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
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.
Table 1. struct v4l2_cropcap
| enum v4l2_buf_type | 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_OUTPUT,
V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver
defined) types with code V4L2_BUF_TYPE_PRIVATE
and higher. |
| 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 |
Table 2. 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. |
| __s32 | width | Width of the rectangle, in pixels. |
| __s32 | height | Height of the rectangle, in pixels. Width and height cannot be negative, the fields are signed for hysterical reasons. |
On success 0 is returned, on error -1 and the errno variable is set appropriately:
The struct v4l2_cropcap type is
invalid or the ioctl is not supported. This is not permitted for
video capture, output and overlay devices, which must support
VIDIOC_CROPCAP.