A palette refers to a subset of the color space. Palette prediction is used to reduce the redundancy in coding pixel information within a given block. Palette prediction is suitable for applications where most colors come from a very small subset of the color space (e.g. PC screen content), there is little or no noise in the pictures, and where the picture may involve repetitive patterns.
In AV1, a palette specifies the most likely colors present in a given block. Palette prediction makes use of separate color index maps with 2 to 8 base colors for each of the Y, U and V planes, and is applicable only to block sizes that are at least the size of 8x8 blocks and have dimensions smaller than 128.
The context associated with each pixel in the block is used to encode the palette index for that pixel. Wavefront processing of the pixels is considered to allow for parallel processing. The bitstream includes the number of base colors and the base colors used in encoding a given block.
To illustrate the idea behind palette prediction, consider the 4x4 block shown in the figure below. In this case the palette consists of three colors with index 0, 1 and 2. Based on the selected palette, the index map of the block can be generated as shown below. The encoding of the indices then proceeds in a wavefront manner as indicated below.
Figure 1. Example of a 4x4 source block, corresponding palette, index map and wavefront processing pattern. The 4x4 block is considered here only for illustration purposes, as the block size needs to be at least 8x8 for palette prediction to be allowed.
The index for each pixel is encoded using the top and left encoded indices as context, as shown in the table below.
| Pixel | Context |
|---|---|
| 0 | - |
| 1 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 1, 2 |
| 5 | 2 |
| 6 | 3 |
| … | |
| 15 | 13, 14 |
Inputs: Input source video
Outputs: Palette information (palette size, colors and map tokens)
Control tokens/flags:
The feature is currently active only when screen content encoding is active, either through:
-
Setting screen content encoding to Auto mode, where screen-content-type of pictures are flagged based on detector information, or
-
Setting the screen content encoding to Manual mode, where the input sequence is encoded as screen content (occurs when “
—scm 1” is specified in the command line).
| Flag | Level (Sequence/Picture) | Description |
|---|---|---|
| --scm | Sequence | Command line token. 0: No SC, 1: SC ON 2: Auto mode (detector based) |
| --palette | Configuration | To enable palette from the command-line interface. 0: OFF; 1: Slow; 2: Fastest. Auto mode=-1 if not set from the encoder configuration |
| palette_level | Picture based | Set based on the configuration palette mode. For auto mode it is set to 6 for M0. |
Details of the implementation
The main function calls associated with palette mode prediction are indicated in Figure 2 below.
The following steps are then considered in the generation of palette prediction candidates.
-
In the function
generate_md_stage_0_cand, a candidate for palette prediction is first evaluated to determine if the palette mode is allowed (svt_av1_allow_palette). The use of palette prediction mode is allowed if (palette_level different from 0 AND block width <= 64 AND block height <= 64 AND block size at least 8x8.) -
For blocks where palette prediction mode is allowed, the function
inject_palette_candidatesis invoked to create and inject palette candidates.The candidates are signaled using the Intra DC mode. This function calls another function (search_palette_luma) in order to determine all palette candidates for luma. The palette prediction candidates are determined by performing two types of search, namely a search based on the most dominant colors and a search based on the K-means clustering of the colors in the block.-
Most dominant colors search: In this search, a histogram of the colors in the source block is generated. The number of the most used colors to select from the histogram depends on the number of colors it is desired to include in the palette, which ranges from a minimum of 2 to a maximum of 8.
-
The K-means clustering of the colors: A K-means clustering algorithm is used to cluster the set of input source block colors into a fixed number of colors. The number of clusters to consider depends on the number of colors it is desired to include in the palette. The algorithm goes through a maximum of 50 iterations to converge on the best clustering for a given number of clusters.
-
Given that the number of colors should be in the range from 2 to 8, up to 7 candidates can be generated from each of the two search methods described above. In total, up to 14 candidates are created for each source block.
In mode decision, palette prediction candidates are assigned to a special MD candidate class.
| Signal | Description |
|---|---|
| enabled | |
| dominant_color_step | In the dominant color search, test a subset of the most dominant color combinations by testing every nth combo. For example, with step size of 2, if the block involves 7 colors, then only 3 candidates with palettes based on the most dominant 7, 5 and 3 colors are tested. Range: [1 (test all), 7 (test one)] |
The most important signals/parameters which are sent in the bit stream regarding palette prediction:
- Size of the palette: The number of colors used for the current block.
- The palette colors:
- Special techniques are used to code the colors in the bit-stream;
- The encoder and decoder maintain a cache of colors computed based on the neighboring colors. Information on whether some colors could be re-used from the cache is sent in the bit-stream using fewer bits as compared to signaling the colors directly.
- Moreover, in the search, some colors are changed to use one of the cache elements if they are close to each-others.
- The palette indices map
The feature settings that are described in this document were compiled at v1.4.0 of the code and may not reflect the current status of the code. The description in this document represents an example showing how features would interact with the SVT architecture. For the most up-to-date settings, it's recommended to review the section of the code implementing this feature.