REC 709-A Exposed: Navigating the Gamma Minefield in Post-Production

In the color grading world, nothing can throw off your game like an unexpected gamma shift. REC 709-A, a specialized color space in DaVinci Resolve, aims to tackle gamma inconsistencies head-on, especially those arising between Apple and non-Apple workflows. However, it opens up a can of worms, such as what happens when REC 709-A materials are mistakenly interpreted as BT.1886.

The Mathematical Anatomy of Linear Segment in sRGB and Apple Display P3 EOTF

The sRGB EOTF and Apple Display P3 EOTF have a linear segment, often described by:

L = a · V

Where:

• L is the luminance
• V is the video signal level
• a is a constant

This linear segment is a legacy from CRT displays and is considered undesirable in modern flat-panel displays. For sRGB, the linear segment applies to values of V < 0.04045.

REC 709-A: The DaVinci Resolve Special

REC 709-A is engineered to manage gamma more consistently across different systems. It effectively bypasses the linear segment and adapts the gamma curve, particularly in the shadows and highlights, to mitigate the gamma shift issues commonly experienced in Apple workflows.

When REC 709-A Meets BT.1886: A Case of Mistaken Identity

Inaccurately interpreting REC 709-A footage as BT.1886 can have noticeable repercussions, especially in the shadow details. This is because BT.1886 and REC 709-A have different EOTF characteristics.

Mathematical Example

Let’s consider a simplified representation of the EOTFs for both:

• BT.1886 EOTF is typically expressed as:L = (V + b)^cwhere c = 2.4 is commonly used.
• REC 709-A EOTF might look like:
  • For V ≥ Vthreshold:L = V^γ'
  • For V < Vthreshold:L = a · V
  where γ' is the gamma value excluding the linear segment, Vthreshold is the threshold below which the linear segment kicks in, and a is a constant.

If REC 709-A footage is incorrectly interpreted as BT.1886, then for V < Vthreshold, the originally intended linear segment L = a · V would be erroneously interpreted through the BT.1886 formula L = (V + b)^2.4.

This mismatch could result in lifted blacks and a shift in the luminance, especially noticeable in the shadows, just as you experienced in your color grading work.

Practical Implications and Challenges

• Gamma Shift Pitfall: The gamma shift could severely impact the creative intent, especially in scenes with complex lighting or critical skin tones.
• Educational Complexity: The need to understand these nuances adds another layer of complexity to an already intricate field.
• Metadata Mess: The issue underscores the pressing need for a standardized metadata ecosystem in digital video production.

References and Standards

• DaVinci Resolve User Manual: Link to DaVinci Resolve User Manual
• ITU-R BT.709-6: Link to ITU-R BT.709
• ITU-R BT.1886: Link to ITU-R BT.1886