7.4. ioctl VIDIOC_CROPCAP¶

7.4.1. Name¶

VIDIOC_CROPCAP - Information about the video cropping and scaling abilities

7.4.2. Synopsis¶

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.

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 above.
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 co-ordinate 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.