Back in the olden days of CRT displays, along with carriers having less bandwidth for transmitting images, and video, carriers wanted to be able to achieve the effect of sending 60 frames per second.
Of course, bandwidth was not available at the time to send images at this temporal resolution (60 fps), so interlacing was used.
Interlacing is a technique to increase the temporal resolution. That is, we perceive the updates to be more frequent in a period of time. With interlacing, this is mostly perceptual, and is just a trick to convince our visual system to think we are getting more updates a second.
Each frame is divided into 2 fields. Each field may consist of only even scanlines, or odd scanlines. Note, that the term scanlines here is used in the context of CRT displays.
By restricting each update to a single field, a carrier would only need to transmit 30 images at full spatial resolution (number of pixels in WxH) per second when the fields are effectively merged. However, the receiver will perceive it to be 60 frames per second.
Nowadays, we can send images at their full spatial resolution. This is also known as progressive scan.
Notes
- Weaving is a way to naively merge the scanlines together to create the image at full spatial resolution
- Bob is another way to do the merge of scanlines