[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 1. 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 2. |