Cropping is a way of identifying a specific region of the video to use in the compression, excluding the other areas of the source frame. TVs do not display the entire image that is transmitted. Instead, they are overscanned, meaning slightly larger than the viewable area of a standard TV. This is done for several reasons, all of which culminate in needing to hide irregularities that exist in the edges of the video frames. Production people are aware of this and have created three regions of a video image that affect how they frame shots and incorporate graphic overlays: overscan, action safe, and title safe (Figure 4.2).
Figure 4.2 This type of image will be familiar to anyone who has looked through a video camera viewfinder. It depicts the action- and title-safe areas that a camera operator needs to be conscious of when framing a shot. Compressionists need to be aware of these regions as well and use them as a general guideline for cropping nonbroadcast content.
The outermost region of the video is known as the overscan region. It will not appear on standard consumer TV screens, and it is often used to store information that isn't meant to be viewed by the general public, such as the edge of the set or cables and other equipment. Professional-grade monitors have a mode that allows the overscan to be viewed, known as underscan mode. These monitors may also include white lines like the viewfinder that show where the title-safe and action-safe areas are located.
The action-safe area is the larger rectangle within the image area. This area displays approximately 90 percent of the video image and is where camera operators will make sure to keep the primary action framed up in order to keep it viewable on TVs. As of 2007, most TV stations and networks will also place information within this area, such as news tickers, station IDs, and advertisements.
The smaller rectangle in the image is the title-safe area (comprising about 80 percent of the visible image). It is far enough in from the four edges of a standard CRT TV set that text or graphics should show neatly without any distortion caused by the curved glass at the edges of the screen. Modern TVs—CRTs included—display more than the title-safe area without any distortion, so though not specifically necessary, it is still applied as a good rule of thumb in many productions.
When compressing video not meant for rebroadcast or playback on a standard TV, it's necessary to crop the active picture area at minimum to the action-safe area. However, depending on the action in the frame, it may be possible to crop even more aggressively, thus focusing on the action better (and possibly improving the picture quality).