Based on kernel version 4.7.2. Page generated on 2016-08-22 22:45 EST.
1 <section id="selection-api"> 2 3 <title>API for cropping, composing and scaling</title> 4 5 <section> 6 <title>Introduction</title> 7 8 <para>Some video capture devices can sample a subsection of a picture and 9 shrink or enlarge it to an image of arbitrary size. Next, the devices can 10 insert the image into larger one. Some video output devices can crop part of an 11 input image, scale it up or down and insert it at an arbitrary scan line and 12 horizontal offset into a video signal. We call these abilities cropping, 13 scaling and composing.</para> 14 15 <para>On a video <emphasis>capture</emphasis> device the source is a video 16 signal, and the cropping target determine the area actually sampled. The sink 17 is an image stored in a memory buffer. The composing area specifies which part 18 of the buffer is actually written to by the hardware. </para> 19 20 <para>On a video <emphasis>output</emphasis> device the source is an image in a 21 memory buffer, and the cropping target is a part of an image to be shown on a 22 display. The sink is the display or the graphics screen. The application may 23 select the part of display where the image should be displayed. The size and 24 position of such a window is controlled by the compose target.</para> 25 26 <para>Rectangles for all cropping and composing targets are defined even if the 27 device does supports neither cropping nor composing. Their size and position 28 will be fixed in such a case. If the device does not support scaling then the 29 cropping and composing rectangles have the same size.</para> 30 31 </section> 32 33 <section> 34 <title>Selection targets</title> 35 36 <para> 37 <figure id="sel-targets-capture"> 38 <title>Cropping and composing targets</title> 39 <mediaobject> 40 <imageobject> 41 <imagedata fileref="selection.png" format="PNG" /> 42 </imageobject> 43 <textobject> 44 <phrase>Targets used by a cropping, composing and scaling 45 process</phrase> 46 </textobject> 47 </mediaobject> 48 </figure> 49 </para> 50 51 <para>See <xref linkend="v4l2-selection-targets" /> for more 52 information.</para> 53 </section> 54 55 <section> 56 57 <title>Configuration</title> 58 59 <para>Applications can use the <link linkend="vidioc-g-selection">selection 60 API</link> to select an area in a video signal or a buffer, and to query for 61 default settings and hardware limits.</para> 62 63 <para>Video hardware can have various cropping, composing and scaling 64 limitations. It may only scale up or down, support only discrete scaling 65 factors, or have different scaling abilities in the horizontal and vertical 66 directions. Also it may not support scaling at all. At the same time the 67 cropping/composing rectangles may have to be aligned, and both the source and 68 the sink may have arbitrary upper and lower size limits. Therefore, as usual, 69 drivers are expected to adjust the requested parameters and return the actual 70 values selected. An application can control the rounding behaviour using <link 71 linkend="v4l2-selection-flags"> constraint flags </link>.</para> 72 73 <section> 74 75 <title>Configuration of video capture</title> 76 77 <para>See figure <xref linkend="sel-targets-capture" /> for examples of the 78 selection targets available for a video capture device. It is recommended to 79 configure the cropping targets before to the composing targets.</para> 80 81 <para>The range of coordinates of the top left corner, width and height of 82 areas that can be sampled is given by the <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant> 83 target. It is recommended for the driver developers to put the 84 top/left corner at position <constant>(0,0)</constant>. The rectangle's 85 coordinates are expressed in pixels.</para> 86 87 <para>The top left corner, width and height of the source rectangle, that is 88 the area actually sampled, is given by the <constant>V4L2_SEL_TGT_CROP</constant> 89 target. It uses the same coordinate system as <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant>. 90 The active cropping area must lie completely inside the capture boundaries. The 91 driver may further adjust the requested size and/or position according to hardware 92 limitations.</para> 93 94 <para>Each capture device has a default source rectangle, given by the 95 <constant>V4L2_SEL_TGT_CROP_DEFAULT</constant> target. This rectangle shall 96 over what the driver writer considers the complete picture. Drivers shall set 97 the active crop rectangle to the default when the driver is first loaded, but 98 not later.</para> 99 100 <para>The composing targets refer to a memory buffer. The limits of composing 101 coordinates are obtained using <constant>V4L2_SEL_TGT_COMPOSE_BOUNDS</constant>. 102 All coordinates are expressed in pixels. The rectangle's top/left 103 corner must be located at position <constant>(0,0)</constant>. The width and 104 height are equal to the image size set by <constant>VIDIOC_S_FMT</constant>. 105 </para> 106 107 <para>The part of a buffer into which the image is inserted by the hardware is 108 controlled by the <constant>V4L2_SEL_TGT_COMPOSE</constant> target. 109 The rectangle's coordinates are also expressed in the same coordinate system as 110 the bounds rectangle. The composing rectangle must lie completely inside bounds 111 rectangle. The driver must adjust the composing rectangle to fit to the 112 bounding limits. Moreover, the driver can perform other adjustments according 113 to hardware limitations. The application can control rounding behaviour using 114 <link linkend="v4l2-selection-flags"> constraint flags</link>.</para> 115 116 <para>For capture devices the default composing rectangle is queried using 117 <constant>V4L2_SEL_TGT_COMPOSE_DEFAULT</constant>. It is usually equal to the 118 bounding rectangle.</para> 119 120 <para>The part of a buffer that is modified by the hardware is given by 121 <constant>V4L2_SEL_TGT_COMPOSE_PADDED</constant>. It contains all pixels 122 defined using <constant>V4L2_SEL_TGT_COMPOSE</constant> plus all 123 padding data modified by hardware during insertion process. All pixels outside 124 this rectangle <emphasis>must not</emphasis> be changed by the hardware. The 125 content of pixels that lie inside the padded area but outside active area is 126 undefined. The application can use the padded and active rectangles to detect 127 where the rubbish pixels are located and remove them if needed.</para> 128 129 </section> 130 131 <section> 132 133 <title>Configuration of video output</title> 134 135 <para>For output devices targets and ioctls are used similarly to the video 136 capture case. The <emphasis>composing</emphasis> rectangle refers to the 137 insertion of an image into a video signal. The cropping rectangles refer to a 138 memory buffer. It is recommended to configure the composing targets before to 139 the cropping targets.</para> 140 141 <para>The cropping targets refer to the memory buffer that contains an image to 142 be inserted into a video signal or graphical screen. The limits of cropping 143 coordinates are obtained using <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant>. 144 All coordinates are expressed in pixels. The top/left corner is always point 145 <constant>(0,0)</constant>. The width and height is equal to the image size 146 specified using <constant>VIDIOC_S_FMT</constant> ioctl.</para> 147 148 <para>The top left corner, width and height of the source rectangle, that is 149 the area from which image date are processed by the hardware, is given by the 150 <constant>V4L2_SEL_TGT_CROP</constant>. Its coordinates are expressed 151 in in the same coordinate system as the bounds rectangle. The active cropping 152 area must lie completely inside the crop boundaries and the driver may further 153 adjust the requested size and/or position according to hardware 154 limitations.</para> 155 156 <para>For output devices the default cropping rectangle is queried using 157 <constant>V4L2_SEL_TGT_CROP_DEFAULT</constant>. It is usually equal to the 158 bounding rectangle.</para> 159 160 <para>The part of a video signal or graphics display where the image is 161 inserted by the hardware is controlled by <constant>V4L2_SEL_TGT_COMPOSE</constant> 162 target. The rectangle's coordinates are expressed in pixels. The composing 163 rectangle must lie completely inside the bounds rectangle. The driver must 164 adjust the area to fit to the bounding limits. Moreover, the driver can 165 perform other adjustments according to hardware limitations.</para> 166 167 <para>The device has a default composing rectangle, given by the 168 <constant>V4L2_SEL_TGT_COMPOSE_DEFAULT</constant> target. This rectangle shall cover what 169 the driver writer considers the complete picture. It is recommended for the 170 driver developers to put the top/left corner at position <constant>(0,0)</constant>. 171 Drivers shall set the active composing rectangle to the default 172 one when the driver is first loaded.</para> 173 174 <para>The devices may introduce additional content to video signal other than 175 an image from memory buffers. It includes borders around an image. However, 176 such a padded area is driver-dependent feature not covered by this document. 177 Driver developers are encouraged to keep padded rectangle equal to active one. 178 The padded target is accessed by the <constant>V4L2_SEL_TGT_COMPOSE_PADDED</constant> 179 identifier. It must contain all pixels from the <constant>V4L2_SEL_TGT_COMPOSE</constant> 180 target.</para> 181 182 </section> 183 184 <section> 185 186 <title>Scaling control</title> 187 188 <para>An application can detect if scaling is performed by comparing the width 189 and the height of rectangles obtained using <constant>V4L2_SEL_TGT_CROP</constant> 190 and <constant>V4L2_SEL_TGT_COMPOSE</constant> targets. If 191 these are not equal then the scaling is applied. The application can compute 192 the scaling ratios using these values.</para> 193 194 </section> 195 196 </section> 197 198 <section> 199 200 <title>Comparison with old cropping API</title> 201 202 <para>The selection API was introduced to cope with deficiencies of previous 203 <link linkend="crop"> API</link>, that was designed to control simple capture 204 devices. Later the cropping API was adopted by video output drivers. The ioctls 205 are used to select a part of the display were the video signal is inserted. It 206 should be considered as an API abuse because the described operation is 207 actually the composing. The selection API makes a clear distinction between 208 composing and cropping operations by setting the appropriate targets. The V4L2 209 API lacks any support for composing to and cropping from an image inside a 210 memory buffer. The application could configure a capture device to fill only a 211 part of an image by abusing V4L2 API. Cropping a smaller image from a larger 212 one is achieved by setting the field 213 &v4l2-pix-format;<structfield>::bytesperline</structfield>. Introducing an image offsets 214 could be done by modifying field &v4l2-buffer;<structfield>::m_userptr</structfield> 215 before calling <constant>VIDIOC_QBUF</constant>. Those 216 operations should be avoided because they are not portable (endianness), and do 217 not work for macroblock and Bayer formats and mmap buffers. The selection API 218 deals with configuration of buffer cropping/composing in a clear, intuitive and 219 portable way. Next, with the selection API the concepts of the padded target 220 and constraints flags are introduced. Finally, &v4l2-crop; and &v4l2-cropcap; 221 have no reserved fields. Therefore there is no way to extend their functionality. 222 The new &v4l2-selection; provides a lot of place for future 223 extensions. Driver developers are encouraged to implement only selection API. 224 The former cropping API would be simulated using the new one.</para> 225 226 </section> 227 228 <section> 229 <title>Examples</title> 230 <example> 231 <title>Resetting the cropping parameters</title> 232 233 <para>(A video capture device is assumed; change 234 <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> for other devices; change target to 235 <constant>V4L2_SEL_TGT_COMPOSE_*</constant> family to configure composing 236 area)</para> 237 238 <programlisting> 239 240 &v4l2-selection; sel = { 241 .type = V4L2_BUF_TYPE_VIDEO_CAPTURE, 242 .target = V4L2_SEL_TGT_CROP_DEFAULT, 243 }; 244 ret = ioctl(fd, &VIDIOC-G-SELECTION;, &sel); 245 if (ret) 246 exit(-1); 247 sel.target = V4L2_SEL_TGT_CROP; 248 ret = ioctl(fd, &VIDIOC-S-SELECTION;, &sel); 249 if (ret) 250 exit(-1); 251 252 </programlisting> 253 </example> 254 255 <example> 256 <title>Simple downscaling</title> 257 <para>Setting a composing area on output of size of <emphasis> at most 258 </emphasis> half of limit placed at a center of a display.</para> 259 <programlisting> 260 261 &v4l2-selection; sel = { 262 .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, 263 .target = V4L2_SEL_TGT_COMPOSE_BOUNDS, 264 }; 265 struct v4l2_rect r; 266 267 ret = ioctl(fd, &VIDIOC-G-SELECTION;, &sel); 268 if (ret) 269 exit(-1); 270 /* setting smaller compose rectangle */ 271 r.width = sel.r.width / 2; 272 r.height = sel.r.height / 2; 273 r.left = sel.r.width / 4; 274 r.top = sel.r.height / 4; 275 sel.r = r; 276 sel.target = V4L2_SEL_TGT_COMPOSE; 277 sel.flags = V4L2_SEL_FLAG_LE; 278 ret = ioctl(fd, &VIDIOC-S-SELECTION;, &sel); 279 if (ret) 280 exit(-1); 281 282 </programlisting> 283 </example> 284 285 <example> 286 <title>Querying for scaling factors</title> 287 <para>A video output device is assumed; change 288 <constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant> for other devices</para> 289 <programlisting> 290 291 &v4l2-selection; compose = { 292 .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, 293 .target = V4L2_SEL_TGT_COMPOSE, 294 }; 295 &v4l2-selection; crop = { 296 .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, 297 .target = V4L2_SEL_TGT_CROP, 298 }; 299 double hscale, vscale; 300 301 ret = ioctl(fd, &VIDIOC-G-SELECTION;, &compose); 302 if (ret) 303 exit(-1); 304 ret = ioctl(fd, &VIDIOC-G-SELECTION;, &crop); 305 if (ret) 306 exit(-1); 307 308 /* computing scaling factors */ 309 hscale = (double)compose.r.width / crop.r.width; 310 vscale = (double)compose.r.height / crop.r.height; 311 312 </programlisting> 313 </example> 314 315 </section> 316 317 </section>
