7.4. ioctl VIDIOC_CROPCAP

7.4.1. Name

VIDIOC_CROPCAP - Information about the video cropping and scaling abilities

7.4.2. Synopsis

VIDIOC_CROPCAP

int ioctl(int fd, VIDIOC_CROPCAP, struct v4l2_cropcap *argp)

7.4.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct v4l2_cropcap.

7.4.4. Description

Applications use this function to query the cropping limits, the pixel aspect of images and to calculate scale factors. They set the type field of a v4l2_cropcap structure to the respective buffer (stream) type and call the ioctl VIDIOC_CROPCAP ioctl with a pointer to this structure. Drivers fill the rest of the structure. The results are constant except when switching the video standard. Remember this switch can occur implicit when switching the video input or output.

This ioctl must be implemented for video capture or output devices that support cropping and/or scaling and/or have non-square pixels, and for overlay devices.

type v4l2_cropcap

struct v4l2_cropcap

__u32	type	Type of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE and V4L2_BUF_TYPE_VIDEO_OVERLAY. See v4l2_buf_type and the note below.
struct v4l2_rect	bounds	Defines the window within capturing or output is possible, this may exclude for example the horizontal and vertical blanking areas. The cropping rectangle cannot exceed these limits. Width and height are defined in pixels, the driver writer is free to choose origin and units of the coordinate system in the analog domain.
struct v4l2_rect	defrect	Default cropping rectangle, it shall cover the "whole picture". Assuming pixel aspect 1/1 this could be for example a 640 × 480 rectangle for NTSC, a 768 × 576 rectangle for PAL and SECAM centered over the active picture area. The same coordinate system as for bounds is used.
struct v4l2_fract	pixelaspect	This is the pixel aspect (y / x) when no scaling is applied, the ratio of the actual sampling frequency and the frequency required to get square pixels. When cropping coordinates refer to square pixels, the driver sets pixelaspect to 1/1. Other common values are 54/59 for PAL and SECAM, 11/10 for NTSC sampled according to [ITU BT.601].

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_cropcap 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.

struct v4l2_rect

__s32	left	Horizontal offset of the top, left corner of the rectangle, in pixels.
__s32	top	Vertical offset of the top, left corner of the rectangle, in pixels.
__u32	width	Width of the rectangle, in pixels.
__u32	height	Height of the rectangle, in pixels.

7.4.5. 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

The struct v4l2_cropcap type is invalid.

ENODATA

Cropping is not supported for this input or output.