text_src/a.01_vp8-bitstream__frame_tag.txt

#### A.1 Uncompressed Data Chunk                             {#h-0a-01}


| Frame Tag                                             | Type  |
|-------------------------------------------------------|-------|
| `frame_tag`                                           | f(24) |
| `if (key_frame) {`                                    |       |
| `    start_code`                                      | f(24) |
| `    horizontal_size_code`                            | f(16) |
| `    vertical_size_code`                              | f(16) |
| `}`                                                   |       |
{:.col1-pre}


The 3-byte frame tag can be parsed as follows:


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
unsigned char *c = pbi->source;
unsigned int tmp;

tmp = (c[2] << 16) | (c[1] << 8) | c[0];

key_frame = tmp & 0x1;
version = (tmp >> 1) & 0x7;
show_frame = (tmp >> 4) & 0x1;
first_part_size = (tmp >> 5) & 0x7FFFF;

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


Where:

  * `key_frame` indicates if the current frame is a key frame or not.
  * `version` determines the bitstream version.
  * `show_frame` indicates if the current frame is meant to be displayed
    or not.
  * `first_part_size` determines the size of the first partition
    (control partition), excluding the uncompressed data chunk.

The `start_code` is a constant 3-byte pattern having value 0x9d012a. The
latter part of the uncompressed chunk (after the `start_code`) can be
parsed as follows:


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
unsigned char *c = pbi->source + 6;
unsigned int tmp;

tmp = (c[1] << 8) | c[0];

width = tmp & 0x3FFF;
horizontal_scale = tmp >> 14;

tmp = (c[3] << 8) | c[2];

height = tmp & 0x3FFF;
vertical_scale = tmp >> 14;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}