Name

VIDIOC_G_SELECTION, VIDIOC_S_SELECTION — Get or set one of the selection rectangles

Synopsis

int ioctl(int fd,
 int request,
 struct v4l2_selection *argp);
 

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_SELECTION, VIDIOC_S_SELECTION

argp

Description

Experimental

This is an experimental interface and may change in the future.

The ioctls are used to query and configure selection rectangles.

To query the cropping (composing) rectangle set struct v4l2_selection::type to the respective buffer type. Do not use multiplanar buffers. Use V4L2_BUF_TYPE_VIDEO_CAPTURE instead of V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE . Use V4L2_BUF_TYPE_VIDEO_OUTPUT instead of V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE . The next step is setting struct v4l2_selection::target to value V4L2_SEL_TGT_CROP_ACTIVE ( V4L2_SEL_TGT_COMPOSE_ACTIVE ). Please refer to table Table A.74, “Selection targets.” or the section called “Experimental API for cropping, composing and scaling” for additional targets. Fields struct v4l2_selection::flags and struct v4l2_selection::reserved 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, structure struct v4l2_selection::r 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 struct v4l2_selection::type to the respective buffer type. Do not use multiplanar buffers. Use V4L2_BUF_TYPE_VIDEO_CAPTURE instead of V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE . Use V4L2_BUF_TYPE_VIDEO_OUTPUT instead of V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE . The next step is setting struct v4l2_selection::target to value V4L2_SEL_TGT_CROP_ACTIVE ( V4L2_SEL_TGT_COMPOSE_ACTIVE ). Please refer to table Table A.74, “Selection targets.” or the section called “Experimental API for cropping, composing and scaling” for additional targets. Set desired active area into the field struct v4l2_selection::r . Field struct v4l2_selection::reserved is ignored and must be filled with zeros. The driver may adjust the rectangle coordinates. An application may introduce constraints to control rounding behaviour. Set the field struct v4l2_selection::flags to one of values:

  • 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 Figure A.1, “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 field struct v4l2_selection::r 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.

Table A.74. Selection targets.

V4L2_SEL_TGT_CROP_ACTIVE0area that is currently cropped by hardware
V4L2_SEL_TGT_CROP_DEFAULT1suggested cropping rectangle that covers the "whole picture"
V4L2_SEL_TGT_CROP_BOUNDS2limits for the cropping rectangle
V4L2_SEL_TGT_COMPOSE_ACTIVE256area to which data are composed by hardware
V4L2_SEL_TGT_COMPOSE_DEFAULT257suggested composing rectangle that covers the "whole picture"
V4L2_SEL_TGT_COMPOSE_BOUNDS258limits for the composing rectangle
V4L2_SEL_TGT_COMPOSE_PADDED259the active area and all padding pixels that are inserted or modified by the hardware

Table A.75. Selection constraint flags

V4L2_SEL_FLAG_GE0x00000001indicate that adjusted rectangle must contain a rectangle from struct v4l2_selection::r
V4L2_SEL_FLAG_LE0x00000002indicate that adjusted rectangle must be inside a rectangle from struct v4l2_selection::r

Figure A.1. Size adjustments with constraint flags.

Behaviour of rectangle adjustment for different constraint flags.

Table A.76. struct v4l2_selection

__u32typeType of the buffer (from enum v4l2_buf_type)
__u32targetused to select between cropping and composing rectangles
__u32flagscontrol over coordinates adjustments, refer to selection flags
struct v4l2_rectrselection rectangle
__u32reserved[9]Reserved fields for future use

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

The buffer struct v4l2_selection::type or struct v4l2_selection::target is not supported, or the struct v4l2_selection::flags are invalid.

ERANGE

it is not possible to adjust a rectangle struct v4l2_selection::r that satisfies all contraints from struct v4l2_selection::flags .

EBUSY

it is not possible to apply change of selection rectangle at the moment. Usually because streaming is in progress.