Digital cinema projection: choosing the right technology - Christie

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Digital cinema projectionIt’s always good to have choices.Today, when deciding on which digital cinema projection system will best serve the needs of yourbusiness and the expectations of your customers, you have more choices than ever. First, youhave a choice between projection technologies; two today and potentially more in the future. Youcan choose between two approved pixel formats (resolutions). You have a choice of technologiesfor 3D, a high value add-on for digital cinema projection. And lastly, you have a choice of vendors.Not all of these choices are independent. And it’s not always apparent which of them is right for agiven theater. To clear up the confusion, this whitepaper will discuss the relevant technologies,their fundamental differences and show how they impact the factors that are most important fortheatrical exhibition: image quality, functionality, reliability and cost of ownership.What makes a good image?What makes a motion-picture image lookgood? Technically, projected images can bedefined by a number of different parameterssuch as brightness, contrast, color space, colordepth, resolution, device refresh rate and,for motion pictures, temporal resolution. Thesubjective evaluation of an image as “good”or “poor” is based on the overall effect of allof these parameters working together.Nevertheless, resolution in particular isoften sold as the most important parameter.However, most people would agree that adim, washed-out image with poor colors, asan extreme example, wouldn’t look good nomatter how sharp. Everything else has to be inplace for resolution to make a difference. Notall projection technologies are equally adeptat getting everything right.Counting pixelsSince resolution has garnered so muchattention, let us examine it in detail. Resolutionis often equated with the pixel format ofthe display device, hence it is said that theresolution of a “2K” projector is 2048 x 1080while that of a “4K” projector is 4096 x 2160.However, a display’s pixel format is not thesame as its resolution. The pixel format tellsyou only how much information an image cancontain, whereas resolution tells you how muchof that information is actually conveyed to thescreen and can (potentially) be seen bythe audience.Resolution depends on both the projectiontechnology and on the capabilities of thehuman visual system (hereafter HVS) in atheater environment.With respect to the HVS, optometristscommonly use the Snellen eye chart (Figure 1)to determine approximate visual acuity basedon the finest line of text that can be read froma certain reference distance. Normal acuity isidentified as 20/20 (or 6/6 in metric), signifyingthat a specific line on the chart can be readat a distance of 20 feet (6 meters). The finestdetails of the letters on that line (e.g., thestrokes in the letter E) are one angular minuteof arc wide, or 1/60th of a degree. If the chartwere presented as a digital image usingdiscrete pixels, 20/20 vision would then equateto resolving 60 pixels per degree.However, “resolving” in this context meansonly that we can identify a letter made up ofpixels. To clearly see the shape of each pixel(whether it is a square or a round dot, forexample), or any separation or gap betweenadjacent pixels that may exist, would likelyrequire even greater visual acuity.This is important in the context of digitalcinema, since all currently practical projectiontechnologies use square pixels with a smallinter-pixel gap.Brightness is also a consideration. In theSnellen test the chart should be adequatelyilluminated, such that the minimum luminanceof the white background is 120 candelas persquare meter (cd/m²) [1].This ensures dilation of the eye’s pupil to about3mm, resulting in maximum acuity [2]. Thenominal luminance of peak white specified fordigital cinema images, on the other hand, isonly 48 cd/m², more commonly expressed as14 foot-lamberts (fL) [3]. Combined with theeffect of a dark surround, this results in a pupildilation in the range of 4-6mm [2]. Becauseoptical aberrations increase with lens aperture,this causes a roughly 30% drop in visual acuity,equivalent to 20/28 vision or about 42 pixelsper degree [4].However, some studies have suggested thatnormal visual acuity could be as low as 44pixels per degree, which in a darkened theaterenvironment becomes just 32 pixels perdegree [5].Note that the above numbers apply onlyto a predominantly white image containingblack text, like the Snellen chart. The averageluminance of most real images in a movie,which contain a mix of colors and gray levels,is far lower, resulting in pupil dilations at thelarge end of the range and hence, even lessacuity. Very dark scenes in particular maycause the HVS to approach or even enter the“mesopic” region, in which it is more sensitivethan normal to light, but with less colordiscrimination and with far less acuity.Looking sharpRegardless of what specific acuity numberapplies while watching a movie in a theater,it tells us only about the finest details wecan resolve. As it turns out, due to anotherproperty of the HVS the finest details donot contribute much to our perception ofsharpness. A relationship called the ContrastSensitivity Function (Figure 2) describes howdifferent spatial frequencies (the rate at whichlight and dark lines alternate) are seen withdifferent amounts of contrast. We see detailsmost clearly – that is, with the greatest1
digital cinema projectionFigure 1Typical Snellen Eye ChartFigure 2Contrast Sensitivity Functioncontrast – in a range between 4 and 9 linepairs line per degree, equivalent to between8 and 18 pixels per degree [6]. An image thatreproduces spatial frequencies in this rangewith high contrast will look sharp. Higherspatial frequencies in the image contributesignificantly less to that perception.To understand the affect of contrast sensitivity,we must examine seating distances in atheater. Despite different screen sizes,the distances of seats from the screen intoday’s stadium-seating venues vary over aremarkably similar range when expressedas multiples of screen height. The closestseats are usually a little less than one screenheight away, while the farthest are somewhatcloser than three screen heights (Figure 3).From one screen height away, the screenwill span 53 degrees of our vertical field ofview. Horizontally, it will be anywhere fromalmost 100 degrees to nearly 130 degrees,well beyond what most viewers prefer toexperience. Choosing the more demandingend of the range of spatial frequencies thatare important for sharpness – 18 pixels perdegree – results in a minimum requirementof 956 vertical pixels (pixel rows). In thehorizontal direction, a flat (1.85:1) picturewould require a minimum of 1769 pixels.(For scope, the requirements will dependon whether the screen is constant widthor constant height.) If the pixel-to-pixelcontrast is very high for the most importantspatial frequencies, an image with thesepixel dimensions or greater will look sharp.This conclusion is consistent with thesubjective experiences of countless theaterpatrons viewing DLP Cinema ® images overthe past several years. The 2K images fromthese projectors, which fit within a full-frameimage “container” of 2048 x 1080, do indeedlook sharp, even from the front row of thetheater. This wouldn’t be possible wereit not for the high performance of DLP ®technology itself, which provides the extremelyhigh pixel-to-pixel contrast required.I see pixelsWhat then of 4K? As we have seen, it isnot necessary for achieving sharp imagessince even 2K projection provides sufficientresolution, assuming the use of DLPtechnology. The primary advantage of 4K is,rather, the reduction it offers in the visibility ofpixels, an issue that is entirely different thanresolution. For certain images (the closingcredits of a movie, for example) viewers seatedin the first few rows of a theater can easilydetect the square shape of the pixels in a 2Kimage of characters as they roll up the screen.The closest seats have traditionally alwaysbeen the least desirable for most patrons,since viewing angles are overly wide and neckinclinations are severe. In the case of a 35mmpresentation, film grain is also much moreobvious. However, because of its randomnature film grain may be less objectionable toviewers than a static pixel structure.A 4K image has twice as many pixels in boththe horizontal and vertical directions as a 2Kimage, which results in four pixels in the spacea single pixel occupies in a 2K image. Sincethe pixels are half the size in each directionthey are proportionally harder to detect. Thisbecomes more important for wider screens,since the increased width of the theater placesmore seats closer to the screen.One argument made for 4K is simply that ithas far more pixels than 1080p high-definitiontelevisions. Why would patrons come to atheater to see almost the same number ofpixels they can watch at home? This onceagain incorrectly equates image qualityto a single parameter: pixel count, and itconveniently ignores the fact that digitalcinema images are far, far better in other, moreimportant respects than even the best homevideo formats (e.g., Blu-ray Disc).Compared to Blu-ray Disc, digital cinemaimages have a wider color gamut, fourtimes the color information, up to sixteentimes finer gray shade resolution and havebeen compressed far more lightly with acompression scheme that inherently createsless visible artifacts (JPEG 2000).Another essential difference between cinemaand home theater is that very few homes arelikely to have a 20-foot wide or larger screen(most are less than four feet wide). Theargument that sitting proportionally closerto a smaller screen is equivalent to viewing alarge screen in a cinema ignores the fact thatour inter-ocular distance does not change –we sense the screen’s real size and distance.2
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Digital cinema projection: choosing the right technology - Christie

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