Iowa Public Television

Monday, December 28, 2009

What's Wrong With This Picture?

One of the questions that I get most frequently is what's wrong with the picture. It seldom relates to the quality of the picture but the content of the picture. Why are the titles cut off? Why a titles in the middle of the screen rather than at the edge of the picture like they used to be? The issue is aspect ratio which is the relationship between the height and width of the picture. This is an important concept to grasp because in the transition from analog television to digital television, the aspect ratio changed. Traditional analog television has an aspect ratio of 4x3 which means that the picture is 33% wider than it is tall. Digital television's have an aspect ratio of 16x9 which means that the picture is about 78% wider than it is tall. This represents a pretty significant difference in aspect ratio. In my office I have an analog television with a screen that is 16 inches wide and 12 inches all. If you do a little basic math that confirms that the screen is 33% wider than it is tall. I also have a digital television in my office with a screen that is about 22.5 inches wide and about 12.7 inches tall. The same basic math will show that the screen is about 78% wider than it is tall.

If you think about it in terms of pictures and the frames we mount them in, it is a little easier to understand. Thanks to some friends at NBC, I'll use a picture that many of us are familiar with. Here is Da Vinci's Last Supper. Don't focus on the poor image quality but on the shape and the size relationship of the picture height to picture width. That is the aspect ratio. This particular image has an aspect ratio that is very close to the 16x9 digital aspect ratio. If you were to frame this picture for display you would measure the height and width of the canvas and then build the frame to those specifications. But what if you had to make it fit in a frame that you already have? This is the issue that many people are dealing with now as they try to display digital content on their old analog televisions.

If the desire is to show the complete 16x9 image in the existing 4x3 television frame, there are only two options. The first is to do what is called letterboxing the image. What this means isthat we scale or adjust the width of the 16x9 picture to fill the width of the 4x3 frame. The side effect of this scaling is that there is a portion of the 4x3 frame that the picture does not fill. In essence a black matte is generated to fill from the top and bottom edges of the picture to the top and bottom edges of the frame. As you can see from the example the entire 16x9 picture is displayed with the proper width to height relationship and is made to fit within the 4x3 frame. Image integrity is maintained but the actual image on the screen is smaller than a 4x3 picture would be.

The other option for displaying a 16x9 image on a 4x3 display is to forget about maintaining image integrity. in this case we would scale the width of the image to fill the width of the 4x3 frame and then independently scale the height of the image to fill the 4x3 frame. As you can see from the example, the 16x9 image fills the 4x3 screen but the picture is distorted. The characters are vertically stretched so that they appear taller and skinnier than normal. Conversely, if you look at the horizontal table, it appears to be shorter and than normal. So the 4x3 screen is filled but the picture content is compromised.

As I said, there are really only two ways to display the entire 16x9 image on a 4x3 screen. There are however other options that can be used. One popular one is called cropping or center cutting. If you have every taken a picture that is too large for a frame and trimmed it to fit, you have cropped. In television, because most of the important visual information is in the center of the screen, a 4x3 image is cut from the center of the 16x9 picture. Once again, the 4x3 screen is filled but the image is compromised. In the example image, although the characters in the original image maintain their proper width to height relationship, three of the original characters are no longer displayed so in this case we would see Jesus and the nine apostles. I am certain that Leonardo Da Vinci would not believe that this was an accurate interpretation of his work or an accurate representation of the historical event he is recording.

One technique that is often suggested is what is called center or 4x3 protect. What this means is that when the picture is being created, the action should be framed to all take place within the 4x3 center of the 16x9 frame. In some cases this is being done with some success but it is not always possible. In the example picture, this is what the Last Supper might have looked like if Da Vinci had created on a 16x9 canvas for display in both a 16x9 and 4x3 frames. As you can, all of the characters are in the image and their width to height ratio is proper but the image is unbalanced because of the compression of all of the action to the center of the frame while the left and right edges are more vacant and unused. In this case the image would look unusual on both displays.

It is challenging and sometimes impossible to create content that makes full use of the wider digital aspect ratio while trying to insure the content's integrity on legacy displays. Realizing that 4x3 displays are the legacy device and will eventually be gone and 16x9 or wider displays will be the norm, content needs to be created to take advantage of the benefits of new display technologies. For display on legacy devices, the first choice should be to maintain the integrity of the wide screen images through letterboxing.