7.34. ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

7.34.1. Name

VIDIOC_G_JPEGCOMP - VIDIOC_S_JPEGCOMP

7.34.2. Synopsis

VIDIOC_G_JPEGCOMP

int ioctl(int fd, VIDIOC_G_JPEGCOMP, v4l2_jpegcompression *argp)

VIDIOC_S_JPEGCOMP

int ioctl(int fd, VIDIOC_S_JPEGCOMP, const v4l2_jpegcompression *argp)

7.34.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct v4l2_jpegcompression.

7.34.4. 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.

type v4l2_jpegcompression
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 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.
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

7.34.5. 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.