Deinterlacing requires the display to buffer one or more fields and recombine them into full frames. In theory this would be as simple as capturing one field and combining it with the next field to be received, producing a single frame. However, the originally recorded signal was produced from two fields at different points in time, and without special processing any motion across the fields usually results in a "combing" effect where alternate lines are slightly displaced from each other. There are various methods to deinterlace video, each producing different problems or
artifacts of its own. Some methods are much cleaner in artifacts than other methods. Most deinterlacing techniques fall under three broad groups: •
Field combination deinterlacing which takes the even and odd fields and combine them into one frame. This halves the perceived frame-rate (the temporal resolution) whereby 50i or 60i is converted to 25p or 30p. •
Field extension deinterlacing which takes each field (with only half the lines) and extend it to the entire screen to make a frame. This halves the vertical resolution of the image but maintains the original field-rate (50i or 60i is converted to 50p or 60p). •
Motion compensation deinterlacing which uses more advanced algorithms to detect motion across fields, switching techniques when necessary. This produces the best quality result, but requires the most processing power. Modern deinterlacing systems therefore buffer several fields and use techniques like
edge detection in an attempt to find the motion between the fields. This is then used to interpolate the missing lines from the original field, reducing the combing effect.
Field combination deinterlacing These methods take the even and odd fields and combine them into one frame. They retain the full vertical resolution at the expense of the temporal resolution (perceived frame-rate) whereby 50i/60i is converted to 24p/25p/30p which may lose the smooth, fluid feel of the original. However, if the interlaced signal was originally produced from a lower frame-rate source such as film, then no information is lost and these methods may suffice. • Weaving is the simplest and most rudimentary method, performed by interleaving ("weaving") the consecutive fields together into a single frame. This method does not cause any problems when the image has not changed between fields, but any motion will result in artifacts known as "combing" when the pixels in one field do not line up with the pixels in the other, forming a jagged edge. •
Blending is done by
blending, or
averaging consecutive fields to be displayed as one frame. Combing is avoided because the images are on top of each other. This instead leaves an artifact known as ghosting. The image loses both vertical resolution and temporal resolution. Although video produced with this technique only requires half the number of pixels vertically, it is often combined with a vertical resize so that the output has no numerical loss in vertical pixels. When interpolation is used, it can result in an even softer image. Blending also loses half the temporal resolution since two motion fields are combined into one frame. •
Selective blending, or
smart blending or
motion adaptive blending, is a combination of weaving and blending. As areas that haven't changed from frame to frame don't need any processing, the frames are woven and only the areas that need it are blended. This retains the full vertical resolution and half the temporal resolution, and it has fewer artifacts than weaving or blending because of the selective combination of both techniques. •
Inverse Telecine:
Telecine is used to convert a motion picture source at 24 frames per second to interlaced TV video in countries that use NTSC video system at 30 frames per second. Countries which use PAL at 25 frames per second do not require Telecine – motion picture sources are merely sped up 4% to achieve the needed 25 frames per second. If Telecine was used then it is possible to reverse the algorithm to obtain the original non-interlaced footage, which has a slower frame rate. In order for this to work, the exact telecine pattern must be known or guessed. Unlike most other deinterlacing methods, when it works, inverse telecine can perfectly recover the original progressive video stream. • Telecine-style algorithms: If the interlaced footage was generated from progressive frames at a slower frame rate (e.g. "cartoon pulldown"), then the exact original frames can be recovered by copying the missing field from a matching previous/next frame. In cases where there is no match (e.g. brief cartoon sequences with an elevated frame rate), then the filter falls back on another deinterlacing method such as blending or line-doubling. This means that the worst case for Telecine is occasional frames with ghosting or reduced resolution. By contrast, when more sophisticated motion-detection algorithms fail, they can introduce pixel artifacts that are unfaithful to the original material. For
telecine video,
decimation can be applied as a post-process to reduce the frame rate, and this combination is generally more robust than a simple inverse telecine, which fails when differently interlaced footage is spliced together.
Field extension deinterlacing These methods take each field (with only half the lines) and extend it to the entire screen to make a frame. This may halve the vertical resolution of the image but aims to maintain the original field-rate (50i or 60i is converted to 50p or 60p). •
Half-sizing displays each interlaced field on its own, resulting in a video with half the vertical resolution of the original, unscaled. While this method retains all original pixels and all temporal resolution it is understandably not used for regular viewing because of its false aspect ratio. However, it can be successfully used to apply
video filters which expect a noninterlaced frame, such as those exploiting information from neighbouring pixels (e.g., sharpening). •
Line doubling or "bobbing" takes the lines of each interlaced field (consisting of only even or odd lines) and doubles them, filling the entire frame. This results in the video having a frame rate identical to the original field rate, but each frame having half the vertical resolution, or resolution equal to that of each field that the frame was made from. Line doubling prevents combing artifacts and maintains smooth motion but can cause a noticeable reduction in picture quality from the loss of vertical resolution and visual anomalies whereby stationary objects can appear to bob up and down as the odd and even lines alternate. These techniques are also called
bob deinterlacing and
linear deinterlacing for this reason. A variant of this method discards one field out of each frame, halving temporal resolution. Line doubling is sometimes confused with deinterlacing in general, or with
interpolation (image scaling) which uses spatial filtering to generate extra lines and hence reduce the visibility of pixelation on any type of display. The terminology 'line doubler' is used more frequently in high end consumer electronics, while 'deinterlacing' is used more frequently in the computer and digital video arena.
Motion compensation deinterlacing More advanced deinterlacing algorithms combine the traditional field combination methods (weaving and blending) and frame extension methods (bob or line doubling) to create a high quality progressive video sequence. One of the basic hints to the direction and amount of motion would be the direction and length of combing artifacts in the interlaced signal. The best algorithms also try to predict the direction and the amount of image motion between subsequent fields in order to better blend the two fields together. They may employ algorithms similar to
block motion compensation used in video compression. For example, if two fields had a person's face moving to the left, weaving would create combing, and blending would create ghosting. Advanced motion compensation (ideally) would see that the face in several fields is the same image, just moved to a different position, and would try to detect direction and amount of such motion. The algorithm would then try to reconstruct the full detail of the face in both output frames by combining the images together, moving parts of each field along the detected direction by the detected amount of movement. Deinterlacers that use this technique are often superior because they can use information from many fields, as opposed to just one or two, however they require powerful hardware to achieve this in real-time. Motion compensation needs to be combined with
scene change detection (which has its own challenges), otherwise it will attempt to find motion between two completely different scenes. A poorly implemented motion compensation algorithm would interfere with natural motion and could lead to visual artifacts which manifest as "jumping" parts in what should be a stationary or a smoothly moving image. ==Quality Measurement==