text_src/09.07__vp8-bitstream__refresh-golden-frame-and-altref-frame.txt

#### 9.7 Refresh Golden Frame and Altref Frame             {#h-09-07}


For key frames, both the golden frame and the altref frame are
refreshed/ replaced by the current reconstructed frame, by default.
For non-key frames, VP8 uses two bits to indicate whether the two
frame buffers are refreshed, using the reconstructed current frame:


| Index     | Description
| --------- | -------------------------------
| `L(1)`    | Whether golden frame is refreshed (`0` for no, `1` for yes).
| `L(1)`    | Whether altref frame is refreshed (`0` for no, `1` for yes).


When the flag for the golden frame is `0`, VP8 uses 2 more bits in the
bitstream to indicate whether the buffer (and which buffer) is copied
to the golden frame, or if no buffer is copied:


| Index     | Description
| --------- | -------------------------------
| `L(2)`    | Buffer copy flag for golden frame buffer


Where:

* `0` means no buffer is copied to the golden frame

* `1` means `last_frame` is copied to the golden frame

* `2` means `alt_ref_frame` is copied to the golden frame


Similarly, when the flag for altref is `0`, VP8 uses 2 bits in the
bitstream to indicate which buffer is copied to `alt_ref_frame`.


| Index     | Description
| --------- | -------------------------------
| `L(2)`    | Buffer copy flag for altref frame buffer

Where: 

* `0` means no buffer is copied to the altref frame

* `1` means `last_frame` is copied to the altref frame

* `2` means `golden_frame` is copied to the altref frame


Two bits are transmitted for `ref_frame_sign_bias` for `golden_frame` and
`alt_ref_frame`, respectively.


| Index     | Description
| --------- | -------------------------------
| `L(1)`    | Sign bias flag for golden frame
| `L(1)`    | Sign bias flag for altref frame


These values are used to control the sign of the motion vectors when
a golden frame or an altref frame is used as the reference frame for
a macroblock.