Image Scanning Techniques: Interlaced vs. Progressive

Interlaced scanning and progressive scanning are the two image scanning techniques available today for reading and displaying information produced by image sensors. Interlaced scanning is used mainly in CCDs. Progressive scanning is used in either CCD or CMOS sensors. Network cameras can make use of either scanning technique. (Analog cameras, however, can only make use of the interlaced scanning technique for transferring images over a coaxial cable and for displaying them on analog monitors.)

Interlaced scanning

When an interlaced image from a CCD is produced, two fields of lines are generated: a field displaying the odd lines, and a second field displaying the even lines. However, to create the odd field, information from both the odd and even lines on a CCD sensor is combined. The same goes for the even field, where information from both the even and odd lines is combined to form an image on every other line.

When transmitting an interlaced image, only half the number of lines (alternating between odd and even lines) of an image is sent at a time, which reduces the use of bandwidth by half. The monitor, for example, a traditional TV, must also use the interlaced technique. First the odd lines and then the even lines of an image are displayed and then refreshed alternately at 25 (PAL) or 30 (NTSC) frames per second so that the human visual system interprets them as complete images. All analog video formats and some modern HDTV formats are interlaced. Although the interlacing technique creates artifacts or distortions as a result of ‘missing’ data, they are not very noticeable on an interlaced monitor.

However, when interlaced video is shown on progressive scan monitors such as computer monitors, which scan lines of an image consecutively, the artifacts become noticeable. The artifacts, which can be seen as “tearing”, are caused by the slight delay between odd and even line refreshes as only half the lines keep up with a moving image while the other half waits to be refreshed. It is especially noticeable when the video is stopped and a freeze frame of the video is analyzed.

Progressive scanning

With a progressive scan image sensor, values are obtained for each pixel on the sensor and each line of image data is scanned sequentially, producing a full frame image. In other words, captured images are not split into separate fields as with interlaced scanning. With progressive scan, an entire image frame is sent over a network and when displayed on a progressive scan computer monitor, each line of an image is put on the screen one at a time in perfect order. Moving objects are, therefore, better presented on computer screens using the progressive scan technique. In a video surveillance application, it can be critical in viewing details of a moving subject (e.g., a person running away). Most Axis network cameras use the progressive scan technique.

At left, an interlaced scan image shown on a progressive (computer) monitor. At right, a progressive scan image on a computer monitor.

At left: a full-sized JPEG image (704x576 pixels) from an analog camera using interlaced scanning. At right: a full-sized JPEG image (640x480 pixels) from an Axis network camera using progressive scan technology.

Both cameras used the same type of lens and the speed of the car was the same at 20 km/h (15 mph). The background is clear in both images. However, the driver is clearly visible only in the image using progressive scan technology.

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