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Based on kernel version 4.0. Page generated on 2015-04-14 21:23 EST.

1	<section id="selection-api">
2	
3	  <title>Experimental API for cropping, composing and scaling</title>
4	
5	      <note>
6		<title>Experimental</title>
7	
8		<para>This is an <link linkend="experimental">experimental</link>
9	interface and may change in the future.</para>
10	      </note>
11	
12	  <section>
13	    <title>Introduction</title>
14	
15	<para>Some video capture devices can sample a subsection of a picture and
16	shrink or enlarge it to an image of arbitrary size. Next, the devices can
17	insert the image into larger one. Some video output devices can crop part of an
18	input image, scale it up or down and insert it at an arbitrary scan line and
19	horizontal offset into a video signal. We call these abilities cropping,
20	scaling and composing.</para>
21	
22	<para>On a video <emphasis>capture</emphasis> device the source is a video
23	signal, and the cropping target determine the area actually sampled. The sink
24	is an image stored in a memory buffer.  The composing area specifies which part
25	of the buffer is actually written to by the hardware. </para>
26	
27	<para>On a video <emphasis>output</emphasis> device the source is an image in a
28	memory buffer, and the cropping target is a part of an image to be shown on a
29	display. The sink is the display or the graphics screen. The application may
30	select the part of display where the image should be displayed. The size and
31	position of such a window is controlled by the compose target.</para>
32	
33	<para>Rectangles for all cropping and composing targets are defined even if the
34	device does supports neither cropping nor composing. Their size and position
35	will be fixed in such a case. If the device does not support scaling then the
36	cropping and composing rectangles have the same size.</para>
37	
38	  </section>
39	
40	    <section>
41	      <title>Selection targets</title>
42	
43	      <para>
44	      <figure id="sel-targets-capture">
45		<title>Cropping and composing targets</title>
46		<mediaobject>
47		  <imageobject>
48		    <imagedata fileref="selection.png" format="PNG" />
49		  </imageobject>
50		  <textobject>
51		    <phrase>Targets used by a cropping, composing and scaling
52	            process</phrase>
53		  </textobject>
54		</mediaobject>
55	      </figure>
56	      </para>
57	
58	      <para>See <xref linkend="v4l2-selection-targets" /> for more
59	    information.</para>
60	    </section>
61	
62	  <section>
63	
64	  <title>Configuration</title>
65	
66	<para>Applications can use the <link linkend="vidioc-g-selection">selection
67	API</link> to select an area in a video signal or a buffer, and to query for
68	default settings and hardware limits.</para>
69	
70	<para>Video hardware can have various cropping, composing and scaling
71	limitations. It may only scale up or down, support only discrete scaling
72	factors, or have different scaling abilities in the horizontal and vertical
73	directions. Also it may not support scaling at all. At the same time the
74	cropping/composing rectangles may have to be aligned, and both the source and
75	the sink may have arbitrary upper and lower size limits. Therefore, as usual,
76	drivers are expected to adjust the requested parameters and return the actual
77	values selected. An application can control the rounding behaviour using <link
78	linkend="v4l2-selection-flags"> constraint flags </link>.</para>
79	
80	   <section>
81	
82	   <title>Configuration of video capture</title>
83	
84	<para>See figure <xref linkend="sel-targets-capture" /> for examples of the
85	selection targets available for a video capture device.  It is recommended to
86	configure the cropping targets before to the composing targets.</para>
87	
88	<para>The range of coordinates of the top left corner, width and height of
89	areas that can be sampled is given by the <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant>
90	target. It is recommended for the driver developers to put the
91	top/left corner at position <constant>(0,0)</constant>.  The rectangle's
92	coordinates are expressed in pixels.</para>
93	
94	<para>The top left corner, width and height of the source rectangle, that is
95	the area actually sampled, is given by the <constant>V4L2_SEL_TGT_CROP</constant>
96	target. It uses the same coordinate system as <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant>.
97	The active cropping area must lie completely inside the capture boundaries. The
98	driver may further adjust the requested size and/or position according to hardware
99	limitations.</para>
100	
101	<para>Each capture device has a default source rectangle, given by the
102	<constant>V4L2_SEL_TGT_CROP_DEFAULT</constant> target. This rectangle shall
103	over what the driver writer considers the complete picture.  Drivers shall set
104	the active crop rectangle to the default when the driver is first loaded, but
105	not later.</para>
106	
107	<para>The composing targets refer to a memory buffer. The limits of composing
108	coordinates are obtained using <constant>V4L2_SEL_TGT_COMPOSE_BOUNDS</constant>.
109	All coordinates are expressed in pixels. The rectangle's top/left
110	corner must be located at position <constant>(0,0)</constant>. The width and
111	height are equal to the image size set by <constant>VIDIOC_S_FMT</constant>.
112	</para>
113	
114	<para>The part of a buffer into which the image is inserted by the hardware is
115	controlled by the <constant>V4L2_SEL_TGT_COMPOSE</constant> target.
116	The rectangle's coordinates are also expressed in the same coordinate system as
117	the bounds rectangle. The composing rectangle must lie completely inside bounds
118	rectangle. The driver must adjust the composing rectangle to fit to the
119	bounding limits. Moreover, the driver can perform other adjustments according
120	to hardware limitations. The application can control rounding behaviour using
121	<link linkend="v4l2-selection-flags"> constraint flags</link>.</para>
122	
123	<para>For capture devices the default composing rectangle is queried using
124	<constant>V4L2_SEL_TGT_COMPOSE_DEFAULT</constant>. It is usually equal to the
125	bounding rectangle.</para>
126	
127	<para>The part of a buffer that is modified by the hardware is given by
128	<constant>V4L2_SEL_TGT_COMPOSE_PADDED</constant>. It contains all pixels
129	defined using <constant>V4L2_SEL_TGT_COMPOSE</constant> plus all
130	padding data modified by hardware during insertion process. All pixels outside
131	this rectangle <emphasis>must not</emphasis> be changed by the hardware. The
132	content of pixels that lie inside the padded area but outside active area is
133	undefined. The application can use the padded and active rectangles to detect
134	where the rubbish pixels are located and remove them if needed.</para>
135	
136	   </section>
137	
138	   <section>
139	
140	   <title>Configuration of video output</title>
141	
142	<para>For output devices targets and ioctls are used similarly to the video
143	capture case. The <emphasis>composing</emphasis> rectangle refers to the
144	insertion of an image into a video signal. The cropping rectangles refer to a
145	memory buffer. It is recommended to configure the composing targets before to
146	the cropping targets.</para>
147	
148	<para>The cropping targets refer to the memory buffer that contains an image to
149	be inserted into a video signal or graphical screen. The limits of cropping
150	coordinates are obtained using <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant>.
151	All coordinates are expressed in pixels. The top/left corner is always point
152	<constant>(0,0)</constant>.  The width and height is equal to the image size
153	specified using <constant>VIDIOC_S_FMT</constant> ioctl.</para>
154	
155	<para>The top left corner, width and height of the source rectangle, that is
156	the area from which image date are processed by the hardware, is given by the
157	<constant>V4L2_SEL_TGT_CROP</constant>. Its coordinates are expressed
158	in in the same coordinate system as the bounds rectangle. The active cropping
159	area must lie completely inside the crop boundaries and the driver may further
160	adjust the requested size and/or position according to hardware
161	limitations.</para>
162	
163	<para>For output devices the default cropping rectangle is queried using
164	<constant>V4L2_SEL_TGT_CROP_DEFAULT</constant>. It is usually equal to the
165	bounding rectangle.</para>
166	
167	<para>The part of a video signal or graphics display where the image is
168	inserted by the hardware is controlled by <constant>V4L2_SEL_TGT_COMPOSE</constant>
169	target.  The rectangle's coordinates are expressed in pixels. The composing
170	rectangle must lie completely inside the bounds rectangle.  The driver must
171	adjust the area to fit to the bounding limits.  Moreover, the driver can
172	perform other adjustments according to hardware limitations.</para>
173	
174	<para>The device has a default composing rectangle, given by the
175	<constant>V4L2_SEL_TGT_COMPOSE_DEFAULT</constant> target. This rectangle shall cover what
176	the driver writer considers the complete picture. It is recommended for the
177	driver developers to put the top/left corner at position <constant>(0,0)</constant>.
178	Drivers shall set the active composing rectangle to the default
179	one when the driver is first loaded.</para>
180	
181	<para>The devices may introduce additional content to video signal other than
182	an image from memory buffers.  It includes borders around an image. However,
183	such a padded area is driver-dependent feature not covered by this document.
184	Driver developers are encouraged to keep padded rectangle equal to active one.
185	The padded target is accessed by the <constant>V4L2_SEL_TGT_COMPOSE_PADDED</constant>
186	identifier.  It must contain all pixels from the <constant>V4L2_SEL_TGT_COMPOSE</constant>
187	target.</para>
188	
189	   </section>
190	
191	   <section>
192	
193	     <title>Scaling control</title>
194	
195	<para>An application can detect if scaling is performed by comparing the width
196	and the height of rectangles obtained using <constant>V4L2_SEL_TGT_CROP</constant>
197	and <constant>V4L2_SEL_TGT_COMPOSE</constant> targets. If
198	these are not equal then the scaling is applied. The application can compute
199	the scaling ratios using these values.</para>
200	
201	   </section>
202	
203	  </section>
204	
205	  <section>
206	
207	    <title>Comparison with old cropping API</title>
208	
209	<para>The selection API was introduced to cope with deficiencies of previous
210	<link linkend="crop"> API</link>, that was designed to control simple capture
211	devices. Later the cropping API was adopted by video output drivers. The ioctls
212	are used to select a part of the display were the video signal is inserted. It
213	should be considered as an API abuse because the described operation is
214	actually the composing.  The selection API makes a clear distinction between
215	composing and cropping operations by setting the appropriate targets.  The V4L2
216	API lacks any support for composing to and cropping from an image inside a
217	memory buffer.  The application could configure a capture device to fill only a
218	part of an image by abusing V4L2 API.  Cropping a smaller image from a larger
219	one is achieved by setting the field
220	&v4l2-pix-format;<structfield>::bytesperline</structfield>.  Introducing an image offsets
221	could be done by modifying field &v4l2-buffer;<structfield>::m_userptr</structfield>
222	before calling <constant>VIDIOC_QBUF</constant>. Those
223	operations should be avoided because they are not portable (endianness), and do
224	not work for macroblock and Bayer formats and mmap buffers.  The selection API
225	deals with configuration of buffer cropping/composing in a clear, intuitive and
226	portable way.  Next, with the selection API the concepts of the padded target
227	and constraints flags are introduced.  Finally, &v4l2-crop; and &v4l2-cropcap;
228	have no reserved fields. Therefore there is no way to extend their functionality.
229	The new &v4l2-selection; provides a lot of place for future
230	extensions.  Driver developers are encouraged to implement only selection API.
231	The former cropping API would be simulated using the new one.</para>
232	
233	  </section>
234	
235	   <section>
236	      <title>Examples</title>
237	      <example>
238		<title>Resetting the cropping parameters</title>
239	
240		<para>(A video capture device is assumed; change
241	<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> for other devices; change target to
242	<constant>V4L2_SEL_TGT_COMPOSE_*</constant> family to configure composing
243	area)</para>
244	
245		<programlisting>
246	
247		&v4l2-selection; sel = {
248			.type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
249			.target = V4L2_SEL_TGT_CROP_DEFAULT,
250		};
251		ret = ioctl(fd, &VIDIOC-G-SELECTION;, &amp;sel);
252		if (ret)
253			exit(-1);
254		sel.target = V4L2_SEL_TGT_CROP;
255		ret = ioctl(fd, &VIDIOC-S-SELECTION;, &amp;sel);
256		if (ret)
257			exit(-1);
258	
259	        </programlisting>
260	      </example>
261	
262	      <example>
263		<title>Simple downscaling</title>
264		<para>Setting a composing area on output of size of <emphasis> at most
265	</emphasis> half of limit placed at a center of a display.</para>
266		<programlisting>
267	
268		&v4l2-selection; sel = {
269			.type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
270			.target = V4L2_SEL_TGT_COMPOSE_BOUNDS,
271		};
272		struct v4l2_rect r;
273	
274		ret = ioctl(fd, &VIDIOC-G-SELECTION;, &amp;sel);
275		if (ret)
276			exit(-1);
277		/* setting smaller compose rectangle */
278		r.width = sel.r.width / 2;
279		r.height = sel.r.height / 2;
280		r.left = sel.r.width / 4;
281		r.top = sel.r.height / 4;
282		sel.r = r;
283		sel.target = V4L2_SEL_TGT_COMPOSE;
284		sel.flags = V4L2_SEL_FLAG_LE;
285		ret = ioctl(fd, &VIDIOC-S-SELECTION;, &amp;sel);
286		if (ret)
287			exit(-1);
288	
289	        </programlisting>
290	      </example>
291	
292	      <example>
293		<title>Querying for scaling factors</title>
294		<para>A video output device is assumed; change
295	<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant> for other devices</para>
296		<programlisting>
297	
298		&v4l2-selection; compose = {
299			.type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
300			.target = V4L2_SEL_TGT_COMPOSE,
301		};
302		&v4l2-selection; crop = {
303			.type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
304			.target = V4L2_SEL_TGT_CROP,
305		};
306		double hscale, vscale;
307	
308		ret = ioctl(fd, &VIDIOC-G-SELECTION;, &amp;compose);
309		if (ret)
310			exit(-1);
311		ret = ioctl(fd, &VIDIOC-G-SELECTION;, &amp;crop);
312		if (ret)
313			exit(-1);
314	
315		/* computing scaling factors */
316		hscale = (double)compose.r.width / crop.r.width;
317		vscale = (double)compose.r.height / crop.r.height;
318	
319		</programlisting>
320	      </example>
321	
322	   </section>
323	
324	</section>
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