Interlacing is a technique developed in the early days of television to work around the limitations of the equipment available that time. The basic idea is that instead of broadcasting the whole video image 50 (or 60 in the NTSC standard) times per second to produce acceptable motion during playback, only half of the image - each even or each odd line - is transmitted in 1/50 of a second followed by the other half of the the next image in the next 1/50 second, then the first half of the third image in the next 1/50 second and so on. Conventional TV broadcasts still use that method, and lots of digital video cameras record interlaced video as well.
When you convert a video, it is important to know whether it's interlaced because it has to be handled differently when resizing and encoding.
In DVDforger's title editor window, you can set whether the video you're about to convert is interlaced in the video file properties:
The default setting is 'Auto-deinterlace', which means that DVDforger attempts to remove all interlacing artifacts from the video before processing. Although this provides acceptable results most of the time, it's highly recommended to set it according the actual video properties.
When you play the video in Media Player, you usually can't see any difference between interlaced and non-interlaced video, because Media Player automatically de-interlaces the video when playing it back. That leads us to the next question:
1. At the video file properties, click 'Take a look':
2. The video preview window shall appear, in which you shall see the first 100 frames of the video:
Note 1: For some videos, you'll see the image upside down; the reason is a bug in some codecs (like MS MPEG-4). To correct it, check 'Upside down' in the video file properties; otherwise the image will be upside down in the converted DVD as well.
Note 2: Some videos may cause DVDforger to misteriously crash after clicking the 'Take a look' button (by 'misteriously' I mean that I haven't found a way to prevent it yet). I have experienced this with DV-AVI files saved from Windows Movie Maker. However, those files can be corrected by opening them in VirtualDub, choosing 'Direct stream copy' from the 'Video' menu, and saving them using 'File'->'Save as AVI'.
3. You can seek forward using the buttons 'Load next 100 frames' and 'Skip 1000 frames'. Using those, find a part in the video where there's some motion.
4. If the image you see is either clear or blurred, it means that the video is not interlaced; in the video file properties, you shall choose the option 'Progressive'. If you see distinctive horizontal lines (like on the screenshot below), the video is interlaced.
5. There are two kinds of interlaced video: 'top field first' and 'bottom field first' (related to whether the even or the odd lines shall be displayed first during playback). To determine which one you have, click 'View - Half frames':
6. You shall see a stretched image; this is half of the video image consisting of every 2nd line. Play the video step by step, and examine the motion. If the motion is smooth, you shall set 'Interlaced - top field first' in the video properties; if the image moves back and forth (like below), you shall set 'Interlaced - bottom field first' in the video properties.