# Interlaced Video

Video buffers have a number of states identifiable through a combination of caps and buffer flags.

Possible states:

- Progressive
- Interlaced
- Plain
- One field
- Two fields
- Three fields - this should be a progressive buffer with a repeated 'first' field that can be used for telecine pulldown
- Telecine
  - One field
  - Two fields
  - Progressive
  - Interlaced (a.k.a. 'mixed'; the fields are from different frames)
  - Three fields - this should be a progressive buffer with a repeated 'first' field that can be used for telecine pulldown

Note: It can be seen that the difference between the plain interlaced and telecine states is that in the latter, buffers containing two fields may be progressive.

Tools for identification:

- `GstVideoInfo`
- `GstVideoInterlaceMode` - enum `GST_VIDEO_INTERLACE_MODE_...`
- PROGRESSIVE
- INTERLEAVED
- MIXED
- Buffers flags - `GST_VIDEO_BUFFER_FLAG_...`
  - TFF
  - RFF
  - ONEFIELD
  - INTERLACED

## Identification of Buffer States

Note that flags are not necessarily interpreted in the same way for all different states nor are they necessarily required nor make sense in all cases.

### Progressive

If the interlace mode in the video info corresponding to a buffer is "progressive", then the buffer is progressive.

### Plain Interlaced

If the video info interlace mode is "interleaved", then the buffer is plain interlaced.

`GST_VIDEO_BUFFER_FLAG_TFF` indicates whether the top or bottom field is to be displayed first. The timestamp on the buffer corresponds to the first field.

`GST_VIDEO_BUFFER_FLAG_RFF` indicates that the first field (indicated by the TFF flag) should be repeated. This is generally only used for telecine purposes but as the telecine state was added long after the interlaced state was added and defined, this flag remains valid for plain interlaced buffers.

`GST_VIDEO_BUFFER_FLAG_ONEFIELD` means that only the field indicated through the TFF flag is to be used. The other field should be ignored.

### Telecine

If video info interlace mode is "mixed" then the buffers are in some form of telecine state.

The `TFF` and `ONEFIELD` flags have the same semantics as for the plain interlaced state.

`GST_VIDEO_BUFFER_FLAG_RFF` in the telecine state indicates that the buffer contains only repeated fields that are present in other buffers and are as such unneeded. For example, in a sequence of three telecined frames, we might have:

    AtAb AtBb BtBb

In this situation, we only need the first and third buffers as the second buffer contains fields present in the first and third.

Note that the following state can have its second buffer identified using the `ONEFIELD` flag (and `TFF` not set):

    AtAb AtBb BtCb

The telecine state requires one additional flag to be able to identify progressive buffers.

The presence of the `GST_VIDEO_BUFFER_FLAG_INTERLACED` means that the buffer is an 'interlaced' or 'mixed' buffer that contains two fields that, when combined with fields from adjacent buffers, allow reconstruction of progressive frames. The absence of the flag implies the buffer containing two fields is a progressive frame.

For example in the following sequence, the third buffer would be mixed (yes, it is a strange pattern, but it can happen):

    AtAb AtBb BtCb CtDb DtDb

### Alternate fields

Since: 1.16

If the video info interlace mode is "alternate", then each buffer carries a single field of interlaced video.

`GST_VIDEO_BUFFER_FLAG_TOP_FIELD` and `GST_VIDEO_BUFFER_FLAG_BOTTOM_FIELD` indicate whether the buffer carries a top or bottom field. The order of buffers/fields in the stream and the timestamps on the buffers indicate the temporal order of the fields.

Top and bottom fields are expected to alternate in this mode.

Caps for this interlace mode must also carry a `format:Interlaced` caps feature (`GST_CAPS_FEATURE_FORMAT_INTERLACED`) to ensure backwards compatibility for the new mode.

The frame rate in the caps still signals the frame rate, so the notional field rate will be twice the frame rate from the caps (see `GST_VIDEO_INFO_FIELD_RATE_N`).

In the same vein the width and height in the caps will indicate frame dimensions not field dimensions, meaning the height of the video data inside the buffers (1 field) will be half of the height in the caps.

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