testcases/kernel/device-drivers/v4l/user_space/doc/spec/x6386.htm - platform/external/ltp - Gitiles

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 >Video for Linux Two API Specification: Revision 0.24</TH
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 ><DIV
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 ><H1
 CLASS="SECTION"
 ><A
 NAME="FIELD-ORDER"
 >3.6. Field Order</A
 ></H1
 ><P
 >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.</P
 ><P
 >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 <SPAN
 CLASS="emphasis"
 ><I
 CLASS="EMPHASIS"
 >temporal
 order</I
 ></SPAN
 >.</P
 ><P
 >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 and
 bottom fields, the <SPAN
 CLASS="emphasis"
 ><I
 CLASS="EMPHASIS"
 >spatial order</I
 ></SPAN
 >: 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.</P
 ><P
 >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.&nbsp;g. when transferring film to video, two fields may come from
 the same frame, creating a natural order.</P
 ><P
 >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.</P
 ><P
 >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
 <CODE
 CLASS="STRUCTFIELD"
 >field</CODE
 > field of struct&nbsp;<A
 HREF="c2030.htm#V4L2-PIX-FORMAT"
 >v4l2_pix_format</A
 > before
 calling the <A
 HREF="r10944.htm"
 ><CODE
 CLASS="CONSTANT"
 >VIDIOC_S_FMT</CODE
 ></A
 > ioctl. If this is not desired it should
 have the value <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_ANY</CODE
 > (0).</P
 ><DIV
 CLASS="TABLE"
 ><A
 NAME="V4L2-FIELD"
 ></A
 ><P
 ><B
 >Table 3-8. enum v4l2_field</B
 ></P
 ><TABLE
 BORDER="0"
 FRAME="void"
 WIDTH="100%"
 CLASS="CALSTABLE"
 ><COL
 WIDTH="38%"
 TITLE="C1"><COL
 WIDTH="12%"
 TITLE="C2"><COL
 WIDTH="50%"
 TITLE="C3"><TBODY
 VALIGN="TOP"
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_ANY</CODE
 ></TD
 ><TD
 >0</TD
 ><TD
 >Applications request this field order when any
 one of the <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_NONE</CODE
 >,
 <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_TOP</CODE
 >,
 <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_BOTTOM</CODE
 >, or
 <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_INTERLACED</CODE
 > formats is acceptable.
 Drivers choose depending on hardware capabilities or e.&nbsp;g. the
 requested image size, and return the actual field order. struct&nbsp;<A
 HREF="x5953.htm#V4L2-BUFFER"
 >v4l2_buffer</A
 >
 <CODE
 CLASS="STRUCTFIELD"
 >field</CODE
 > can never be
 <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_ANY</CODE
 >.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_NONE</CODE
 ></TD
 ><TD
 >1</TD
 ><TD
 >Images are in progressive format, not interlaced.
 The driver may also indicate this order when it cannot distinguish
 between <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_TOP</CODE
 > and
 <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_BOTTOM</CODE
 >.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_TOP</CODE
 ></TD
 ><TD
 >2</TD
 ><TD
 >Images consist of the top field only.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_BOTTOM</CODE
 ></TD
 ><TD
 >3</TD
 ><TD
 >Images consist of the bottom field only.
 Applications may wish to prevent a device from capturing interlaced
 images because they will have "comb" or "feathering" artefacts around
 moving objects.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_INTERLACED</CODE
 ></TD
 ><TD
 >4</TD
 ><TD
 >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.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_SEQ_TB</CODE
 ></TD
 ><TD
 >5</TD
 ><TD
 >Images 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.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_SEQ_BT</CODE
 ></TD
 ><TD
 >6</TD
 ><TD
 >Images 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.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_ALTERNATE</CODE
 ></TD
 ><TD
 >7</TD
 ><TD
 >The two fields of a frame are passed in separate
 buffers, in temporal order, i.&nbsp;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&nbsp;<A
 HREF="x5953.htm#V4L2-BUFFER"
 >v4l2_buffer</A
 >
 <CODE
 CLASS="STRUCTFIELD"
 >field</CODE
 > to
 <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_TOP</CODE
 > or
 <CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_BOTTOM</CODE
 >. 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&nbsp;<A
 HREF="x5953.htm#V4L2-BUFFER"
 >v4l2_buffer</A
 > <CODE
 CLASS="STRUCTFIELD"
 >sequence</CODE
 > field. Image
 sizes refer to the frame, not fields. This format cannot be selected
 when using the read/write I/O method.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_INTERLACED_TB</CODE
 ></TD
 ><TD
 >8</TD
 ><TD
 >Images contain both fields, interleaved line by
 line, top field first. The top field is transmitted first.</TD
 ></TR
 ><TR
 ><TD
 ><CODE
 CLASS="CONSTANT"
 >V4L2_FIELD_INTERLACED_BT</CODE
 ></TD
 ><TD
 >9</TD
 ><TD
 >Images contain both fields, interleaved line by
 line, top field first. The bottom field is transmitted first.</TD
 ></TR
 ></TBODY
 ></TABLE
 ></DIV
 ><DIV
 CLASS="FIGURE"
 ><A
 NAME="FIELDSEQ-TB"
 ></A
 ><P
 ><B
 >Figure 3-1. Field Order, Top Field First Transmitted</B
 ></P
 ><DIV
 CLASS="MEDIAOBJECT"
 ><P
 ><IMG
 SRC="fieldseq_tb.gif"></P
 ></DIV
 ></DIV
 ><DIV
 CLASS="FIGURE"
 ><A
 NAME="FIELDSEQ-BT"
 ></A
 ><P
 ><B
 >Figure 3-2. Field Order, Bottom Field First Transmitted</B
 ></P
 ><DIV
 CLASS="MEDIAOBJECT"
 ><P
 ><IMG
 SRC="fieldseq_bt.gif"></P
 ></DIV
 ></DIV
 ></DIV
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	<HTML
	><HEAD
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	>Field Order</TITLE
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	>Video for Linux Two API Specification: Revision 0.24</TH
	></TR
	><TR
	><TD
	WIDTH="10%"
	ALIGN="left"
	VALIGN="bottom"
	><A
	HREF="x5953.htm"
	ACCESSKEY="P"
	>Prev</A
	></TD
	><TD
	WIDTH="80%"
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	><HR
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	><DIV
	CLASS="SECTION"
	><H1
	CLASS="SECTION"
	><A
	NAME="FIELD-ORDER"
	>3.6. Field Order</A
	></H1
	><P
	>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.</P
	><P
	>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 <SPAN
	CLASS="emphasis"
	><I
	CLASS="EMPHASIS"
	>temporal
	order</I
	></SPAN
	>.</P
	><P
	>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 and
	bottom fields, the <SPAN
	CLASS="emphasis"
	><I
	CLASS="EMPHASIS"
	>spatial order</I
	></SPAN
	>: 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.</P
	><P
	>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.</P
	><P
	>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.</P
	><P
	>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
	<CODE
	CLASS="STRUCTFIELD"
	>field</CODE
	> field of struct <A
	HREF="c2030.htm#V4L2-PIX-FORMAT"
	>v4l2_pix_format</A
	> before
	calling the <A
	HREF="r10944.htm"
	><CODE
	CLASS="CONSTANT"
	>VIDIOC_S_FMT</CODE
	></A
	> ioctl. If this is not desired it should
	have the value <CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_ANY</CODE
	> (0).</P
	><DIV
	CLASS="TABLE"
	><A
	NAME="V4L2-FIELD"
	></A
	><P
	><B
	>Table 3-8. enum v4l2_field</B
	></P
	><TABLE
	BORDER="0"
	FRAME="void"
	WIDTH="100%"
	CLASS="CALSTABLE"
	><COL
	WIDTH="38%"
	TITLE="C1"><COL
	WIDTH="12%"
	TITLE="C2"><COL
	WIDTH="50%"
	TITLE="C3"><TBODY
	VALIGN="TOP"
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_ANY</CODE
	></TD
	><TD
	>0</TD
	><TD
	>Applications request this field order when any
	one of the <CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_NONE</CODE
	>,
	<CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_TOP</CODE
	>,
	<CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_BOTTOM</CODE
	>, or
	<CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_INTERLACED</CODE
	> formats is acceptable.
	Drivers choose depending on hardware capabilities or e. g. the
	requested image size, and return the actual field order. struct <A
	HREF="x5953.htm#V4L2-BUFFER"
	>v4l2_buffer</A
	>
	<CODE
	CLASS="STRUCTFIELD"
	>field</CODE
	> can never be
	<CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_ANY</CODE
	>.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_NONE</CODE
	></TD
	><TD
	>1</TD
	><TD
	>Images are in progressive format, not interlaced.
	The driver may also indicate this order when it cannot distinguish
	between <CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_TOP</CODE
	> and
	<CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_BOTTOM</CODE
	>.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_TOP</CODE
	></TD
	><TD
	>2</TD
	><TD
	>Images consist of the top field only.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_BOTTOM</CODE
	></TD
	><TD
	>3</TD
	><TD
	>Images consist of the bottom field only.
	Applications may wish to prevent a device from capturing interlaced
	images because they will have "comb" or "feathering" artefacts around
	moving objects.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_INTERLACED</CODE
	></TD
	><TD
	>4</TD
	><TD
	>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.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_SEQ_TB</CODE
	></TD
	><TD
	>5</TD
	><TD
	>Images 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.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_SEQ_BT</CODE
	></TD
	><TD
	>6</TD
	><TD
	>Images 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.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_ALTERNATE</CODE
	></TD
	><TD
	>7</TD
	><TD
	>The 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 <A
	HREF="x5953.htm#V4L2-BUFFER"
	>v4l2_buffer</A
	>
	<CODE
	CLASS="STRUCTFIELD"
	>field</CODE
	> to
	<CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_TOP</CODE
	> or
	<CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_BOTTOM</CODE
	>. 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 <A
	HREF="x5953.htm#V4L2-BUFFER"
	>v4l2_buffer</A
	> <CODE
	CLASS="STRUCTFIELD"
	>sequence</CODE
	> field. Image
	sizes refer to the frame, not fields. This format cannot be selected
	when using the read/write I/O method.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_INTERLACED_TB</CODE
	></TD
	><TD
	>8</TD
	><TD
	>Images contain both fields, interleaved line by
	line, top field first. The top field is transmitted first.</TD
	></TR
	><TR
	><TD
	><CODE
	CLASS="CONSTANT"
	>V4L2_FIELD_INTERLACED_BT</CODE
	></TD
	><TD
	>9</TD
	><TD
	>Images contain both fields, interleaved line by
	line, top field first. The bottom field is transmitted first.</TD
	></TR
	></TBODY
	></TABLE
	></DIV
	><DIV
	CLASS="FIGURE"
	><A
	NAME="FIELDSEQ-TB"
	></A
	><P
	><B
	>Figure 3-1. Field Order, Top Field First Transmitted</B
	></P
	><DIV
	CLASS="MEDIAOBJECT"
	><P
	><IMG
	SRC="fieldseq_tb.gif"></P
	></DIV
	></DIV
	><DIV
	CLASS="FIGURE"
	><A
	NAME="FIELDSEQ-BT"
	></A
	><P
	><B
	>Figure 3-2. Field Order, Bottom Field First Transmitted</B
	></P
	><DIV
	CLASS="MEDIAOBJECT"
	><P
	><IMG
	SRC="fieldseq_bt.gif"></P
	></DIV
	></DIV
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