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1 <title>Video Overlay Interface</title>
2 <subtitle>Also known as Framebuffer Overlay or Previewing</subtitle>
3
4 <para>Video overlay devices have the ability to genlock (TV-)video
5into the (VGA-)video signal of a graphics card, or to store captured
6images directly in video memory of a graphics card, typically with
7clipping. This can be considerable more efficient than capturing
8images and displaying them by other means. In the old days when only
9nuclear power plants needed cooling towers this used to be the only
10way to put live video into a window.</para>
11
12 <para>Video overlay devices are accessed through the same character
13special files as <link linkend="capture">video capture</link> devices.
14Note the default function of a <filename>/dev/video</filename> device
15is video capturing. The overlay function is only available after
16calling the &VIDIOC-S-FMT; ioctl.</para>
17
18 <para>The driver may support simultaneous overlay and capturing
19using the read/write and streaming I/O methods. If so, operation at
20the nominal frame rate of the video standard is not guaranteed. Frames
21may be directed away from overlay to capture, or one field may be used
22for overlay and the other for capture if the capture parameters permit
23this.</para>
24
25 <para>Applications should use different file descriptors for
26capturing and overlay. This must be supported by all drivers capable
27of simultaneous capturing and overlay. Optionally these drivers may
28also permit capturing and overlay with a single file descriptor for
29compatibility with V4L and earlier versions of V4L2.<footnote>
30 <para>A common application of two file descriptors is the
31XFree86 <link linkend="xvideo">Xv/V4L</link> interface driver and
32a V4L2 application. While the X server controls video overlay, the
33application can take advantage of memory mapping and DMA.</para>
34 <para>In the opinion of the designers of this API, no driver
35writer taking the efforts to support simultaneous capturing and
36overlay will restrict this ability by requiring a single file
37descriptor, as in V4L and earlier versions of V4L2. Making this
38optional means applications depending on two file descriptors need
39backup routines to be compatible with all drivers, which is
40considerable more work than using two fds in applications which do
41not. Also two fd's fit the general concept of one file descriptor for
42each logical stream. Hence as a complexity trade-off drivers
43<emphasis>must</emphasis> support two file descriptors and
44<emphasis>may</emphasis> support single fd operation.</para>
45 </footnote></para>
46
47 <section>
48 <title>Querying Capabilities</title>
49
50 <para>Devices supporting the video overlay interface set the
51<constant>V4L2_CAP_VIDEO_OVERLAY</constant> flag in the
52<structfield>capabilities</structfield> field of &v4l2-capability;
53returned by the &VIDIOC-QUERYCAP; ioctl. The overlay I/O method specified
54below must be supported. Tuners and audio inputs are optional.</para>
55 </section>
56
57 <section>
58 <title>Supplemental Functions</title>
59
60 <para>Video overlay devices shall support <link
61linkend="audio">audio input</link>, <link
62linkend="tuner">tuner</link>, <link linkend="control">controls</link>,
63<link linkend="crop">cropping and scaling</link> and <link
64linkend="streaming-par">streaming parameter</link> ioctls as needed.
65The <link linkend="video">video input</link> and <link
66linkend="standard">video standard</link> ioctls must be supported by
67all video overlay devices.</para>
68 </section>
69
70 <section>
71 <title>Setup</title>
72
73 <para>Before overlay can commence applications must program the
74driver with frame buffer parameters, namely the address and size of
75the frame buffer and the image format, for example RGB 5:6:5. The
76&VIDIOC-G-FBUF; and &VIDIOC-S-FBUF; ioctls are available to get
77and set these parameters, respectively. The
78<constant>VIDIOC_S_FBUF</constant> ioctl is privileged because it
79allows to set up DMA into physical memory, bypassing the memory
80protection mechanisms of the kernel. Only the superuser can change the
81frame buffer address and size. Users are not supposed to run TV
82applications as root or with SUID bit set. A small helper application
83with suitable privileges should query the graphics system and program
84the V4L2 driver at the appropriate time.</para>
85
86 <para>Some devices add the video overlay to the output signal
87of the graphics card. In this case the frame buffer is not modified by
88the video device, and the frame buffer address and pixel format are
89not needed by the driver. The <constant>VIDIOC_S_FBUF</constant> ioctl
90is not privileged. An application can check for this type of device by
91calling the <constant>VIDIOC_G_FBUF</constant> ioctl.</para>
92
93 <para>A driver may support any (or none) of five clipping/blending
94methods:<orderedlist>
95 <listitem>
96 <para>Chroma-keying displays the overlaid image only where
97pixels in the primary graphics surface assume a certain color.</para>
98 </listitem>
99 <listitem>
100 <para>A bitmap can be specified where each bit corresponds
101to a pixel in the overlaid image. When the bit is set, the
102corresponding video pixel is displayed, otherwise a pixel of the
103graphics surface.</para>
104 </listitem>
105 <listitem>
106 <para>A list of clipping rectangles can be specified. In
107these regions <emphasis>no</emphasis> video is displayed, so the
108graphics surface can be seen here.</para>
109 </listitem>
110 <listitem>
111 <para>The framebuffer has an alpha channel that can be used
112to clip or blend the framebuffer with the video.</para>
113 </listitem>
114 <listitem>
115 <para>A global alpha value can be specified to blend the
116framebuffer contents with video images.</para>
117 </listitem>
118 </orderedlist></para>
119
120 <para>When simultaneous capturing and overlay is supported and
121the hardware prohibits different image and frame buffer formats, the
122format requested first takes precedence. The attempt to capture
123(&VIDIOC-S-FMT;) or overlay (&VIDIOC-S-FBUF;) may fail with an
124&EBUSY; or return accordingly modified parameters..</para>
125 </section>
126
127 <section>
128 <title>Overlay Window</title>
129
130 <para>The overlaid image is determined by cropping and overlay
131window parameters. The former select an area of the video picture to
132capture, the latter how images are overlaid and clipped. Cropping
133initialization at minimum requires to reset the parameters to
134defaults. An example is given in <xref linkend="crop" />.</para>
135
136 <para>The overlay window is described by a &v4l2-window;. It
137defines the size of the image, its position over the graphics surface
138and the clipping to be applied. To get the current parameters
139applications set the <structfield>type</structfield> field of a
140&v4l2-format; to <constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant> and
141call the &VIDIOC-G-FMT; ioctl. The driver fills the
142<structname>v4l2_window</structname> substructure named
143<structfield>win</structfield>. It is not possible to retrieve a
144previously programmed clipping list or bitmap.</para>
145
146 <para>To program the overlay window applications set the
147<structfield>type</structfield> field of a &v4l2-format; to
148<constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant>, initialize the
149<structfield>win</structfield> substructure and call the
150&VIDIOC-S-FMT; ioctl. The driver adjusts the parameters against
151hardware limits and returns the actual parameters as
152<constant>VIDIOC_G_FMT</constant> does. Like
153<constant>VIDIOC_S_FMT</constant>, the &VIDIOC-TRY-FMT; ioctl can be
154used to learn about driver capabilities without actually changing
155driver state. Unlike <constant>VIDIOC_S_FMT</constant> this also works
156after the overlay has been enabled.</para>
157
158 <para>The scaling factor of the overlaid image is implied by the
159width and height given in &v4l2-window; and the size of the cropping
160rectangle. For more information see <xref linkend="crop" />.</para>
161
162 <para>When simultaneous capturing and overlay is supported and
163the hardware prohibits different image and window sizes, the size
164requested first takes precedence. The attempt to capture or overlay as
165well (&VIDIOC-S-FMT;) may fail with an &EBUSY; or return accordingly
166modified parameters.</para>
167
168 <table pgwide="1" frame="none" id="v4l2-window">
169 <title>struct <structname>v4l2_window</structname></title>
170 <tgroup cols="3">
171 &cs-str;
172 <tbody valign="top">
173 <row>
174 <entry>&v4l2-rect;</entry>
175 <entry><structfield>w</structfield></entry>
176 <entry>Size and position of the window relative to the
177top, left corner of the frame buffer defined with &VIDIOC-S-FBUF;. The
178window can extend the frame buffer width and height, the
179<structfield>x</structfield> and <structfield>y</structfield>
180coordinates can be negative, and it can lie completely outside the
181frame buffer. The driver clips the window accordingly, or if that is
182not possible, modifies its size and/or position.</entry>
183 </row>
184 <row>
185 <entry>&v4l2-field;</entry>
186 <entry><structfield>field</structfield></entry>
187 <entry>Applications set this field to determine which
188video field shall be overlaid, typically one of
189<constant>V4L2_FIELD_ANY</constant> (0),
190<constant>V4L2_FIELD_TOP</constant>,
191<constant>V4L2_FIELD_BOTTOM</constant> or
192<constant>V4L2_FIELD_INTERLACED</constant>. Drivers may have to choose
193a different field order and return the actual setting here.</entry>
194 </row>
195 <row>
196 <entry>__u32</entry>
197 <entry><structfield>chromakey</structfield></entry>
198 <entry>When chroma-keying has been negotiated with
199&VIDIOC-S-FBUF; applications set this field to the desired pixel value
200for the chroma key. The format is the same as the pixel format of the
201framebuffer (&v4l2-framebuffer;
202<structfield>fmt.pixelformat</structfield> field), with bytes in host
203order. E.&nbsp;g. for <link
204linkend="V4L2-PIX-FMT-BGR32"><constant>V4L2_PIX_FMT_BGR24</constant></link>
205the value should be 0xRRGGBB on a little endian, 0xBBGGRR on a big
206endian host.</entry>
207 </row>
208 <row>
209 <entry>&v4l2-clip; *</entry>
210 <entry><structfield>clips</structfield></entry>
211 <entry>When chroma-keying has <emphasis>not</emphasis>
212been negotiated and &VIDIOC-G-FBUF; indicated this capability,
213applications can set this field to point to an array of
214clipping rectangles.</entry>
215 </row>
216 <row>
217 <entry></entry>
218 <entry></entry>
219 <entry>Like the window coordinates
220<structfield>w</structfield>, clipping rectangles are defined relative
221to the top, left corner of the frame buffer. However clipping
222rectangles must not extend the frame buffer width and height, and they
223must not overlap. If possible applications should merge adjacent
224rectangles. Whether this must create x-y or y-x bands, or the order of
225rectangles, is not defined. When clip lists are not supported the
226driver ignores this field. Its contents after calling &VIDIOC-S-FMT;
227are undefined.</entry>
228 </row>
229 <row>
230 <entry>__u32</entry>
231 <entry><structfield>clipcount</structfield></entry>
232 <entry>When the application set the
233<structfield>clips</structfield> field, this field must contain the
234number of clipping rectangles in the list. When clip lists are not
235supported the driver ignores this field, its contents after calling
236<constant>VIDIOC_S_FMT</constant> are undefined. When clip lists are
237supported but no clipping is desired this field must be set to
238zero.</entry>
239 </row>
240 <row>
241 <entry>void *</entry>
242 <entry><structfield>bitmap</structfield></entry>
243 <entry>When chroma-keying has
244<emphasis>not</emphasis> been negotiated and &VIDIOC-G-FBUF; indicated
245this capability, applications can set this field to point to a
246clipping bit mask.</entry>
247 </row>
248 <row>
249 <entry spanname="hspan"><para>It must be of the same size
250as the window, <structfield>w.width</structfield> and
251<structfield>w.height</structfield>. Each bit corresponds to a pixel
252in the overlaid image, which is displayed only when the bit is
253<emphasis>set</emphasis>. Pixel coordinates translate to bits like:
254<programlisting>
255((__u8 *) <structfield>bitmap</structfield>)[<structfield>w.width</structfield> * y + x / 8] &amp; (1 &lt;&lt; (x &amp; 7))</programlisting></para><para>where <structfield>0</structfield> &le; x &lt;
256<structfield>w.width</structfield> and <structfield>0</structfield> &le;
257y &lt;<structfield>w.height</structfield>.<footnote>
258 <para>Should we require
259 <structfield>w.width</structfield> to be a multiple of
260 eight?</para>
261 </footnote></para><para>When a clipping
262bit mask is not supported the driver ignores this field, its contents
263after calling &VIDIOC-S-FMT; are undefined. When a bit mask is supported
264but no clipping is desired this field must be set to
265<constant>NULL</constant>.</para><para>Applications need not create a
266clip list or bit mask. When they pass both, or despite negotiating
267chroma-keying, the results are undefined. Regardless of the chosen
268method, the clipping abilities of the hardware may be limited in
269quantity or quality. The results when these limits are exceeded are
270undefined.<footnote>
271 <para>When the image is written into frame buffer
272memory it will be undesirable if the driver clips out less pixels
273than expected, because the application and graphics system are not
274aware these regions need to be refreshed. The driver should clip out
275more pixels or not write the image at all.</para>
276 </footnote></para></entry>
277 </row>
278 <row>
279 <entry>__u8</entry>
280 <entry><structfield>global_alpha</structfield></entry>
281 <entry>The global alpha value used to blend the
282framebuffer with video images, if global alpha blending has been
283negotiated (<constant>V4L2_FBUF_FLAG_GLOBAL_ALPHA</constant>, see
284&VIDIOC-S-FBUF;, <xref linkend="framebuffer-flags" />).</entry>
285 </row>
286 <row>
287 <entry></entry>
288 <entry></entry>
289 <entry>Note this field was added in Linux 2.6.23, extending the structure. However
290the <link linkend="vidioc-g-fmt">VIDIOC_G/S/TRY_FMT</link> ioctls,
291which take a pointer to a <link
292linkend="v4l2-format">v4l2_format</link> parent structure with padding
293bytes at the end, are not affected.</entry>
294 </row>
295 </tbody>
296 </tgroup>
297 </table>
298
299 <table pgwide="1" frame="none" id="v4l2-clip">
300 <title>struct <structname>v4l2_clip</structname><footnote>
301 <para>The X Window system defines "regions" which are
302vectors of struct BoxRec { short x1, y1, x2, y2; } with width = x2 -
303x1 and height = y2 - y1, so one cannot pass X11 clip lists
304directly.</para>
305 </footnote></title>
306 <tgroup cols="3">
307 &cs-str;
308 <tbody valign="top">
309 <row>
310 <entry>&v4l2-rect;</entry>
311 <entry><structfield>c</structfield></entry>
312 <entry>Coordinates of the clipping rectangle, relative to
313the top, left corner of the frame buffer. Only window pixels
314<emphasis>outside</emphasis> all clipping rectangles are
315displayed.</entry>
316 </row>
317 <row>
318 <entry>&v4l2-clip; *</entry>
319 <entry><structfield>next</structfield></entry>
320 <entry>Pointer to the next clipping rectangle, NULL when
321this is the last rectangle. Drivers ignore this field, it cannot be
322used to pass a linked list of clipping rectangles.</entry>
323 </row>
324 </tbody>
325 </tgroup>
326 </table>
327
328 <!-- NB for easier reading this table is duplicated
329 in the vidioc-cropcap chapter.-->
330
331 <table pgwide="1" frame="none" id="v4l2-rect">
332 <title>struct <structname>v4l2_rect</structname></title>
333 <tgroup cols="3">
334 &cs-str;
335 <tbody valign="top">
336 <row>
337 <entry>__s32</entry>
338 <entry><structfield>left</structfield></entry>
339 <entry>Horizontal offset of the top, left corner of the
340rectangle, in pixels.</entry>
341 </row>
342 <row>
343 <entry>__s32</entry>
344 <entry><structfield>top</structfield></entry>
345 <entry>Vertical offset of the top, left corner of the
346rectangle, in pixels. Offsets increase to the right and down.</entry>
347 </row>
348 <row>
349 <entry>__s32</entry>
350 <entry><structfield>width</structfield></entry>
351 <entry>Width of the rectangle, in pixels.</entry>
352 </row>
353 <row>
354 <entry>__s32</entry>
355 <entry><structfield>height</structfield></entry>
356 <entry>Height of the rectangle, in pixels. Width and
357height cannot be negative, the fields are signed for hysterical
358reasons. <!-- video4linux-list@redhat.com on 22 Oct 2002 subject
359"Re:[V4L][patches!] Re:v4l2/kernel-2.5" --></entry>
360 </row>
361 </tbody>
362 </tgroup>
363 </table>
364 </section>
365
366 <section>
367 <title>Enabling Overlay</title>
368
369 <para>To start or stop the frame buffer overlay applications call
370the &VIDIOC-OVERLAY; ioctl.</para>
371 </section>
372
373 <!--
374Local Variables:
375mode: sgml
376sgml-parent-document: "v4l2.sgml"
377indent-tabs-mode: nil
378End:
379 -->
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