Field Order

We have to distinguish between progressive and interlaced video. Progressive video transmits all lines of a video image sequentially. Interlaced video divides an image into two fields, containing only the odd and even lines of the image, respectively. Alternating the so called odd and even field are transmitted, and due to a small delay between fields a cathode ray TV displays the lines interleaved, yielding the original frame. This curious technique was invented because at refresh rates similar to film the image would fade out too quickly. Transmitting fields reduces the flicker without the necessity of doubling the frame rate and with it the bandwidth required for each channel.

It is important to understand a video camera does not expose one frame at a time, merely transmitting the frames separated into fields. The fields are in fact captured at two different instances in time. An object on screen may well move between one field and the next. For applications analysing motion it is of paramount importance to recognize which field of a frame is older, the temporal order.

When the driver provides or accepts images field by field rather than interleaved, it is also important applications understand how the fields combine to frames. We distinguish between top (aka odd) and bottom (aka even) fields, the spatial order: The first line of the top field is the first line of an interlaced frame, the first line of the bottom field is the second line of that frame.

However because fields were captured one after the other, arguing whether a frame commences with the top or bottom field is pointless. Any two successive top and bottom, or bottom and top fields yield a valid frame. Only when the source was progressive to begin with, e. g. when transferring film to video, two fields may come from the same frame, creating a natural order.

Counter to intuition the top field is not necessarily the older field. Whether the older field contains the top or bottom lines is a convention determined by the video standard. Hence the distinction between temporal and spatial order of fields. The diagrams below should make this clearer.

All video capture and output devices must report the current field order. Some drivers may permit the selection of a different order, to this end applications initialize the field field of struct v4l2_pix_format before calling the VIDIOC_S_FMT ioctl. If this is not desired it should have the value V4L2_FIELD_ANY (0).

Table 3.9. enum v4l2_field

V4L2_FIELD_ANY0Applications request this field order when any one of the V4L2_FIELD_NONE, V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM, or V4L2_FIELD_INTERLACED formats is acceptable. Drivers choose depending on hardware capabilities or e. g. the requested image size, and return the actual field order. Drivers must never return V4L2_FIELD_ANY. If multiple field orders are possible the driver must choose one of the possible field orders during VIDIOC_S_FMT or VIDIOC_TRY_FMT. struct v4l2_buffer field can never be V4L2_FIELD_ANY.
V4L2_FIELD_NONE1Images are in progressive format, not interlaced. The driver may also indicate this order when it cannot distinguish between V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM.
V4L2_FIELD_TOP2Images consist of the top (aka odd) field only.
V4L2_FIELD_BOTTOM3Images consist of the bottom (aka even) field only. Applications may wish to prevent a device from capturing interlaced images because they will have "comb" or "feathering" artefacts around moving objects.
V4L2_FIELD_INTERLACED4Images contain both fields, interleaved line by line. The temporal order of the fields (whether the top or bottom field is first transmitted) depends on the current video standard. M/NTSC transmits the bottom field first, all other standards the top field first.
V4L2_FIELD_SEQ_TB5Images contain both fields, the top field lines are stored first in memory, immediately followed by the bottom field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields.
V4L2_FIELD_SEQ_BT6Images contain both fields, the bottom field lines are stored first in memory, immediately followed by the top field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields.
V4L2_FIELD_ALTERNATE7The two fields of a frame are passed in separate buffers, in temporal order, i. e. the older one first. To indicate the field parity (whether the current field is a top or bottom field) the driver or application, depending on data direction, must set struct v4l2_buffer field to V4L2_FIELD_TOP or V4L2_FIELD_BOTTOM. Any two successive fields pair to build a frame. If fields are successive, without any dropped fields between them (fields can drop individually), can be determined from the struct v4l2_buffer sequence field. This format cannot be selected when using the read/write I/O method since there is no way to communicate if a field was a top or bottom field.
V4L2_FIELD_INTERLACED_TB8Images contain both fields, interleaved line by line, top field first. The top field is transmitted first.
V4L2_FIELD_INTERLACED_BT9Images contain both fields, interleaved line by line, top field first. The bottom field is transmitted first.

Figure 3.1. Field Order, Top Field First Transmitted

Field Order, Top Field First Transmitted

Figure 3.2. Field Order, Bottom Field First Transmitted

Field Order, Bottom Field First Transmitted