bits per colorplane. 150:1 contrast will probably be the minimum acceptable grayscale,although daylight film stocks can achieve 1,000:1 contrast.”

And what is the minimum resolution necessary to approximate film projection?

“Probably a 2K image to be acceptable-2048 x 1556 pixels,” says Kuttner.”That works out to an enormous file, typically 75 MB per frame. It alsoprovides more color shadings at the low end of the grayscale, key tofilm-like quality.”

Since none of the cinema-quality projectors currently for sale use 12-bitRGB processing, that would appear to be the next big hurdle in theevolution of e-cinema.

What about viewing colors in a transfer suite? Dowdell uses a BARCO 20-inchmonitor with a MegaCalibrator system for precise color matching andrecently ordered a Digital Projection POWER 3gv for projecting footage ontoa 9-foot-wide front screen.

“The colorimetry of the POWER 3gv is the best I’ve seen. I can get a goodmatch to the 601 color space, plus the digital modulation system results inmore accurate grayscale reproduction,” he says. “And grayscales are whatit’s all about if electronic projection is to approach film projection.”

Run the GamutIt would appear that consensus, not edict, is determining a “standard”color space for film-to-data transfers and eventual electronic projection.There is already a move to push for a 1080p, 24fps HDTV standard, whichwould be 100 percent compatible with film projection and eliminate any needfor a 3:2 pulldown in data transfer. Consensus will also dictate theevolution of this particular standard.

And so it goes with color space. Staying within the CCIR-601 color gamutmakes sense for another reason: Many cinema-grade electronic projectionsystems are adding direct serial digital inputs for both D1 and HDTV signalsources to ensure that colors in the final projected image will match thecolors seen in the film-to-data transfers regardless of how many copies aremade or when they are distributed.

With the eventual adoption of 10-bit and 12-bit pixel sampling, electronicprojection systems will be able to show a far wider and richer palette ofcolors-and that will help the close the gap between electronic and 35mmfilm projection.

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