How librav1e Maps BT.2020 and sRGB Color Spaces

This article explains how the Rust-based AV1 encoder, librav1e, handles and maps standard color spaces like BT.2020 and sRGB. You will learn how the encoder configures color metadata parameters—specifically color primaries, transfer characteristics, and matrix coefficients—to ensure playback devices accurately render the intended colors.

How librav1e Handles Color Spaces

In video encoding, librav1e (the C-compatible library interface for the rav1e AV1 encoder) does not typically perform active color space conversion on the raw pixel data. Instead, it relies on the application feeding it the video frames (such as FFmpeg or a custom pipeline) to handle the pixel conversions (e.g., RGB to YUV).

librav1e’s primary job is to write the correct VUI (Video Usability Information) metadata into the AV1 sequence headers. This metadata tells the decoder precisely how to interpret the YUV or RGB pixel values.

To define a color space, librav1e utilizes the ITU-T H.273 / ISO/IEC 23091-2 standard, which breaks down color description into three distinct variables:

1. Color Primaries: Defines the red, green, blue, and white point coordinates (the color gamut).
2. Transfer Characteristics: Defines the opto-electronic transfer function (the gamma curve).
3. Matrix Coefficients: Defines the mathematical formula used to transition between RGB and YUV color spaces.

Mapping sRGB in librav1e

The sRGB color space is the standard for web graphics, monitors, and standard-definition digital content. In librav1e, mapping an sRGB source requires configuring the following metadata values:

• Color Primaries: sRGB shares the exact same chromaticity coordinates as the ITU-R BT.709 standard. Therefore, librav1e maps this to ColorPrimaries::BT709 (Value: 1).
• Transfer Characteristics: Standard sRGB uses a specific gamma curve defined by IEC 61966-2-1. In librav1e, this maps to TransferCharacteristics::SRGB (Value: 13).
• Matrix Coefficients:
  • If the video is converted to YUV (standard practice for compression), it maps to MatrixCoefficients::BT709 (Value: 1).
  • If the video is encoded as raw RGB (lossless/high-fidelity), it maps to MatrixCoefficients::Identity (Value: 0).

Mapping BT.2020 in librav1e

BT.2020 is the standard for Ultra-High-Definition (UHD) and High Dynamic Range (HDR) video, featuring a much wider color gamut than sRGB. When mapping BT.2020, the configuration depends on whether the content is Standard Dynamic Range (SDR) or High Dynamic Range (HDR).

• Color Primaries: For all BT.2020 content, this is mapped to ColorPrimaries::BT2020 (Value: 9).
• Transfer Characteristics:
  • For BT.2020 SDR: Mapped to TransferCharacteristics::BT2020_10 (Value: 14) or BT2020_12 (Value: 15).
  • For HDR (PQ / HDR10): Mapped to TransferCharacteristics::SMPTE2084 (Value: 16).
  • For HDR (HLG): Mapped to TransferCharacteristics::HLG (Value: 18).
• Matrix Coefficients: BT.2020 uses a distinct color matrix for YUV conversion. This is mapped to MatrixCoefficients::BT2020NCL (Non-Constant Luminance, Value: 9).

Configuring the Parameters in the Code

When programmatically configuring librav1e or using the rav1e API in Rust, you define these settings inside the Sequence configuration using the ColorDescription struct.

use rav1e::prelude::*;

let mut config = Config::default();

// Example: Configuring for HDR BT.2020 (PQ)
config.enc.color_description = Some(ColorDescription {
    color_primaries: ColorPrimaries::BT2020,
    transfer_characteristics: TransferCharacteristics::SMPTE2084,
    matrix_coefficients: MatrixCoefficients::BT2020NCL,
});

By explicitly setting these properties in the encoder configuration, librav1e embeds the correct flags into the AV1 bitstream. This guarantees that downstream video players, web browsers, and TV displays read the metadata and map the colors to your screen exactly as intended.