7.39. ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION

7.39.1. Name

VIDIOC_G_SELECTION - VIDIOC_S_SELECTION - Get or set one of the selection rectangles

7.39.2. Synopsis

int ioctl(int fd, VIDIOC_G_SELECTION, struct v4l2_selection *argp)
int ioctl(int fd, VIDIOC_S_SELECTION, struct v4l2_selection *argp)

7.39.3. Arguments

fd
File descriptor returned by open().
argp
Pointer to struct v4l2_selection.

7.39.4. Description

The ioctls are used to query and configure selection rectangles.

To query the cropping (composing) rectangle set struct v4l2_selection type field to the respective buffer type. The next step is setting the value of struct v4l2_selection target field to V4L2_SEL_TGT_CROP (V4L2_SEL_TGT_COMPOSE). Please refer to table Common selection definitions or API for cropping, composing and scaling for additional targets. The flags and reserved fields of struct v4l2_selection are ignored and they must be filled with zeros. The driver fills the rest of the structure or returns EINVAL error code if incorrect buffer type or target was used. If cropping (composing) is not supported then the active rectangle is not mutable and it is always equal to the bounds rectangle. Finally, the struct v4l2_rect r rectangle is filled with the current cropping (composing) coordinates. The coordinates are expressed in driver-dependent units. The only exception are rectangles for images in raw formats, whose coordinates are always expressed in pixels.

To change the cropping (composing) rectangle set the struct v4l2_selection type field to the respective buffer type. The next step is setting the value of struct v4l2_selection target to V4L2_SEL_TGT_CROP (V4L2_SEL_TGT_COMPOSE). Please refer to table Common selection definitions or API for cropping, composing and scaling for additional targets. The struct v4l2_rect r rectangle need to be set to the desired active area. Field struct v4l2_selection reserved is ignored and must be filled with zeros. The driver may adjust coordinates of the requested rectangle. An application may introduce constraints to control rounding behaviour. The struct v4l2_selection flags field must be set to one of the following:

  • 0 - The driver can adjust the rectangle size freely and shall choose a crop/compose rectangle as close as possible to the requested one.
  • V4L2_SEL_FLAG_GE - The driver is not allowed to shrink the rectangle. The original rectangle must lay inside the adjusted one.
  • V4L2_SEL_FLAG_LE - The driver is not allowed to enlarge the rectangle. The adjusted rectangle must lay inside the original one.
  • V4L2_SEL_FLAG_GE | V4L2_SEL_FLAG_LE - The driver must choose the size exactly the same as in the requested rectangle.

Please refer to Size adjustments with constraint flags..

The driver may have to adjusts the requested dimensions against hardware limits and other parts as the pipeline, i.e. the bounds given by the capture/output window or TV display. The closest possible values of horizontal and vertical offset and sizes are chosen according to following priority:

  1. Satisfy constraints from struct v4l2_selection flags.
  2. Adjust width, height, left, and top to hardware limits and alignments.
  3. Keep center of adjusted rectangle as close as possible to the original one.
  4. Keep width and height as close as possible to original ones.
  5. Keep horizontal and vertical offset as close as possible to original ones.

On success the struct v4l2_rect r field contains the adjusted rectangle. When the parameters are unsuitable the application may modify the cropping (composing) or image parameters and repeat the cycle until satisfactory parameters have been negotiated. If constraints flags have to be violated at then ERANGE is returned. The error indicates that there exist no rectangle that satisfies the constraints.

Selection targets and flags are documented in Common selection definitions.

constraints.svg

Size adjustments with constraint flags.

Behaviour of rectangle adjustment for different constraint flags.
v4l2_selection
struct v4l2_selection
__u32 type Type of the buffer (from enum v4l2_buf_type).
__u32 target Used to select between cropping and composing rectangles.
__u32 flags Flags controlling the selection rectangle adjustments, refer to selection flags.
struct v4l2_rect r The selection rectangle.
__u32 reserved[9] Reserved fields for future use. Drivers and applications must zero this array.

Note

Unfortunately in the case of multiplanar buffer types (V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE and V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE) this API was messed up with regards to how the v4l2_selection type field should be filled in. Some drivers only accepted the _MPLANE buffer type while other drivers only accepted a non-multiplanar buffer type (i.e. without the _MPLANE at the end).

Starting with kernel 4.13 both variations are allowed.

7.39.5. Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.

EINVAL
Given buffer type type or the selection target target is not supported, or the flags argument is not valid.
ERANGE
It is not possible to adjust struct v4l2_rect r rectangle to satisfy all constraints given in the flags argument.
ENODATA
Selection is not supported for this input or output.
EBUSY
It is not possible to apply change of the selection rectangle at the moment. Usually because streaming is in progress.