1.. Permission is granted to copy, distribute and/or modify this 2.. document under the terms of the GNU Free Documentation License, 3.. Version 1.1 or any later version published by the Free Software 4.. Foundation, with no Invariant Sections, no Front-Cover Texts 5.. and no Back-Cover Texts. A copy of the license is included at 6.. Documentation/userspace-api/media/fdl-appendix.rst. 7.. 8.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections 9 10************* 11Configuration 12************* 13 14Applications can use the :ref:`selection API <VIDIOC_G_SELECTION>` to 15select an area in a video signal or a buffer, and to query for default 16settings and hardware limits. 17 18Video hardware can have various cropping, composing and scaling 19limitations. It may only scale up or down, support only discrete scaling 20factors, or have different scaling abilities in the horizontal and 21vertical directions. Also it may not support scaling at all. At the same 22time the cropping/composing rectangles may have to be aligned, and both 23the source and the sink may have arbitrary upper and lower size limits. 24Therefore, as usual, drivers are expected to adjust the requested 25parameters and return the actual values selected. An application can 26control the rounding behaviour using 27:ref:`constraint flags <v4l2-selection-flags>`. 28 29 30Configuration of video capture 31============================== 32 33See figure :ref:`sel-targets-capture` for examples of the selection 34targets available for a video capture device. It is recommended to 35configure the cropping targets before to the composing targets. 36 37The range of coordinates of the top left corner, width and height of 38areas that can be sampled is given by the ``V4L2_SEL_TGT_CROP_BOUNDS`` 39target. It is recommended for the driver developers to put the top/left 40corner at position ``(0,0)``. The rectangle's coordinates are expressed 41in pixels. 42 43The top left corner, width and height of the source rectangle, that is 44the area actually sampled, is given by the ``V4L2_SEL_TGT_CROP`` target. 45It uses the same coordinate system as ``V4L2_SEL_TGT_CROP_BOUNDS``. The 46active cropping area must lie completely inside the capture boundaries. 47The driver may further adjust the requested size and/or position 48according to hardware limitations. 49 50Each capture device has a default source rectangle, given by the 51``V4L2_SEL_TGT_CROP_DEFAULT`` target. This rectangle shall cover what the 52driver writer considers the complete picture. Drivers shall set the 53active crop rectangle to the default when the driver is first loaded, 54but not later. 55 56The composing targets refer to a memory buffer. The limits of composing 57coordinates are obtained using ``V4L2_SEL_TGT_COMPOSE_BOUNDS``. All 58coordinates are expressed in pixels. The rectangle's top/left corner 59must be located at position ``(0,0)``. The width and height are equal to 60the image size set by :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. 61 62The part of a buffer into which the image is inserted by the hardware is 63controlled by the ``V4L2_SEL_TGT_COMPOSE`` target. The rectangle's 64coordinates are also expressed in the same coordinate system as the 65bounds rectangle. The composing rectangle must lie completely inside 66bounds rectangle. The driver must adjust the composing rectangle to fit 67to the bounding limits. Moreover, the driver can perform other 68adjustments according to hardware limitations. The application can 69control rounding behaviour using 70:ref:`constraint flags <v4l2-selection-flags>`. 71 72For capture devices the default composing rectangle is queried using 73``V4L2_SEL_TGT_COMPOSE_DEFAULT``. It is usually equal to the bounding 74rectangle. 75 76The part of a buffer that is modified by the hardware is given by 77``V4L2_SEL_TGT_COMPOSE_PADDED``. It contains all pixels defined using 78``V4L2_SEL_TGT_COMPOSE`` plus all padding data modified by hardware 79during insertion process. All pixels outside this rectangle *must not* 80be changed by the hardware. The content of pixels that lie inside the 81padded area but outside active area is undefined. The application can 82use the padded and active rectangles to detect where the rubbish pixels 83are located and remove them if needed. 84 85 86Configuration of video output 87============================= 88 89For output devices targets and ioctls are used similarly to the video 90capture case. The *composing* rectangle refers to the insertion of an 91image into a video signal. The cropping rectangles refer to a memory 92buffer. It is recommended to configure the composing targets before to 93the cropping targets. 94 95The cropping targets refer to the memory buffer that contains an image 96to be inserted into a video signal or graphical screen. The limits of 97cropping coordinates are obtained using ``V4L2_SEL_TGT_CROP_BOUNDS``. 98All coordinates are expressed in pixels. The top/left corner is always 99point ``(0,0)``. The width and height is equal to the image size 100specified using :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. 101 102The top left corner, width and height of the source rectangle, that is 103the area from which image date are processed by the hardware, is given 104by the ``V4L2_SEL_TGT_CROP``. Its coordinates are expressed in in the 105same coordinate system as the bounds rectangle. The active cropping area 106must lie completely inside the crop boundaries and the driver may 107further adjust the requested size and/or position according to hardware 108limitations. 109 110For output devices the default cropping rectangle is queried using 111``V4L2_SEL_TGT_CROP_DEFAULT``. It is usually equal to the bounding 112rectangle. 113 114The part of a video signal or graphics display where the image is 115inserted by the hardware is controlled by ``V4L2_SEL_TGT_COMPOSE`` 116target. The rectangle's coordinates are expressed in pixels. The 117composing rectangle must lie completely inside the bounds rectangle. The 118driver must adjust the area to fit to the bounding limits. Moreover, the 119driver can perform other adjustments according to hardware limitations. 120 121The device has a default composing rectangle, given by the 122``V4L2_SEL_TGT_COMPOSE_DEFAULT`` target. This rectangle shall cover what 123the driver writer considers the complete picture. It is recommended for 124the driver developers to put the top/left corner at position ``(0,0)``. 125Drivers shall set the active composing rectangle to the default one when 126the driver is first loaded. 127 128The devices may introduce additional content to video signal other than 129an image from memory buffers. It includes borders around an image. 130However, such a padded area is driver-dependent feature not covered by 131this document. Driver developers are encouraged to keep padded rectangle 132equal to active one. The padded target is accessed by the 133``V4L2_SEL_TGT_COMPOSE_PADDED`` identifier. It must contain all pixels 134from the ``V4L2_SEL_TGT_COMPOSE`` target. 135 136 137Scaling control 138=============== 139 140An application can detect if scaling is performed by comparing the width 141and the height of rectangles obtained using ``V4L2_SEL_TGT_CROP`` and 142``V4L2_SEL_TGT_COMPOSE`` targets. If these are not equal then the 143scaling is applied. The application can compute the scaling ratios using 144these values. 145