Aspect Ratio

Aspect ratio is a term used to describe the proportional difference between width and height. The values are often normalized so that the number on the right is 1. Sometimes they are normalized so that only integer values are used.

5.9.1 Common TV Aspect Ratio Formats

If you look at the back of DVD or video packaging you will see aspect ratios quoted.

Normal aspect ratio is taken to be 4:3 and widescreen TV is 16:9. Some broadcasters com-promise and deliver the content as 14:9. This allows for a small portion to be cropped on 4:3 aspect ratio TV sets, and a small black bar is present on either size of the picture on a 16:9 TV set. Figure 5-14 illustrates the various common aspect ratios in use.

Some common TV aspect ratios are summarized in Table 5-4.

5.9.2 Normal Aspect Ratio

Normal aspect ratio material is presented in a raster where the width to height ratio is 4:3. On a wide-screen monitor this is distorted to fill the screen, so a border is placed on each side to retain the original aspect ratio. Center cutout on a wide-screen TV sacrifices horizontal resolution. Letterboxing does the same when playing wide-screen content on a normal TV except that it wastes lines. In both cases, cropping is necessary to remove the wasted space if you are compressing for the Web. That will help to achieve the best possible compression ratio. There is an inevitable trade-off between quality and efficiency.

4:3 (12:9) 14:9 16:9

Figure 5-14 Aspect ratios.

5.9.3 Wide-Screen Aspect Ratio

Wide-screen presentation is based on the width to height ratio being 16:9 and uses the full visible raster area. There is no additional picture detail being provided to add pixels to either side of the 4:3 aspect ratio picture. It is just the standard 720-pixel width image with the pixels being stretched horizontally even more than they were for normal presentation.

Fortunately, the human optical mechanisms are very tolerant of a loss of detail in the horizontal axis when watching TV.

5.9.4 Stretched Display Mode

Manufacturers of wide-screen TV sets added a “justify” display mode where the horizontal scanning is purposely nonlinear. This is another way to work the compromise between wide-screen and normal aspect ratio. The middle section of the wide-screen is not stretched as much as the sides. A circle placed in the middle of the screen still looks roughly circular. Objects placed nearer the edges are stretched more and more as they move away from the center of the screen. Fortunately, you can turn this stretching feature on and off and select the viewing mode you prefer. The original linear geometry is shown in Figure 5-16 for reference.

The effects of the “justify” distortion are visible in Figure 5-17. This is fine most of the time but shows up when text crawls horizontally across the screen. It moves faster at the sides and slows down in the middle. Because you are able to see the whole screen, Table 5-4 Common Aspect Ratios

Description Aspect ratio Normalized value

Standard TV 4:3 1.33:1

Widescreen TV 16:9 1.76:1

Compromised broadcast 14:9 1.56:1

4:3 image padded each side to fill 16:9

Blank BlankVisible region on 16:9 screen

Figure 5-15 Ratio of 4:3 on a wide-screen raster (center cutout).

your eyes focus on a single word at a time and it appears to squash and stretch as you follow it.

Sometimes a presenter will walk across the screen and appear to go on a very rapid slimming regime as the person reaches the center, and then experience a massive weight gain as he or she moves toward the side again. You are strongly advised to let the TV set do this and not try to implement this stretched mode in your content.

Oddly enough, the justify mode seems to work quite well most of the time and is an improvement over simply stretching the 4:3 raster across the 16:9 tube front. It is definitely better than a center cutout with black sides that look very odd after watching stretched 4:3 for a little while. However, purists may disapprove of this technique, and it will very likely offend experienced TV engineers.

5.9.5 Wide-Screen Signaling

Broadcasters transmit signals in hidden TV lines that the TV detects and uses to switch the display mode to a suitable setting for the program. It is quite disconcerting when the TV Figure 5-16 Original geometry.

Figure 5-17 Example of warped nonlinear scaling.

does this as the switch takes place in the scanning circuits in some models. Sometimes the electronics make a loud electrical crackling noise as the voltages change.

Ideally this would be done digitally and the TV scan circuits would simply continue scanning a constant-sized raster.

This is covered in more detail in Chapter 39, which discusses some advanced topics related to hidden data signals.

5.9.6 Film Aspect Ratio Formats on TV

Many movies are delivered on DVD in a wide-screen letterbox format that shows the entire picture. These days this is often standardized to cinemascope or Panavision format, which has an aspect ratio of 2.35:1. Adjusting the vertical zoom factor of the TV presents a larger picture, but it sacrifices consistent scaling in X and Y. Figure 5-18 shows how these formats are fitted onto a TV screen.

Figure 5-18 Panavision aspect ratio.

2.35:1 on 4:3 with letterbox cutout

2.35:1 on 16:9 with letterbox cutout

With the plethora of aspect ratios and built-in zoom and stretch modes on modern TV sets, it is very likely that almost no TV services and movies are broadcast in the aspect ratio that the program maker or movie director intended. But the human eye and vision system is remarkably good at compensating for this and consumers seem to not be unduly concerned. But viewers are occasionally surprised when meeting a celebrity face-to-face to find that the person is not in fact as fat or thin as he or she appears to be on TV.

5.9.7 Aspect Ratio Conversion

Compensation for aspect ratio is sometimes necessary when you compress the video.

Broadcasters use a device called an aspect ratio converter (ARC), and while this is very convenient, it is possible for material to be converted backward and forward several times.

The result is a rather low-quality picture that appears to be over-cropped. This is because the top and bottom are trimmed when going from 4:3 to 16:9, and the sides are trimmed when going back to 4:3. Figure 5-19 shows the commonly used ARC output settings when cropping the input to fit.

An ARC will also pad an image rather than cropping it. Figure 5-20 shows some commonly used padding setups.