VIDIOC_G_JPEGCOMP - VIDIOC_S_JPEGCOMP
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 |
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.