ACES Color Management

What is ACES

ACES (Academy Color Encoding System) is an open color management standard developed by the Academy of Motion Picture Arts and Sciences, aiming to provide a unified color workflow for film, television, and digital media. ACES defines a set of color spaces, conversion functions, and specifications to ensure color consistency from capture through post-production to distribution.

In real-time rendering and game development, ACES often specifically refers to ACES Tone Mapping and ACES color space conversion, used to convert HDR rendering results to displayable colors.

Components of ACES

ACES includes several core components:

• **ACES color space**: A wide-gamut color space that can contain all visible colors, avoiding color information loss.
• **Input Device Transform (IDT)**: Converts various camera color spaces to ACES.
• **Output Device Transform (ODT)**: Converts ACES to specific display device color spaces (e.g., sRGB, Rec.709, HDR10).
• **Reference Rendering Transform (RRT)**: Converts scene-referred linear data to display-referred tone-mapped result.
• **ACES tone mapping**: Common approximate implementations, such as ACES Filmic Tone Mapping, map HDR values to LDR range.

Why Use ACES

• **Color consistency**: Maintains color consistency across different devices and software.
• **High-quality tone mapping**: ACES tone mapping provides natural color and contrast, preserves highlight details, and avoids the dullness of traditional Reinhard.
• **Future compatibility**: ACES supports HDR and wide color gamut, adapting to future display technologies.
• **Industry standard**: Widely adopted in film and game industries, facilitating collaboration.

ACES Tone Mapping

In game engines, ACES tone mapping is most commonly used. Its function takes linear HDR color and outputs displayable LDR color. Common implementations include:

• **ACES Filmic Tone Mapping** (Narkowicz 2015 approximation): Simple and efficient, widely used.
• **ACES Central**: Closer to the official RRT but slightly more complex computationally.

For example, Unity's Post Processing Stack and Unreal Engine offer ACES tone mapping options.

Implementing ACES Color Management

In a real-time rendering pipeline, ACES is typically integrated by:

1. **Linear space rendering**: Ensure scene lighting is computed in linear space.
2. **HDR floating-point render target**: Store HDR colors.
3. **Apply ACES tone mapping**: Convert HDR colors to display colors in post-processing.
4. **Color grading**: Adjust colors in ACES space or display space.
5. **Output transform**: Apply ODT based on target display device (e.g., sRGB or HDR10).

Many engines already have ACES built-in; developers just select the appropriate color management configuration.

Difference between ACES and sRGB

sRGB is a specific color space for consumer displays; ACES is a broader color management system that includes color spaces and conversion workflows. ACES can output to sRGB as one of its targets.

Summary

ACES color management provides a standardized color processing workflow for modern rendering, and its tone mapping algorithm is particularly popular in games and films.