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 Cropping, composing and scaling -- the SELECTION API 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 Cropping, composing and scaling -- the SELECTION API 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:
- Satisfy constraints from struct
v4l2_selection
flags
. - Adjust width, height, left, and top to hardware limits and alignments.
- Keep center of adjusted rectangle as close as possible to the original one.
- Keep width and height as close as possible to original ones.
- 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.
-
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 targettarget
is not supported, or theflags
argument is not valid. - ERANGE
- It is not possible to adjust struct
v4l2_rect
r
rectangle to satisfy all constraints given in theflags
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.