About Interlaced Video
Interlaced video formats capture each frame of a moving picture as two separate images known as fields. One field consists of the first active and subsequent odd horizontal lines in the frame, while the other field consists of the second and subsequent even lines. During the recording process, the two fields are captured consecutively at slightly different moments. The field recorded first is called the dominant field. For example, when recording NTSC using Field 1 as the dominant field, Field 2 is recorded a 1/60th of a second after Field 1. When the interlaced video is played, the two fields of each frame are then displayed with a temporal offset, causing the human eye to perceive a smooth, continuous moving image.
Because of this time difference however, interlaced video can sometimes display jagged horizontal edges, particularly when showing subjects in fast motion. In addition, not all such video formats interlace frames in the same way. When different interlaced formats are mixed, the field dominance of the clips must be changed to avoid visual artifacts and jitter. In Flame, use the video field management tools to do so.
By default when editing or compositing clips that have conflicting scan formats, Flame displays warning messages and requires confirmation of potentially problematic operations. These warnings can be disabled if desired by enabling Ignore Scan Format in the Clip Select section of the General Preferences.
About Field Dominance
Most interlaced video formats are Field 1 dominant, with a few exceptions. This means that for a given frame, Field 1 is recorded earlier in time, and therefore should be displayed first during broadcast. Field 2 is recorded after Field 1 and is displayed second.
Interlaced video formats may also differ in the first active line. The first active line is the uppermost line in the frame. In some interlaced video formats, this uppermost line belongs to Field 1, while in others it belongs to Field 2.
Format: | First active line belongs to: | Displayed in Flame as: | Displayed in Avid as: | Displayed in FCP as: | Other applications: |
---|---|---|---|---|---|
NTSC | Field 2 |
|
Even | Lower | f2
bottom |
PAL | Field 1 | Odd | Upper | f1
top |
|
HD (and other interlaced) |
All clips in Flame have a metadata tag showing their scan mode: F1 indicates that the clip conforms to the same dominant field requirement in the current project settings. F2 shows a mismatch, while P meanwhile indicates a progressive scanned, non-interlaced clip. This tag is assigned when the material is first imported. If the tag scan mode appears incorrectly for any reason, it can be changed to match the actual scan mode of the clip. This is different from other softwares that expose the actual spatiality of a clip's fields.
In projects containing mixed-resolution clips, interlaced video formats may still be combined despite having different dominant fields. The differences between the two formats must be addressed however to avoid outputting a result that displays one of the interlaced video formats improperly and introduces undesirable visuals. Use the procedure below to correct such interlacing issues.
Reversing Field Dominance
When working in a project that uses clips of various interlaced video formats, the field dominance of clips that do not correspond with the project settings should be reversed. For example, if a Field 1 dominant PAL clip is to be included in an NTSC project that is Field 2 dominant, simply reverse the field dominance of the PAL clip.
Mismatched clips show F2 in their metadata.
When reversing the dominance of a clip, the first and last fields of the clip are dropped to account for the offsetting required to reverse the field dominance. The resulting clip becomes a frame shorter. It is therefore a good practice to always capture enough head and tail frames so that the fields dropped on both ends do not adversely affect the visible frames.
Both the media and the scan mode metadata tag are changed by this process.
To reverse the dominance of clips:
- Select Tools, and select the Clip tab.
- Click Reverse Dominance.
- Select the clip to process.
- Select the destination.
The field dominance of the clip is reversed.
Deinterlacing and Interlacing
Deinterlacing and interlacing operations are used primarily to aid in the rotoscoping of field-based material, or in the application of filters to field-based material. Deinterlacing and interlacing are not necessary for processes that support field-based rendering.
Because there is a line-based difference in time per frame of interlaced video, artifacts may be introduced when field-based frames are painted or filtered. For example, if a blur is applied over fields, colour information from the pixels on Field 1 will be smudged into the pixels on Field 2.
Deinterlacing clips prior to applying any paint or filtering operations always provides better results. Once the effects have been added, the clip can be interlaced again, if required.
To deinterlace and interlace clips, see the sections below. Clips can also be deinterlaced and interlaced using Batch and Batch FX. For more information, see Deinterlace and Interlace.
Deinterlacing Clips
Deinterlace clips to separate each field to its own frame. The result clip is twice as long as the original.
To deinterlace clips:
- Select Tools, and select the Clip tab.
- Click DeInterlace.
- If necessary, select Field 1 or Field 2 from the Field Dominance box. By default, Field Dominance is set to Auto, which automatically determines whether the clip is Field 1 or 2 dominant. Manually set a field if Auto does not work.
- Enable or disable Interpolation based on the desired result:
- If clips are deinterlaced with interpolation off, the process repeats each isolated line to fill in the scan lines that would otherwise be vacant in the resulting clip's frames.
- If clips are deinterlaced with interpolation on, the process interpolates a blend between adjacent lines to fill in the isolated scan lines. This option reduces interlacing artifacts.
- Select the clip to deinterlace.
- Select the destination.
The result clip appears, now with double the duration.
Interlacing Clips
Interlace clips to combine de-interlaced fields into interlaced frames. The resulting clip is half as long as the original.
To interlace clips:
- Select Tools, and select the Clip tab.
- Click Interlace.
- From the Interlace Parameter box, select Field 1 or Field 2 to establish the resulting clip's field dominance.
- Select the clip to interlace.
- Select the destination.
The resulting clip appears, now with half the duration.
Merging Fields
Merging fields blends pixels in neighbouring field lines to reduce artifacts caused by motion jitter. Merge fields when there is interlaced material to be output to a progressive scan format.
To merge fields:
- Select Tools, and select the Clip tab.
- Click Field Merge.
- Set the percentage of blending between fields in the Level field.
Value Result 100% The average pixel value is replicated to both fields in an F1/F2 merge-pair, resulting in total field merging. 50% Pixels on each line in an F1/F2 merge-pair are replaced by pixels defined by 50% of their original value and 50% of the blend, or field merge result. 0% No change is applied to the clip. - Select the clip to process.
- Select the destination.
The field dominance of the clip is reversed.
Changing the Scan Mode of a Project
The project scan mode dictates how interlaced video formats are captured from tape, sent to the broadcast monitor in-session, and output to tape. The project scan mode also determines field-based processing.
You can change the scan mode of your project by selecting an alternative scan mode option. Do this, for example, to view a PAL clip in an NTSC project properly before reversing its field dominance. Generally, changing the scan mode of a project is a temporary measure performed only to allow for the monitoring of clips whose format does not match the scan mode of the current project.
Changing the Scan Mode Metadata Tag of a Clip
When material is brought into Flame, it is assigned a particular scan mode that becomes part of the clip metadata.
You can correct this problem using the Dominance option part of the Reformat tool, accessible from the right-click menu. This option changes the scan mode metadata tag for a clip without changing the actual scan mode of the media.