Deinterlace - What's that?

Why and how to deinterlace is an important item to study when editing.

The whole concept of why we need to deinterlace dates back to the origins of TV

Video (or film) consists of a series of images played in succession, each of these images is known as a frame. For a variety of reasons, analog television standards break these frames into a number of lower-resolution fields, each one containing a portion of the image. The fields are then displayed in a fashion that produces the illusion of single frames again. This process of producing complete frames from the lower-resolution fields is known as interlacing. For instance, in the NTSC system used in North America and Japan, frames are made for every 30th of a second, but are recorded as a series of 60th second fields, each containing only the odd or even lines of the display.

Problems with modern technology

Interlacing does not work well with modern display technologies, such as LCDs. Deinterlacing attempts to reverse the interlacing process to produce single higher resolution frames suitable for these displays. However, the original interlaced signal did not contain these frames in a complete form, and de-interlacing is a fundamentally lossy process that generally introduces some image degradation. The exception to this rule is deinterlacing an originally non-interlaced source that was later interlaced for display; movies encoded using telecine are one example.

Examples

So let's see what this looks like in action

In order to deinterlace you have to find a way of merging together the two different fields in a manner that produces a single frame representative of the two pictures. In a fairly static picture that is quite straightforward, however with movement in the picture it creates problems. you see the first field is taken a fraction of a second before the second field. This means that any movement in between will put objects in a slightly different place on the second field. Trying to merge these two fields together will produce undesirable results.

Let's look at the types of deinterlacing procedure.

Weaving is done by adding consecutive fields together. This is fine when the image hasn't changed between fields, but any change will result in artifacts known as "mouse teeth" or "tearing", when the pixels in one frame do not line up with the pixels in the other, forming a jagged edge. This technique retains full vertical resolution at the expense of half the temporal resolution.

This results in a picture that looks like this:

An alternate method of deinterlacing is to Blend. Blending results in a picture which is a merge of he two underlying fields. This removes jagged edges (as seen with the weaving method) but causes a softening and blurring of the picture. Both the jagged edges from weaving and the blurring or ghosting from blending are known as artifacts of deinterlacing. Whichever method you use for deinterlacing, artifacts will occur.

When blending an image you loose quality and sharpness compared with a deinterlaced image This is illustrated by the second picture here

The Other main types of Deinterlaceing are

Selective blending, or smart blending or motion adaptive blending. This 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 weaved and only the areas that need it are blended. This retains full vertical resolution, half the temporal resolution, and has fewer artifacts than weaving or blending because of the combination of them both.

Motion Compensation deinterlacing uses motion compensation techniques in order to align the two fields in a picture. While effective for horizontal motion, some software for this technique does not handle vertical motion at all, and may fall back on selective blending or other techniques when it is unable to resolve the motion vectors. This technique is implemented in the open source tool yuvdeinterlace, part of the mjpeg tools package.

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