7.34. ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

7.34.1. Name

VIDIOC_G_JPEGCOMP - VIDIOC_S_JPEGCOMP

7.34.2. Synopsis

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

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.

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.