ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
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## Name

VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

## Synopsis

`int ioctl(` | int fd,
| int request,
| v4l2_jpegcompression *argp`)`;

`int ioctl(` | int fd,
| int request,
| const v4l2_jpegcompression *argp`)`;

## Arguments

`fd`

  File descriptor returned by `open()`.

`request`

  VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

`argp`

## 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.

Table A.69. 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.70, “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.70. 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

## 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.

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