1 <section id="selection-api">
3 <title>Experimental API for cropping, composing and scaling</title>
6 <title>Experimental</title>
8 <para>This is an <link linkend="experimental">experimental</link>
9 interface and may change in the future.</para>
13 <title>Introduction</title>
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>
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>
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>
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>
41 <title>Selection targets</title>
43 <figure id="sel-targets-capture">
44 <title>Cropping and composing targets</title>
47 <imagedata fileref="selection.png" format="PNG" />
50 <phrase>Targets used by a cropping, composing and scaling
56 For complete list of the available selection targets see table <xref
57 linkend="v4l2-sel-target"/>
63 <title>Configuration</title>
65 <para>Applications can use the <link linkend="vidioc-g-selection">selection
66 API</link> to select an area in a video signal or a buffer, and to query for
67 default settings and hardware limits.</para>
69 <para>Video hardware can have various cropping, composing and scaling
70 limitations. It may only scale up or down, support only discrete scaling
71 factors, or have different scaling abilities in the horizontal and vertical
72 directions. Also it may not support scaling at all. At the same time the
73 cropping/composing rectangles may have to be aligned, and both the source and
74 the sink may have arbitrary upper and lower size limits. Therefore, as usual,
75 drivers are expected to adjust the requested parameters and return the actual
76 values selected. An application can control the rounding behaviour using <link
77 linkend="v4l2-sel-flags"> constraint flags </link>.</para>
81 <title>Configuration of video capture</title>
83 <para>See figure <xref linkend="sel-targets-capture" /> for examples of the
84 selection targets available for a video capture device. It is recommended to
85 configure the cropping targets before to the composing targets.</para>
87 <para>The range of coordinates of the top left corner, width and height of
88 areas that can be sampled is given by the <constant> V4L2_SEL_TGT_CROP_BOUNDS
89 </constant> target. It is recommended for the driver developers to put the
90 top/left corner at position <constant> (0,0) </constant>. The rectangle's
91 coordinates are expressed in pixels.</para>
93 <para>The top left corner, width and height of the source rectangle, that is
94 the area actually sampled, is given by the <constant> V4L2_SEL_TGT_CROP_ACTIVE
95 </constant> target. It uses the same coordinate system as <constant>
96 V4L2_SEL_TGT_CROP_BOUNDS </constant>. The active cropping area must lie
97 completely inside the capture boundaries. The driver may further adjust the
98 requested size and/or position according to hardware limitations.</para>
100 <para>Each capture device has a default source rectangle, given by the
101 <constant> V4L2_SEL_TGT_CROP_DEFAULT </constant> target. This rectangle shall
102 over what the driver writer considers the complete picture. Drivers shall set
103 the active crop rectangle to the default when the driver is first loaded, but
106 <para>The composing targets refer to a memory buffer. The limits of composing
107 coordinates are obtained using <constant> V4L2_SEL_TGT_COMPOSE_BOUNDS
108 </constant>. All coordinates are expressed in pixels. The rectangle's top/left
109 corner must be located at position <constant> (0,0) </constant>. The width and
110 height are equal to the image size set by <constant> VIDIOC_S_FMT </constant>.
113 <para>The part of a buffer into which the image is inserted by the hardware is
114 controlled by the <constant> V4L2_SEL_TGT_COMPOSE_ACTIVE </constant> target.
115 The rectangle's coordinates are also expressed in the same coordinate system as
116 the bounds rectangle. The composing rectangle must lie completely inside bounds
117 rectangle. The driver must adjust the composing rectangle to fit to the
118 bounding limits. Moreover, the driver can perform other adjustments according
119 to hardware limitations. The application can control rounding behaviour using
120 <link linkend="v4l2-sel-flags"> constraint flags </link>.</para>
122 <para>For capture devices the default composing rectangle is queried using
123 <constant> V4L2_SEL_TGT_COMPOSE_DEFAULT </constant>. It is usually equal to the
124 bounding rectangle.</para>
126 <para>The part of a buffer that is modified by the hardware is given by
127 <constant> V4L2_SEL_TGT_COMPOSE_PADDED </constant>. It contains all pixels
128 defined using <constant> V4L2_SEL_TGT_COMPOSE_ACTIVE </constant> plus all
129 padding data modified by hardware during insertion process. All pixels outside
130 this rectangle <emphasis>must not</emphasis> be changed by the hardware. The
131 content of pixels that lie inside the padded area but outside active area is
132 undefined. The application can use the padded and active rectangles to detect
133 where the rubbish pixels are located and remove them if needed.</para>
139 <title>Configuration of video output</title>
141 <para>For output devices targets and ioctls are used similarly to the video
142 capture case. The <emphasis> composing </emphasis> rectangle refers to the
143 insertion of an image into a video signal. The cropping rectangles refer to a
144 memory buffer. It is recommended to configure the composing targets before to
145 the cropping targets.</para>
147 <para>The cropping targets refer to the memory buffer that contains an image to
148 be inserted into a video signal or graphical screen. The limits of cropping
149 coordinates are obtained using <constant> V4L2_SEL_TGT_CROP_BOUNDS </constant>.
150 All coordinates are expressed in pixels. The top/left corner is always point
151 <constant> (0,0) </constant>. The width and height is equal to the image size
152 specified using <constant> VIDIOC_S_FMT </constant> ioctl.</para>
154 <para>The top left corner, width and height of the source rectangle, that is
155 the area from which image date are processed by the hardware, is given by the
156 <constant> V4L2_SEL_TGT_CROP_ACTIVE </constant>. Its coordinates are expressed
157 in in the same coordinate system as the bounds rectangle. The active cropping
158 area must lie completely inside the crop boundaries and the driver may further
159 adjust the requested size and/or position according to hardware
162 <para>For output devices the default cropping rectangle is queried using
163 <constant> V4L2_SEL_TGT_CROP_DEFAULT </constant>. It is usually equal to the
164 bounding rectangle.</para>
166 <para>The part of a video signal or graphics display where the image is
167 inserted by the hardware is controlled by <constant>
168 V4L2_SEL_TGT_COMPOSE_ACTIVE </constant> target. The rectangle's coordinates
169 are expressed in pixels. The composing rectangle must lie completely inside the
170 bounds rectangle. The driver must adjust the area to fit to the bounding
171 limits. Moreover, the driver can perform other adjustments according to
172 hardware limitations. </para>
174 <para>The device has a default composing rectangle, given by the <constant>
175 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)
178 </constant>. Drivers shall set the active composing rectangle to the default
179 one when the driver is first loaded.</para>
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
186 </constant> identifier. It must contain all pixels from the <constant>
187 V4L2_SEL_TGT_COMPOSE_ACTIVE </constant> target.</para>
193 <title>Scaling control</title>
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_ACTIVE
197 </constant> and <constant> V4L2_SEL_TGT_COMPOSE_ACTIVE </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>
207 <title>Comparison with old cropping API</title>
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 <structfield>
220 &v4l2-pix-format;::bytesperline </structfield>. Introducing an image offsets
221 could be done by modifying field <structfield> &v4l2-buffer;::m:userptr
222 </structfield> 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, <structname> &v4l2-crop;
228 </structname> and <structname> &v4l2-cropcap; </structname> have no reserved
229 fields. Therefore there is no way to extend their functionality. The new
230 <structname> &v4l2-selection; </structname> provides a lot of place for future
231 extensions. Driver developers are encouraged to implement only selection API.
232 The former cropping API would be simulated using the new one. </para>
237 <title>Examples</title>
239 <title>Resetting the cropping parameters</title>
241 <para>(A video capture device is assumed; change <constant>
242 V4L2_BUF_TYPE_VIDEO_CAPTURE </constant> for other devices; change target to
243 <constant> V4L2_SEL_TGT_COMPOSE_* </constant> family to configure composing
248 &v4l2-selection; sel = {
249 .type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
250 .target = V4L2_SEL_TGT_CROP_DEFAULT,
252 ret = ioctl(fd, &VIDIOC-G-SELECTION;, &sel);
255 sel.target = V4L2_SEL_TGT_CROP_ACTIVE;
256 ret = ioctl(fd, &VIDIOC-S-SELECTION;, &sel);
264 <title>Simple downscaling</title>
265 <para>Setting a composing area on output of size of <emphasis> at most
266 </emphasis> half of limit placed at a center of a display.</para>
269 &v4l2-selection; sel = {
270 .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
271 .target = V4L2_SEL_TGT_COMPOSE_BOUNDS,
275 ret = ioctl(fd, &VIDIOC-G-SELECTION;, &sel);
278 /* setting smaller compose rectangle */
279 r.width = sel.r.width / 2;
280 r.height = sel.r.height / 2;
281 r.left = sel.r.width / 4;
282 r.top = sel.r.height / 4;
284 sel.target = V4L2_SEL_TGT_COMPOSE_ACTIVE;
285 sel.flags = V4L2_SEL_FLAG_LE;
286 ret = ioctl(fd, &VIDIOC-S-SELECTION;, &sel);
294 <title>Querying for scaling factors</title>
295 <para>A video output device is assumed; change <constant>
296 V4L2_BUF_TYPE_VIDEO_OUTPUT </constant> for other devices</para>
299 &v4l2-selection; compose = {
300 .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
301 .target = V4L2_SEL_TGT_COMPOSE_ACTIVE,
303 &v4l2-selection; crop = {
304 .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
305 .target = V4L2_SEL_TGT_CROP_ACTIVE,
307 double hscale, vscale;
309 ret = ioctl(fd, &VIDIOC-G-SELECTION;, &compose);
312 ret = ioctl(fd, &VIDIOC-G-SELECTION;, &crop);
316 /* computing scaling factors */
317 hscale = (double)compose.r.width / crop.r.width;
318 vscale = (double)compose.r.height / crop.r.height;