Tone Mapping: HDR to LDR Conversion and ACES Application

What is Tone Mapping

Tone mapping is the process of converting high dynamic range (HDR) images into a range displayable on low dynamic range (LDR) devices. HDR scenes contain luminance ranges far beyond display capabilities (e.g., sun vs. shadow). Tone mapping algorithms compress luminance while preserving details and color, simulating how human eyes adapt to high-contrast scenes. It is widely used in photography, film, and real-time rendering.

Why Tone Mapping Is Needed

Rendering pipelines typically compute HDR lighting in linear space, where luminance values can range from 0 to hundreds or more, while ordinary displays can only show 0 to 1 (or 0-255). Directly truncating HDR values leads to overexposed highlights and crushed blacks. Tone mapping compresses the wide luminance range into a displayable range while maintaining visual realism and avoiding detail loss.

Common Tone Mapping Algorithms

Reinhard Tone Mapping

Classic algorithm: `L_display = L / (1 + L)`. It maps infinite luminance to [0,1], simple and efficient, but may reduce contrast and alter color saturation.

Filmic Tone Mapping

Simulates the response curve of film, providing better highlight rolloff and color preservation. Common variants include ACES Filmic and Uncharted 2.

ACES Tone Mapping

ACES (Academy Color Encoding System) is a motion picture industry standard, widely adopted for its tone mapping curve. It achieves cinematic quality with complex fitted curves, preserving highlight details and color accuracy. In Unity and Unreal, ACES is one of the default options.

Other Algorithms

• **Cinematic tone mapping**: Custom curves for artistic control.
• **Local tone mapping**: Adjusts contrast based on local regions, more complex and less common in real-time.

Application in the Rendering Pipeline

In real-time rendering, tone mapping is typically a post-processing step, executed after HDR rendering and before gamma correction. Flow:

1. Scene rendered to an HDR buffer.
2. Tone mapping applied to convert HDR to LDR.
3. Gamma correction (linear to sRGB) applied for screen output.

In Unity, the Post Processing package enables tone mapping with modes like ACES or Neutral. In Unreal, set the Tone Mapper in a Post Process Volume.

Tone Mapping and Exposure

Tone mapping often works with exposure control. Exposure simulates camera aperture or human eye adaptation, adjusting HDR image brightness before tone mapping. For example, auto-exposure adjusts based on average scene luminance to avoid being too dark or bright.

FAQ

What is the difference between tone mapping and gamma correction?

Tone mapping compresses HDR to LDR range, a nonlinear mapping of luminance; gamma correction is an encoding conversion between linear and sRGB, executed sequentially in the pipeline.

Why is ACES tone mapping recommended?

ACES provides cinematic quality, natural highlight rolloff, and good color preservation, becoming an industry standard.

What happens without tone mapping?

HDR values get truncated, causing overexposed highlights, loss of shadow detail, and distorted images.

Can tone mapping be used on mobile?

Yes, but it adds post-processing overhead; evaluate performance. Simplified Reinhard or approximate curves can be used on mobile.

How to adjust tone mapping effects?

Adjust exposure, white point, curve parameters, or choose different algorithms. In Unity, Post Processing provides multiple parameters.