7.34.1. Name


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

File descriptor returned by open().
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.

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.