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 whether the current frame is a key frame
    or not.

  * `version` determines the bitstream version.

  * `show_frame` indicates whether 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"}