2.29. V4L2 JPEG header related functions and data structures¶

struct v4l2_jpeg_reference¶: reference into the JPEG buffer

Definition:

struct v4l2_jpeg_reference {
    u8 *start;
    size_t length;
};

Members

start: pointer to the start of the referenced segment or table
length: size of the referenced segment or table

Description

Wnen referencing marker segments, start points right after the marker code, and length is the size of the segment parameters, excluding the marker code.

struct v4l2_jpeg_frame_component_spec¶: frame component-specification

Definition:

struct v4l2_jpeg_frame_component_spec {
    u8 component_identifier;
    u8 horizontal_sampling_factor;
    u8 vertical_sampling_factor;
    u8 quantization_table_selector;
};

Members

component_identifier: C[i]
horizontal_sampling_factor: H[i]
vertical_sampling_factor: V[i]
quantization_table_selector: quantization table destination selector Tq[i]

struct v4l2_jpeg_frame_header¶: JPEG frame header

Definition:

struct v4l2_jpeg_frame_header {
    u16 height;
    u16 width;
    u8 precision;
    u8 num_components;
    struct v4l2_jpeg_frame_component_spec component[V4L2_JPEG_MAX_COMPONENTS];
    enum v4l2_jpeg_chroma_subsampling subsampling;
};

Members

height: Y
width: X
precision: P
num_components: Nf
component: component-specification, see v4l2_jpeg_frame_component_spec
subsampling: decoded subsampling from component-specification

struct v4l2_jpeg_scan_component_spec¶: scan component-specification

Definition:

struct v4l2_jpeg_scan_component_spec {
    u8 component_selector;
    u8 dc_entropy_coding_table_selector;
    u8 ac_entropy_coding_table_selector;
};

Members

component_selector: Cs[j]
dc_entropy_coding_table_selector: Td[j]
ac_entropy_coding_table_selector: Ta[j]

struct v4l2_jpeg_scan_header¶: JPEG scan header

Definition:

struct v4l2_jpeg_scan_header {
    u8 num_components;
    struct v4l2_jpeg_scan_component_spec component[V4L2_JPEG_MAX_COMPONENTS];
};

Members

num_components: Ns
component: component-specification, see v4l2_jpeg_scan_component_spec

enum v4l2_jpeg_app14_tf¶: APP14 transform flag According to Rec. ITU-T T.872 (06/2012) 6.5.3 APP14 segment is for color encoding, it contains a transform flag, which may have values of 0, 1 and 2 and are interpreted as follows:

Constants

V4L2_JPEG_APP14_TF_CMYK_RGB: CMYK for images encoded with four components RGB for images encoded with three components
V4L2_JPEG_APP14_TF_YCBCR: an image encoded with three components using YCbCr
V4L2_JPEG_APP14_TF_YCCK: an image encoded with four components using YCCK
V4L2_JPEG_APP14_TF_UNKNOWN: indicate app14 is not present

struct v4l2_jpeg_header¶: parsed JPEG header

Definition:

struct v4l2_jpeg_header {
    struct v4l2_jpeg_reference sof;
    struct v4l2_jpeg_reference sos;
    unsigned int num_dht;
    struct v4l2_jpeg_reference dht[V4L2_JPEG_MAX_TABLES];
    unsigned int num_dqt;
    struct v4l2_jpeg_reference dqt[V4L2_JPEG_MAX_TABLES];
    struct v4l2_jpeg_frame_header frame;
    struct v4l2_jpeg_scan_header *scan;
    struct v4l2_jpeg_reference *quantization_tables;
    struct v4l2_jpeg_reference *huffman_tables;
    u16 restart_interval;
    size_t ecs_offset;
    enum v4l2_jpeg_app14_tf app14_tf;
};

Members

sof: pointer to frame header and size
sos: pointer to scan header and size
num_dht: number of entries in dht
dht: pointers to huffman tables and sizes
num_dqt: number of entries in dqt
dqt: pointers to quantization tables and sizes
frame: parsed frame header
scan: pointer to parsed scan header, optional
quantization_tables: references to four quantization tables, optional
huffman_tables: references to four Huffman tables in DC0, DC1, AC0, AC1 order, optional
restart_interval: number of MCU per restart interval, Ri
ecs_offset: buffer offset in bytes to the entropy coded segment
app14_tf: transform flag from app14 data

Description

When this structure is passed to v4l2_jpeg_parse_header, the optional scan, quantization_tables, and huffman_tables pointers must be initialized to NULL or point at valid memory.

int v4l2_jpeg_parse_header(void *buf, size_t len, struct v4l2_jpeg_header *out)¶: locate marker segments and optionally parse headers

Parameters

void *buf: address of the JPEG buffer, should start with a SOI marker
size_t len: length of the JPEG buffer
struct v4l2_jpeg_header *out: returns marker segment positions and optionally parsed headers

Description

The out->scan_header pointer must be initialized to NULL or point to a valid v4l2_jpeg_scan_header structure. The out->huffman_tables and out->quantization_tables pointers must be initialized to NULL or point to a valid array of 4 v4l2_jpeg_reference structures each.