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.70. struct v4l2_jpegcompression

intqualityDeprecated. If V4L2_CID_JPEG_COMPRESSION_QUALITY control is exposed by a driver applications should use it instead and ignore this field.
intAPPn 
intAPP_len 
charAPP_data[60] 
intCOM_len 
charCOM_data[60] 
__u32jpeg_markersSee Table A.71, “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.71. 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.