vidioc-g-parm.rst (8600B)
1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _VIDIOC_G_PARM: 5 6********************************** 7ioctl VIDIOC_G_PARM, VIDIOC_S_PARM 8********************************** 9 10Name 11==== 12 13VIDIOC_G_PARM - VIDIOC_S_PARM - Get or set streaming parameters 14 15Synopsis 16======== 17 18.. c:macro:: VIDIOC_G_PARM 19 20``int ioctl(int fd, VIDIOC_G_PARM, v4l2_streamparm *argp)`` 21 22.. c:macro:: VIDIOC_S_PARM 23 24``int ioctl(int fd, VIDIOC_S_PARM, v4l2_streamparm *argp)`` 25 26Arguments 27========= 28 29``fd`` 30 File descriptor returned by :c:func:`open()`. 31 32``argp`` 33 Pointer to struct :c:type:`v4l2_streamparm`. 34 35Description 36=========== 37 38Applications can request a different frame interval. The capture or 39output device will be reconfigured to support the requested frame 40interval if possible. Optionally drivers may choose to skip or 41repeat frames to achieve the requested frame interval. 42 43For stateful encoders (see :ref:`encoder`) this represents the 44frame interval that is typically embedded in the encoded video stream. 45 46Changing the frame interval shall never change the format. Changing the 47format, on the other hand, may change the frame interval. 48 49Further these ioctls can be used to determine the number of buffers used 50internally by a driver in read/write mode. For implications see the 51section discussing the :c:func:`read()` function. 52 53To get and set the streaming parameters applications call the 54:ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and 55:ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl, respectively. They take a 56pointer to a struct :c:type:`v4l2_streamparm` which contains a 57union holding separate parameters for input and output devices. 58 59.. tabularcolumns:: |p{3.7cm}|p{3.5cm}|p{10.1cm}| 60 61.. c:type:: v4l2_streamparm 62 63.. flat-table:: struct v4l2_streamparm 64 :header-rows: 0 65 :stub-columns: 0 66 :widths: 1 1 2 67 68 * - __u32 69 - ``type`` 70 - The buffer (stream) type, same as struct 71 :c:type:`v4l2_format` ``type``, set by the 72 application. See :c:type:`v4l2_buf_type`. 73 * - union { 74 - ``parm`` 75 * - struct :c:type:`v4l2_captureparm` 76 - ``capture`` 77 - Parameters for capture devices, used when ``type`` is 78 ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or 79 ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``. 80 * - struct :c:type:`v4l2_outputparm` 81 - ``output`` 82 - Parameters for output devices, used when ``type`` is 83 ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``. 84 * - __u8 85 - ``raw_data``\ [200] 86 - A place holder for future extensions. 87 * - } 88 89 90.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}| 91 92.. c:type:: v4l2_captureparm 93 94.. flat-table:: struct v4l2_captureparm 95 :header-rows: 0 96 :stub-columns: 0 97 :widths: 1 1 2 98 99 * - __u32 100 - ``capability`` 101 - See :ref:`parm-caps`. 102 * - __u32 103 - ``capturemode`` 104 - Set by drivers and applications, see :ref:`parm-flags`. 105 * - struct :c:type:`v4l2_fract` 106 - ``timeperframe`` 107 - This is the desired period between successive frames captured by 108 the driver, in seconds. 109 * - :cspan:`2` 110 111 This will configure the speed at which the video source (e.g. a sensor) 112 generates video frames. If the speed is fixed, then the driver may 113 choose to skip or repeat frames in order to achieve the requested 114 frame rate. 115 116 For stateful encoders (see :ref:`encoder`) this represents the 117 frame interval that is typically embedded in the encoded video stream. 118 119 Applications store here the desired frame period, drivers return 120 the actual frame period. 121 122 Changing the video standard (also implicitly by switching 123 the video input) may reset this parameter to the nominal frame 124 period. To reset manually applications can just set this field to 125 zero. 126 127 Drivers support this function only when they set the 128 ``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field. 129 * - __u32 130 - ``extendedmode`` 131 - Custom (driver specific) streaming parameters. When unused, 132 applications and drivers must set this field to zero. Applications 133 using this field should check the driver name and version, see 134 :ref:`querycap`. 135 * - __u32 136 - ``readbuffers`` 137 - Applications set this field to the desired number of buffers used 138 internally by the driver in :c:func:`read()` mode. 139 Drivers return the actual number of buffers. When an application 140 requests zero buffers, drivers should just return the current 141 setting rather than the minimum or an error code. For details see 142 :ref:`rw`. 143 * - __u32 144 - ``reserved``\ [4] 145 - Reserved for future extensions. Drivers and applications must set 146 the array to zero. 147 148 149.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}| 150 151.. c:type:: v4l2_outputparm 152 153.. flat-table:: struct v4l2_outputparm 154 :header-rows: 0 155 :stub-columns: 0 156 :widths: 1 1 2 157 158 * - __u32 159 - ``capability`` 160 - See :ref:`parm-caps`. 161 * - __u32 162 - ``outputmode`` 163 - Set by drivers and applications, see :ref:`parm-flags`. 164 * - struct :c:type:`v4l2_fract` 165 - ``timeperframe`` 166 - This is the desired period between successive frames output by the 167 driver, in seconds. 168 * - :cspan:`2` 169 170 The field is intended to repeat frames on the driver side in 171 :c:func:`write()` mode (in streaming mode timestamps 172 can be used to throttle the output), saving I/O bandwidth. 173 174 For stateful encoders (see :ref:`encoder`) this represents the 175 frame interval that is typically embedded in the encoded video stream 176 and it provides a hint to the encoder of the speed at which raw 177 frames are queued up to the encoder. 178 179 Applications store here the desired frame period, drivers return 180 the actual frame period. 181 182 Changing the video standard (also implicitly by switching 183 the video output) may reset this parameter to the nominal frame 184 period. To reset manually applications can just set this field to 185 zero. 186 187 Drivers support this function only when they set the 188 ``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field. 189 * - __u32 190 - ``extendedmode`` 191 - Custom (driver specific) streaming parameters. When unused, 192 applications and drivers must set this field to zero. Applications 193 using this field should check the driver name and version, see 194 :ref:`querycap`. 195 * - __u32 196 - ``writebuffers`` 197 - Applications set this field to the desired number of buffers used 198 internally by the driver in :c:func:`write()` mode. Drivers 199 return the actual number of buffers. When an application requests 200 zero buffers, drivers should just return the current setting 201 rather than the minimum or an error code. For details see 202 :ref:`rw`. 203 * - __u32 204 - ``reserved``\ [4] 205 - Reserved for future extensions. Drivers and applications must set 206 the array to zero. 207 208 209.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}| 210 211.. _parm-caps: 212 213.. flat-table:: Streaming Parameters Capabilities 214 :header-rows: 0 215 :stub-columns: 0 216 :widths: 3 1 4 217 218 * - ``V4L2_CAP_TIMEPERFRAME`` 219 - 0x1000 220 - The frame period can be modified by setting the ``timeperframe`` 221 field. 222 223 224.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}| 225 226.. _parm-flags: 227 228.. flat-table:: Capture Parameters Flags 229 :header-rows: 0 230 :stub-columns: 0 231 :widths: 3 1 4 232 233 * - ``V4L2_MODE_HIGHQUALITY`` 234 - 0x0001 235 - High quality imaging mode. High quality mode is intended for still 236 imaging applications. The idea is to get the best possible image 237 quality that the hardware can deliver. It is not defined how the 238 driver writer may achieve that; it will depend on the hardware and 239 the ingenuity of the driver writer. High quality mode is a 240 different mode from the regular motion video capture modes. In 241 high quality mode: 242 243 - The driver may be able to capture higher resolutions than for 244 motion capture. 245 246 - The driver may support fewer pixel formats than motion capture 247 (eg; true color). 248 249 - The driver may capture and arithmetically combine multiple 250 successive fields or frames to remove color edge artifacts and 251 reduce the noise in the video data. 252 253 - The driver may capture images in slices like a scanner in order 254 to handle larger format images than would otherwise be 255 possible. 256 257 - An image capture operation may be significantly slower than 258 motion capture. 259 260 - Moving objects in the image might have excessive motion blur. 261 262 - Capture might only work through the :c:func:`read()` call. 263 264Return Value 265============ 266 267On success 0 is returned, on error -1 and the ``errno`` variable is set 268appropriately. The generic error codes are described at the 269:ref:`Generic Error Codes <gen-errors>` chapter.