8.34. ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP¶
8.34.1. Name¶
VIDIOC_G_JPEGCOMP - VIDIOC_S_JPEGCOMP
8.34.2. Synopsis¶
-
int
ioctl
(int fd, int request, v4l2_jpegcompression *argp)¶
-
int
ioctl
(int fd, int request, const v4l2_jpegcompression *argp)¶
8.34.3. Arguments¶
fd
- File descriptor returned by open().
request
- VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
argp
8.34.4. Description¶
These ioctls are deprecated. New drivers and applications should use JPEG class controls for image quality and JPEG markers control.
[to do]
Ronald Bultje elaborates:
APP is some application-specific information. The application can set it itself, and it’ll be stored in the JPEG-encoded fields (eg; interlacing information for in an AVI or so). COM is the same, but it’s comments, like ‘encoded by me’ or so.
jpeg_markers describes whether the huffman tables, quantization tables and the restart interval information (all JPEG-specific stuff) should be stored in the JPEG-encoded fields. These define how the JPEG field is encoded. If you omit them, applications assume you’ve used standard encoding. You usually do want to add them.
int | quality |
Deprecated. If V4L2_CID_JPEG_COMPRESSION_QUALITY control is exposed by a driver applications should use it instead and ignore this field. |
int | APPn |
|
int | APP_len |
|
char | APP_data [60] |
|
int | COM_len |
|
char | COM_data [60] |
|
__u32 | jpeg_markers |
See JPEG Markers Flags. Deprecated. If V4L2_CID_JPEG_ACTIVE_MARKER control is exposed by a driver applications should use it instead and ignore this field. |
V4L2_JPEG_MARKER_DHT |
(1<<3) | Define Huffman Tables |
V4L2_JPEG_MARKER_DQT |
(1<<4) | Define Quantization Tables |
V4L2_JPEG_MARKER_DRI |
(1<<5) | Define Restart Interval |
V4L2_JPEG_MARKER_COM |
(1<<6) | Comment segment |
V4L2_JPEG_MARKER_APP |
(1<<7) | App segment, driver will always use APP0 |
8.34.5. 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.