From a82c3149ad8b4a84f9737a633250815d5cf0cc5e Mon Sep 17 00:00:00 2001 From: Steve Longerbeam Date: Fri, 3 May 2019 19:45:07 -0400 Subject: [PATCH] media: docs-rst: Clarify older field vs. first transmitted field Add a paragraph to make it more clear that video equipment will transmit fields in the same order the fields were captured, and replace some of the "is transmitted first" language with "is the older field", since the latter is the important info for motion compensation applications. Signed-off-by: Steve Longerbeam Signed-off-by: Hans Verkuil Signed-off-by: Mauro Carvalho Chehab --- Documentation/media/uapi/v4l/field-order.rst | 17 +++++++++++------ 1 file changed, 11 insertions(+), 6 deletions(-) diff --git a/Documentation/media/uapi/v4l/field-order.rst b/Documentation/media/uapi/v4l/field-order.rst index d640e92..c422beb 100644 --- a/Documentation/media/uapi/v4l/field-order.rst +++ b/Documentation/media/uapi/v4l/field-order.rst @@ -51,6 +51,11 @@ determined by the video standard. Hence the distinction between temporal and spatial order of fields. The diagrams below should make this clearer. +In V4L it is assumed that all video cameras transmit fields on the media +bus in the same order they were captured, so if the top field was +captured first (is the older field), the top field is also transmitted +first on the bus. + 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 @@ -101,10 +106,10 @@ enum v4l2_field * - ``V4L2_FIELD_INTERLACED`` - 4 - Images 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. + order of the fields (whether the top or bottom field is older) + depends on the current video standard. In M/NTSC the bottom + field is the older field. In all other standards the top field + is the older field. * - ``V4L2_FIELD_SEQ_TB`` - 5 - Images contain both fields, the top field lines are stored first @@ -135,11 +140,11 @@ enum v4l2_field * - ``V4L2_FIELD_INTERLACED_TB`` - 8 - Images contain both fields, interleaved line by line, top field - first. The top field is transmitted first. + first. The top field is the older field. * - ``V4L2_FIELD_INTERLACED_BT`` - 9 - Images contain both fields, interleaved line by line, top field - first. The bottom field is transmitted first. + first. The bottom field is the older field. -- 2.7.4