UNDERSTANDING ASPECT RATIOS

Transcription

1 UNDERSTANDING ASPECT RATIOS The transition to digital video is in full swing. Ten years ago, the first DVD players appeared on the market and the first digital TV broadcasts commenced in North Carolina and Texas. Home theater was a niche category, plasma and LCD displays were expensive luxuries, and the majority of TV sets were still equipped with picture tubes. Today, digital video and audio is ubiquitous. CRT TVs have largely disappeared from store shelves, and more homeowners can afford big-screen displays, including projectors. DVD is one of the most successful consumer electronics formats of all time, and there s a new wave of high-definition DVD players coming to stores. Consumers may not always understand what digital means, but they clearly want it. Of course, there are always some bumps along the way when new standards are adopted, and digital video is no exception. WHAT S MY SIZE? Perhaps no topic is more confusing than image aspect ratio, simply defined as the ratio of the height of a video or film image to its width. The new crop of HDTV front projectors show wider images than most consumers have been used to watching in the past. What s more, there are several different standards for aspect ratios in use for motion pictures, television, and computer displays. As television grew rapidly in popularity during the early 1950s, movie studios worried that attendance at theaters would suffer. To fight back, they experimented with new viewing experiences that television couldn t match, such as 3D projection and multi-screen panoramic movies (Cinerama). There were many problems with these systems. 3D projection required each member of the audience to wear special glasses, which couldn t be used again for hygiene reasons. Cinerama, although breathtaking in its use of superwide screens and clever camera techniques such as filming from a moving rollercoaster, was mechanically complex and expensive, requiring three synchronized projectors in every Cinerama theater. SQUEEZE PLAY Eventually, 20 th Century Fox hit on a more practical solution. Instead of using multiple cameras and projectors to create widescreen images, they instead rigged a single camera with a special lens that would anamorphically distort the recorded image, compressing it severely within the normal width of a 35mm movie frame. During projection, a complimentary lens fitted over the projector would restore the image to its original width. Custom masks in the cameras and projectors cropped the top and bottom of the projected frame, resulting in movies that literally filled with width of the theater. Here was an experience that couldn t be duplicated at home at least, not for another 50 years! For many years, the standard aspect ratio for motion pictures measured roughly 4 units wide by 3 units high, expressed commonly as 4:3. This ratio, which works out to 1.33:1, was the standard size of both 35mm and 16mm motion picture frames. Figure 2a-b: Anamorphic image compression (above) and expansion to correct image size (below) Figure 1-4:3 aspect ratio When television was developed and standardized in the 1930s and 1940s, the 4:3 aspect ratio was also adopted. At one point, the only way to make a recording of a television program was to use a motion picture camera, focused on a tiny TV screen, to produce a kinescope. The close match between aspect ratios essentially covered 100% of the TV images.

2 Numerous companies experimented with a wide range of widescreen image formats during the 1950s and 1960s. One of the first anamorphic films produced by 20 th Century Fox was for AT&T. It used the company s new CinemaScope widescreen image format to promote (of all things) telephones in every room! CinemaScope actually had several different aspect ratios, but the most common is 2.35:1, producing an image that measures 2.35 times as wide as it is tall. Other systems included a 1.66:1 aspect ratio (popular in Europe); 35mm film projected sideways (VistaVision), 70mm film (Todd-AO and IMAX), and selected cropping of the 35mm film frame (Super 35 and 1.85:1.) Today, the majority of motion pictures are shot in the Panavision 2.35:1 format (which eventually replaced CinemaScope) or a 1.85:1 format. Figure 4. White dotted lines show possible slices of the widescreen image during pan-and-scan mode. Center crop: For 1.85:1 films, a center crop of each film image is used, throwing away any information at the extreme left and right edges of the film frame. Only that portion of the film frame that equals the coverage of a TV screen is seen. This brute force technique is also known as full screen. Figure 3. The HDTV 1.78:1 aspect ratio accommodates existing video and film image sizes. Figure 5. Center cropping is simple and effective, but ruins the composition of the original image. With the advent of HDTV, television has a new aspect ratio to deal with, too. The proportions of an HDTV image, regardless of the HD format used, measure 16 units wide by 9 units high, or It may seem like that number came out of the blue, but it s actually a reasonable compromise between 4:3, 1.85:1, and 2.35:1, not to mention some of the new computer display formats, including 16:10 notebook computer screens. Letterboxing: The full width of the original widescreen film is seen on the screen, but much of the height of the screen consists of black bars. While this technique preserves the director s original aspect ratio, it results in a band-aid picture on the screen. Image detail is also lost. When viewing on a standard definition screen, the effective vertical picture resolution of a CinemaScope film is only 274 lines, while a 1.85:1 film is reduced to 340 lines of detail. RECTANGULAR PEGS IN SQUARE HOLES Of course, all of these new aspect ratios create numerous problems when the dimensions of the movie or TV show being viewed doesn t match the screen on which you are watching. If a 4:3 aspect ratio screen is in use, there are several options to enable viewing of widescreen movies none of them satisfactory. Pan and scan: This system was devised by TV networks to broadcast widescreen movies with the rule that the full height and width of the TV screen be filled at all times. During the transfer from film to video, the image is electronically scanned with the point of view shifting from one character to another, or one object in the scene to another, essentially destroying the creative vision of the director. Figure 6. Letterboxing preserves the full width of the image, but leaves black bars above and below.

3 The introduction of 16:9 TV screens and projectors provided some relief. With a 16:9 screen, all of the content in a 1.85:1 film is seen and only thin black bars appear at the top and bottom of the frame. Some HDTVs and projectors actually crop in slightly on 1.85:1 movies to eliminate those bars, cropping out about 6% of the image width. Those 1.85:1 films are actually mastered as anamorphically squeezed frames, and identified on their packaging as being anamorphic widescreen or widescreen version. The HDTV or projector simply expands them back to their full width before they are displayed, allowing conventional projection lenses to be used. CinemaScope movies are also anamorphically squeezed and expanded, but still show up as letterboxed images on a 16:9 screen, just not as a tiny little band-aid. The black bars at the top and bottom now make up only 25% of the total image, unlike the 44% unused area seen on a 4:3 screen. Here are some descriptions of aspect ratios found on DVD movie cases and what they actually mean. Figure 8. A common description for standard (4:3) aspect ratio presentations. Standard The movie was originally shot and presented in 4:3, and the DVD uses the same format. 4:3 was the standard for movies prior to 1953, and some films shot after that date also used this aspect ratio. Standard Version - This film has been modified as follows from its original version. It has been formatted to fit your screen. The original widescreen movie was cropped or panned and scanned to fill a 4:3 screen. Figure 9. This movie is letterboxed into a 4:3 frame and is not true anamorphic widescreen. Figure 7. A CinemaScope image, letterboxed on a 16:9 aspect screen with 25% black bars. It s important to understand that anamorphic squeezing preserves all of the horizontal image resolution in a movie transferred to red or blue laser DVD, unlike a letterboxed widescreen transfer. The latter format consists of 4:3 letterboxed widescreen images (i.e. with black bars top and bottom), and the projector must then enlarge these images to fit the width of the screen. This process, of course, decreases image sharpness noticeably when using a 4:3 projection system. The decrease in resolution is even more pronounced when the image is blown up to fill the available imaging area on an HD projector. Widescreen The movie was originally shot and presented in a widescreen format (often 1.85:1) and the DVD generally uses the 1.85:1 anamorphic format. Some early widescreen DVD releases were actually letterboxed, but not described as such. (See figure 9, above.) Anamorphic Widescreen The movie was shot and presented in 1.85:1 or 2.35:1, and the image is squeezed horizontally so that it appears correctly on a 16:9 screen. Anamorphic DVDs will not be sized correctly when shown on a 4:3 screen. Enhanced Widescreen Version (Matted) This description means the DVD transfer is letterboxed, and may also have to be zoomed-up to fill the width of a widescreen TV. WHAT YOU SEE IS WHAT?? Understanding the actual aspect ratio that has been used on a given DVD can be particularly frustrating. In the early days of DVD releases, it was common to use a full screen (4:3) presentation and letterbox any widescreen movies to fit the width of the screen. Unfortunately, many of these movies were labeled as being widescreen or enhanced widescreen, which many viewers believed to be true widescreen movies not letterboxed. The move to labeling true widescreen DVDs as anamorphic widescreen helped to alleviate some of the confusion. Figure 10. The movie on this DVD happens to be a true anamorphic widescreen transfer. Presented In the Original Theatrical Aspect Ratio (1.85:1 or 2.35:1) The DVD has been mastered with an anamorphic video transfer and requires a 16:9 screen to be viewed correctly. Widescreen TVs and projectors have become very popular in the past half decade, rapidly displacing 4:3 TVs and projectors from the consumer marketplace.

4 As a result, the trend is to use widescreen and anamorphic widescreen to mean the same thing when labeling DVDs, while fullscreen denotes any program presented in a 4:3 aspect ratio, whether a letterboxed movie or a TV show. EXPAND YOUR HORIZONS CinemaScope movies still suffer from a decrease in resolution when projected as letterboxed images. Even when a 1920x1080 projector is used, the resulting vertical image resolution is only 810 pixels. Therefore, for cinema purists, the ultimate quest is to project CinemaScope movies to their full width and height while eliminating any black bars, top and bottom. This may seem like a real technical challenge, but it s actually easy to do and borrows from the playbook of 1950s movie studios. The CinemaScope movie is transferred to DVD as a series of anamorphic frames, with the full width of the 16:9 video frame being used to contain each squeezed image. Because part of the image still contains black bars, the height of the image must also be expanded in the projector. This is accomplished with a non-linear vertical image stretching process, using all available pixels for maximum screen resolution, top to bottom and side to side. Unlike 1.85:1 projection, where the image is simply expanded electronically and a normal lens is used, 2.35:1 projection requires an anamorphic lens to expand the compressed image to its correct proportions. The lens is attached to a precision motorized mount and moves in and out of the optical path automatically. Needless to say, the screen must also be in the 2.35:1 image format. Thus, a CinemaScope movie can be watched at its full height and width at maximum screen resolution as can a subsequent 1.85 movie or 1.78:1 HDTV program, requiring only slight adjustments to the projection screen for each to mask the edges of the projected images (side drapes or top/bottom drapes or masks). There are motorized screens that will automatically do the re-sizing as the anamorphic lens is engaged or disengaged. CONCLUSION The adoption of 16:9 aspect ratios for front projection and the development of affordable anamorphic projection lenses for home theater use have made it possible to enjoy both widescreen movies and high definition television programs using the same front projection system and projection screens with automatic masking. APPENDIX A There are currently two methods of presenting a video image that are used for standard definition and high definition television. The first is interlaced scanning, which was first developed in the 1920s to conserve TV channel bandwidth. In an interlaced scan, half of the information in a video frame is presented to the display in 1/60 th of a second (1/50 th in the PAL system). The scanned image consists of every other odd-numbered line, starting with line 1 at the top of the video frame. The even-numbered scan lines are transmitted next, starting with line #2 and skipping every other line as before. This also takes 1/60 th of a second. Two complete sets of scans (odd fields and even fields) make up an interlaced video frame. Each complete video frame is refreshed at 1/30 th of a second (the actual frame rate is 1/29.97 Hz), or 1/25 th of a second for PAL TV. For 1920x1080i HDTV, 540 lines are present in the odd field and the remaining 540 lines are in the even field with the same field (1/60) and frame (1/30) rates. Progressive scan presents all horizontal rows of picture information in sequence from line 1 to the bottom of the visible frame. This process takes 1/60 of a second in the 1280x720p format (1/50 in PAL). The advantage of progressive scan is that it is easier to match to a projector, which also uses progressive scan, without introducing motion discontinuity errors between the odd and even fields in 480i, 576i, and 1080i TV signals. At the present time, no movies are being mastered to red or blue laser DVDs specifically for CinemaScope projection. Interlaced Scan with ½ Progressive scan with of all scan lines (odd #) all scan lines present These movies will have black bars above and below the active image area, so the combination of nonlinear image stretch and anamorphic lens placement is always required to present these films in the correct aspect ratio.

5 APPENDIX B Table I. Common television and computer display resolutions with aspect ratios Standard Resolution Scan Format Aspect Ratio Total Pixels NTSC 720x525 (483 visible) Interlaced 4:3 347,760 PAL 720x625 (576 visible) Interlaced 4:3 414,720 VGA 640x480 Progressive 4:3 307,200 WVGA* 848x480 Progressive 16:9 407,040 SVGA 800x600 Progressive 4:3 480,000 XGA 1024x768 Progressive 4:3 786,432 HDTV 720p 1280x720 Progressive 16:9 921,600 WXGA** 1366x768 Progressive 16:9 1,049,088 SXGA*** 1365x1024 Progressive 4:3 1,397,760 SXGA+ 1440x1050 Progressive 4:3 1,512,000 UXGA 1600x1200 Progressive 4:3 1,920,000 HDTV 1080i 1920x1080 Interlaced 16:9 2,073,600 HD PC 1920x1200 Progressive 15:9 2,304,000 * - WVGA also includes 852x480 pixel displays ** - WXGA also includes 1280x768 pixel displays *** - SXGA also includes 1280x1024 (5:4) displays Table II. Effective display resolutions of format-converted television signals Native Signal Native Aspect Ratio Resolution of Display Cropping Effective Resolution Unused Area? NTSC 4:3 852x480 Center 4:3 720x480 None NTSC 4:3 1280x768 Center 4:3 1024x768 Side Bars NTSC 4:3 1280x720 Center 4:3 960x720 Side Bars NTSC 4:3 1920x1080 Center 4:3 1440x1080 Side Bars NTSC 4:3 1366x768 Center 4:3 1024x768 Side Bars 720x480p 16:9 1024x768 Letterboxed 1024x576 Top/Bottom Bars 720x480p 16:9 1280x720 Full Frame 1280x720 None 720x480p 16:9 1920x1080 Full Frame 1920x1080 None 1280x720 16:9 1024x768 Letterboxed 1024x576 Top/Bottom Bars 1280x720 16:9 640x480 Letterboxed 852x480 Top/Bottom Bars 1920x :9 640x480 Letterboxed 852x480 Top/Bottom Bars 1920x :9 1280x720 Full Frame 1280x720 None