VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
int ioctl( | int fd, |
| int request, | |
v4l2_jpegcompression *argp); |
int ioctl( | int fd, |
| int request, | |
const v4l2_jpegcompression *argp); |
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.
Table A.74. struct v4l2_jpegcompression
| 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 Table A.75, “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. |
Table A.75. JPEG Markers Flags
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 |
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.