Color Science: Helios Log
What is Log Encoding?
Human vision operates non-linearly. According to the Weber-Fechner law, human perception of brightness is roughly logarithmic—we are much more sensitive to differences in dark shades than in bright highlights. Camera sensors, however, capture light linearly. A modern cinema camera sensor can capture 14 or more stops of dynamic range, but encoding this massive linear data directly into 8-bit or 10-bit integer formats would either clip highlights or throw away shadow detail due to quantization.
Logarithmic (Log) encoding solves this by redistributing the linear sensor data (code values) to match human perception. It compresses the highlights to preserve headroom and expands the shadows to maintain detail, allowing wide dynamic range to fit efficiently into limited bit depths.
General Logarithmic Transfer Function
Where x is linear scene light and y is the encoded log signal.
Helios Log Transfer Function
Helios Log is mathematically identical to ARRI LogC3, widely considered the industry gold standard for digital cinema. It provides an incredibly natural roll-off in the highlights and rich shadow detail. The transfer function uses a piecewise approach: a logarithmic curve for midtones and highlights, and a linear toe section for the deepest shadows to prevent infinite negative values when dealing with sensor noise.
Curve Parameters
| Parameter | Symbol | Value | Purpose |
|---|---|---|---|
| Cut | cut | 0.010591 | Linear/log transition point |
| Linear scale factor | a | 5.555556 | Scales incoming linear light |
| Linear offset | b | 0.052272 | Prevents log(0) at black |
| Log scale factor | c | 0.247190 | Compresses range for encoding |
| Log offset | d | 0.385537 | Mid-gray position |
| Toe slope | e | 5.367655 | Slope of linear shadow region |
| Toe offset | f | 0.092809 | Black point offset in shadows |
Key Reference Points
- 18% Middle Gray (0.18 linear) → 0.391 encoded
- 90% White (0.90 linear) → 0.573 encoded
- Black (0.0 linear) → 0.093 encoded
18% middle gray (0.18 linear) is a key reference — notice how log curves lift shadows while compressing highlights
All Supported Log Profiles
Helios is capable of transforming linear sensor data into multiple industry-standard log curves in real-time. Below are the formulas and parameters for all supported curves.
1. Helios Log / ARRI LogC3 (Primary)
Default encoding, identical to ARRI LogC3.
2. ARRI LogC4 (Extended DR)
Designed for extreme dynamic range (Alexa 35). Base-2 logarithmic curve.
3. Sony S-Log3
Similar to Cineon, but scaled for modern digital sensors.
4. Kodak Cineon
The original film scan log curve.
5. ACEScct
ACES color space working curve with a toe for color correction.
6. RED REDLogFilm
RED's traditional Cineon-style log encoding.
7. Blackmagic Film Gen5
Latest BMD color science curve.
8. Canon C-Log2
Canon's extended dynamic range curve, symmetric around 0.
Curve Comparison
18% middle gray (0.18 linear) is a key reference — notice how log curves lift shadows while compressing highlights
Color Primaries & Gamut
A log transfer function only defines lightness (grayscale). For color, Helios Log pairs natively with ARRI Wide Gamut 3 (AWG3) color primaries. Color primaries define the coordinates in the CIE 1931 chromaticity diagram that represent the absolute extremes of Red, Green, and Blue the system can reproduce—forming a triangle called the gamut. AWG3 is larger than Rec.2020 and encompasses nearly all visible pointer colors.
XYZ(D65) to ARRI Wide Gamut 3 (AWG3)
3×3Matrix for transforming from absolute XYZ color space to Helios native AWG3 primaries.
Supported Output Color Spaces
Helios can output to various standard color spaces using the following 3x3 matrices (from XYZ D65).
Rec.709
3×3Identity matrix (assuming internal gamut matches output).
Rec.2020
3×3UHDTV wide color gamut.
DCI-P3
3×3Digital cinema standard.
ACES AP0
3×3ACES ultra-wide color space (archive).
ACES AP1
3×3ACEScct/ACEScc working space.
Sony S-Gamut3.Cine
3×3Sony's standard wide gamut matching digital cinema needs.
Chromatic Adaptation
Different color spaces often use different illuminant white points (e.g., D65 for Rec.709, ~D60 for ACES, D50 for ICC profiles). To accurately transform color coordinates between spaces without shifting the perceptual appearance of neutral grays and colors, we use a Chromatic Adaptation Transform (CAT), typically using the Bradford method.
Bradford D50 to D65
3×3Adapts standard D50 white point to D65.
Bradford D50 to ACES (~D60)
3×3Adapts D50 to the ACES white point.
McCamy's CCT Formula
To estimate Correlated Color Temperature (CCT) from CIE 1931 xy chromaticity coordinates, Helios uses McCamy's cubic approximation, which is highly accurate and computationally efficient for real-time video processing.
IDT & NLE Workflow
Native NLE Support
Because Helios Log relies on the exact mathematical model of ARRI LogC3 paired with ARRI Wide Gamut 3 (AWG3), you do not necessarily need a custom Input Device Transform (IDT). You can use built-in ARRI color management profiles in any major NLE to accurately develop Helios footage.
DaVinci Resolve Workflow
We highly recommend a scene-referred workflow in DaVinci Resolve using DaVinci Wide Gamut (DWG) or ACES. Transforming directly to a massive working space preserves the entire captured color volume, providing significantly more latitude for creative grading than crushing the image down to Rec.709 immediately.
- Add a Color Space Transform (CST) node at the start of your tree.
- Set Input Color Space: ARRI Wide Gamut 3
- Set Input Gamma: ARRI LogC3
- Set Output Color Space: DaVinci Wide Gamut (or Rec.709)
- Set Output Gamma: DaVinci Intermediate (or Gamma 2.4)
Adobe Premiere Workflow
Premiere Pro users can rely on built-in color management or apply a conversion LUT. Simply apply the official `.cube` LUT to the master clip settings or as the first effect in the Lumetri Color panel to normalize the footage to Rec.709.
65-point 3D LUT designed for converting Helios Log AWG3 footage directly to Rec.709 Gamma 2.4.