Interlacing

Interlacing describes how the picture is created in the system's display unit. An interlaced display creates an image by scanning each line, and in the next scan, it scans the line opposite the previous line. The screen refreshes information at a relatively less cost. It provides a faster refresh rate. The only issue with that method is that the content displayed on the screen can flicker or have noticeable lines in it.

We also have interlaced videos and GIFs. The interlaced GIF is displayed by loading odd or even numbers of lines. Then it loads the next cycle of lines. It may seem faded when the user has a slow internet connection.

Interlaced Video

It is a method that doubles the video's frame rate. It captures two fields of a video frame simultaneously. This increases the motion perception and reduces the flickering of the content by using the phi phenomenon. The benefit of using this method is that the user is not required to add extra bandwidth to increase the frame rate.

Phi Phenomenon

It refers to the phenomenon that creates an illusion of movement. It is done by placing two images simultaneously and lighted one by one.

This time resolution of an interlaced video is almost double that of a non-interlaced video. Interlaced signals need a display that can project the content by displaying them as individual fields in an orderly manner. CRT and ALiS plasma displays are the real-world application of interlaced screens.

The interlaced scan is a technique that paints a video image on the system's display unit. The display is represented by representing each row of pixels one after another. A field consists of an odd number of lines, while the other contains even-numbered lines. The two fields are used to create a single image or video.

Another application of the interlaced scan is PAL-based television. PAL stands for Phase Alternating Line. The television can scan 50 field lines in a single scan (25 odd lines and 25 even field lines). These two sets are combined and thus forming a single frame of an image or the video every 1/25 of a second. This means that the television creates 25 frames each second. But when you use interlacing in the television it creates an additional half frame every 1/50 second. It creates 50 fields each second. If the user wants to show an interlaced video on the displays, the playback must first deinterlace the video or the image.

The European Broadcasting Union has spoken against using interlaced video in production or broadcasting; instead, they preferred 720p 50 frames per second for the production or broadcasting of the video. They were developing the 1080p 50fps to introduce the resolutions as the new standard for video production. The resolutions were higher in the 1080p 50fps. The user can easily change the video format from 1080p 50 fps to 720p 50. It provides better quality video even at lower bit rates. It also makes it easier for the user to convert the video format.

They were against the idea of using interlacing for video production because no matter how complex the algorithm used for interlacing is, the artifacts in the interlaced signal can not be completely removed. Thus, there will always be some loss between the frames in the video.

The new video standards were modified to be implemented with the progressive scan videos, but they often support interlaced videos. But the organizations still used continued interlacing techniques in producing and broadcasting the videos. The technique is implemented in the DV and DVB formats of videos.

Interlaced Scans and Progressive Scans

In interlaced scanning, the display unit represents the content in the green and red lines. The green lines represent the odd field lines, while the red represents the even field lines. It returns blanking lines.

The video or the image is captured, transmitted, or displayed line by line. It is done from the top to the bottom of the image. In Progressive scanning, the image is captured, transmitted, and then displayed in the same path as the text is represented on the page of the document.

In the CRT displays, the interlaced scan finishes the scan two times. The first time the odd-numbered lines are represented. They are represented from the top left corner to the bottommost right corner. The second scan represents the even-numbered lines in the same sequential order. Thus, completing the same scan.

The technique of alternate scanning lines is known as interlacing, and the fields are the image formed by either odd-numbered or even-numbered lines combined to complete the picture.

It uses the persistence of the eyes to make the viewer believe that the combined images are continuous. When the CRT displays were still used, the phosphorus used in the display helped to interlace.

In both techniques, the image returned has the complete vertical detail with the same bandwidth. But the frame and refresh rate are twice as in the case of interlacing than the progressive scan. Interlacing eliminated flickering and therefore was used in analog televisions.

Advantages of Using Interlacing

1. An important characteristic of analog television was that it had a specific bandwidth, and the bandwidth was computed in megahertz. As the bandwidth of the device increases, the production and broadcasting of the video or the image become more difficult and expensive. The devices include cameras, storage broadcast, and reception systems that the cable or the satellite, internet, or any other user display units used to transmit the video or image.
2. If the bandwidth is fixed for a device, the interlacing offers a video signal that has double the frame and refreshes rate compared to a progressive scan with the same line count.
3. This time resolution of an interlaced video is almost double that of a non-interlaced video. Interlaced signals need a display that can project the content by displaying them as individual fields in an orderly manner. CRT and ALiS plasma displays are real-world applications of interlaced screens.
4. For Example, there are two screens with the following specification 1080i at 60half frames per sec and 1080p at 30 full frames per sec, respectively.
   Then, the screen with a higher refresh rate will provide the user with a better picture quality of the object in transition. It is because the object's position will be updated more frequently on the screen with a higher refresh rate. But if the object is stationary, the human vision takes information from several similar half frames to build the image of the image with the same resolutions as it will be built by the progressive full frame.
5. The above technique only works if the source material has higher refresh rates. The movies at that time were recorded at 24 fps. This was why the interlaced video did not affect the display quality much. If the video has a fixed bandwidth and the refresh rate is high, the interlaced video will have a higher spatial resolution than the video that is a progressive scan.
6. The spatial resolution for the low-motion scenes will be twice as high in the interlaced video than in the progressive scan video. This means the definition of the interlaced video will be better than the progressive scan video. For Example, the interlaced video with 1920X1080 pixel resolution with a refresh rate of 60 Hz has comparable bandwidth to the 1280×720 pixel progressive scan HDTV video with a refresh rate of 60 Hz.
7. In all the present television standards, the digital video or compressed interlaced video caused several issues. EBU performed several tests and proved that the bandwidth saved using interlacing on the video instead of progressive video is very small even if the video's frame rate is twice as high as the progressive scanned video.
8. The benefit associated with the bandwidth of the video is only applicable to the analog or uncompressed digital video signal. The 1080p50 signal video will have a bit rate similar to the 1080i50 video. The video with 1080p50 needs less bandwidth to be perceived and is better than the 1080i/25 equivalent.
9. The user can use the interlacing if they want to exploit the programming of the 3D television. This could be specially done in the CRT displays that have color-filtered glasses. It transmits the color-keyed picture for each eye in the changing fields. This method does not require performing any change in the current system equipment. The user can also use the shutter glass instead of the color-filtered glass. The shutter glass can achieve a higher synchronization rate than it.
10. Suppose a progressive scan display enabled the user to the programming. Every field line in the display must be lined, doubled, and represented in different frames. If the user tries to deinterlace the image or video, all the effects will become useless.
11. If the user wants to use color-filtered glass, then the picture has to be either buffered or can be viewed if the image or video is progressive with alternate color-keyed lines. The user must use the above method to effectively use the shutter glass in the progressive display.

Problems with using interlacing

1. Suppose the user wants to view the interlaced video using a progressive monitor without deinterlacing the video or with poor deinterlacing. In that case, the user can use the combing at the moment between the fields in a frame instead.
2. In the image of a moving car tire, interlace combing can reduce the realignment between the even and odd field on the X-axis. If the fields are moved by 16 pixels to the right, this will reduce the combing on the bumper and the outlining of the tire. But on the car's hub cap, there will still be some notable combing because it is turned in between the field.
3. The purpose of using the interlaced video was to record, store, share, and display in the same interlaced format. In interlaced video, the video frame is divided into two separate fields that are recorded in different instances.
4. Suppose the objects recorded in the video move at a certain speed. The interlaced video frames can show certain motion artifacts. These artifacts are referred to as interlacing effects or combing. These artifacts are visible when the interlaced video is placed slower.