videobeaux/docs/_site/programs/utilities/gamma_fix.md

gamma_fix

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Description

Normalizes gamma, brightness, and exposure levels for broadcast-safe or web-safe consistency across diverse footage.

Purpose

gamma_fix is designed to correct inconsistent exposure, contrast, and gamma shifts that often appear when mixing footage from multiple cameras, phones, archives, or lighting conditions.
It provides predictable luminance normalization suitable for:

• web delivery,
• broadcast pipelines,
• archival stabilization,
• color prep before LUTs or grading.

How It Works

1. Luma Analysis
   The program evaluates input brightness and computes adjustments toward target_yavg.
2. Contrast Shaping
   min_contrast and max_contrast constrain the allowed contrast stretch, preventing overcorrection.
3. Gamma Adjustment
   A user-specified gamma curve can brighten or darken midtones without clipping shadows or highlights.
4. Saturation Shaping
   sat provides global saturation control to compensate for washed-out or overly vivid sources.
5. Legalization
   When enabled, legalize ensures output luminance stays within broadcast-safe IRE ranges.
6. Encoding
   Output is encoded using the selected codec, preset, and CRF.

Program Template

videobeaux -P gamma_fix \
  -i input.mp4 \
  -o output.mp4 \
  --target_yavg VALUE \
  --min_contrast VALUE \
  --max_contrast VALUE \
  --gamma VALUE \
  --sat VALUE \
  --legalize VALUE \
  --vcodec VALUE \
  --crf VALUE \
  --preset VALUE \
  --acodec VALUE \
  --ab VALUE

Arguments

• target_yavg — Target brightness (luma average) to normalize toward.
• min_contrast — Minimum allowed contrast level after adjustment.
• max_contrast — Maximum allowed contrast level after adjustment.
• gamma — Gamma correction multiplier for shaping midtones.
• sat — Saturation adjustment factor.
• legalize — Ensures output levels conform to broadcast-safe luminance ranges.
• vcodec — Video codec to use for output (e.g., libx264).
• crf — Constant Rate Factor controlling video quality.
• preset — Encoding speed/quality preset.
• acodec — Audio codec.
• ab — Audio bitrate.

Real World Example

videobeaux -P gamma_fix \
  -i myvideo.mp4 \
  -o gamma_fix_styled.mp4 \
  --target_yavg EXAMPLE \
  --min_contrast EXAMPLE \
  --max_contrast EXAMPLE \
  --gamma EXAMPLE \
  --sat EXAMPLE \
  --legalize EXAMPLE \
  --vcodec EXAMPLE \
  --crf EXAMPLE \
  --preset EXAMPLE \
  --acodec EXAMPLE \
  --ab EXAMPLE

Technical Notes

• Gamma correction subtly adjusts midtone brightness without flattening highlights.
• Luma averaging helps normalize footage from mixed sources with inconsistent exposure.
• Excessive contrast stretching may introduce noise—use moderate ranges for natural results.
• Broadcast-safe legalization prevents clipped peaks in deliverables meant for TV or streaming platforms.
• Saturation adjustments apply globally; fine-grained color correction should be done in grading tools.

Recommended Usage

• Normalizing multi-camera footage before editing.
• Preparing consistent brightness for LUT workflows or grading.
• Fixing underexposed or flat-looking archive material.
• Making levels compliant for broadcast delivery requirements.

Quality Tips

• Start with small gamma adjustments for natural tonal shifts.
• Set target_yavg close to typical mid-gray values (around 40–55% normalized luma).
• Use moderate sat boosts (1.05–1.15) to avoid color clipping.
• Lower CRF (14–18) retains quality when making heavy tonal corrections.
• Always inspect waveform before and after applying legalization.