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); |
[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 86. struct v4l2_jpegcompression
| int | quality | |
| int | APPn | |
| int | APP_len | |
| char | APP_data[60] | |
| int | COM_len | |
| char | COM_data[60] | |
| __u32 | jpeg_markers | See Table 87, “JPEG Markers Flags”. |
Table 87. 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 |