HLG vs Log
HLG (Hybrid Log-Gamma) and Log (Logarithmic gamma curves) are both critical concepts in modern video production, especially when dealing with high dynamic range (HDR) and wide color gamut (WCG) footage. However, they serve different purposes within the imaging pipeline.
1. Log (Logarithmic Gamma Curves)
What it is:
"Log" refers to a family of logarithmic gamma curves used by professional video cameras (like Canon C-Log, Sony S-Log, Panasonic V-Log, ARRI Log C, Blackmagic RAW, etc.). These curves are designed to capture the widest possible dynamic range and color information directly from the camera's sensor.
Purpose:
- Maximize Sensor Data: The primary goal of Log is to efficiently store the vast amount of light and color information a modern camera sensor can capture into a video file. This means compressing a very wide range of light values (from deep shadows to bright highlights) into a smaller digital container, without clipping (losing detail) at either end of the spectrum.
- Post-Production Flexibility: Log footage looks flat, desaturated, and low contrast "out of the box." This is by design! It's not meant for direct viewing. Instead, it provides maximum flexibility for colorists in post-production, allowing them to stretch and reshape the tonal values and colors without introducing artifacts or losing detail.
Characteristics:
- Flat & Desaturated: Images shot in Log typically look dull, low-contrast, and lacking saturation.
- High Dynamic Range (Potential): Contains a massive amount of dynamic range, often 12-15+ stops, which can be "unpacked" during grading.
- Requires Grading: Log footage must be color graded to look presentable and to translate its captured dynamic range into a display-ready format (like Rec. 709 SDR or an HDR standard).
- Scene-Referred: Log footage is often described as "scene-referred" because it directly represents the light ratios of the original scene, rather than how it would appear on a specific display.
Where it sits in the workflow:
- Acquisition: Used during camera recording.
- Editing & Grading: The primary working space for colorists before final output.
2. HLG (Hybrid Log-Gamma)
What it is:
HLG (Hybrid Log-Gamma) is a High Dynamic Range (HDR) Electro-Optical Transfer Function (EOTF), developed jointly by the BBC and NHK. It's a standard for encoding HDR video. Unlike Log, HLG is specifically designed for display and distribution.
Purpose:
- Backward Compatibility with SDR: HLG's "hybrid" nature is its key differentiator. It's designed to be somewhat backward-compatible with Standard Dynamic Range (SDR) displays (Rec. 709). The lower portion of the HLG curve resembles a traditional Rec. 709 gamma curve, while the upper portion is logarithmic, encoding HDR highlights.
- Broadcast-Friendly HDR: HLG is favored for broadcast HDR because it simplifies the workflow. A single HLG signal can be shown on both HDR and SDR screens, with SDR screens showing a reasonably good SDR image (though perhaps a bit darker than a properly graded SDR signal) without needing complex tone mapping.
- "Set and Forget" for some scenarios: For live broadcasting or situations where a full HDR grading pipeline isn't feasible, HLG allows for a more "baked-in" HDR look that works decently on various displays.
Characteristics:
- Looks "Decent" on SDR: While not perfectly matched to an SDR grade, HLG content on an SDR display will look much better than Log content, as it has a reasonable contrast and saturation.
- HDR on HDR Displays: On an HLG-compatible HDR display, it reproduces the full HDR dynamic range and wide color gamut.
- Display-Referred: HLG is a "display-referred" EOTF because it's designed to describe how the image should be shown on a specific type of display (HDR or SDR).
- Less Creative Freedom than Log: While it encodes HDR, HLG is less flexible for extensive creative grading than Log. Its curve is more "fixed" to display characteristics.
Where it sits in the workflow:
- Distribution/Delivery: An output format for HDR video, particularly for broadcast and some streaming platforms.
- Sometimes Acquisition: Some cameras can record directly in HLG, especially for quicker turnaround HDR productions, but this usually limits grading flexibility compared to Log.
HLG vs. Log: Key Differences
| Feature | Log Gamma Curves (e.g., S-Log, C-Log, Log C) | HLG (Hybrid Log-Gamma) |
|---|---|---|
| Purpose | Capture: Maximize sensor data for post-production. | Display/Distribution: Encode HDR for viewing, with SDR compatibility. |
| Appearance (raw) | Very flat, desaturated, low contrast. | Good contrast & saturation on HDR displays; reasonably good on SDR. |
| Dynamic Range | Maximizes captured dynamic range (raw sensor potential). | Encodes HDR dynamic range for display. |
| Color Gamut | Usually captures Wide Color Gamut (e.g., Rec. 2020). | Utilizes Wide Color Gamut (Rec. 2020) for display. |
| Grading Needs | Requires extensive grading to look presentable. | Can be used "as is" or with minimal grading for broadcast HDR. |
| Flexibility | Maximum flexibility for creative grading. | Less flexible for creative grading due to display-oriented curve. |
| Workflow Stage | Acquisition & Working Space in post-production. | Delivery/Output Format for HDR. |
| "Referred" Type | Scene-referred | Display-referred (hybrid) |
When to Use Each
-
Use Log (for acquisition) when:
- You need the absolute maximum dynamic range and color information from your camera.
- You plan to do extensive color grading in post-production.
- You want the flexibility to output to multiple deliverables (SDR Rec. 709, HDR PQ, HDR HLG, etc.) from a single master grade.
- You are working in a controlled post-production environment with calibrated monitoring.
-
Use HLG (for delivery) when:
- Your final deliverable is HDR for broadcast, or for platforms that specifically require HLG.
- You need some degree of backward compatibility so that your HDR content looks acceptable on SDR displays without a separate SDR pass.
- For live HDR production or workflows where a full, custom color grade for every shot is not feasible, and you need a good "out-of-the-box" HDR look.
- Some cameras allow recording directly in HLG for quicker, "baked-in" HDR results, though with less grading latitude than Log.
Typical Professional Workflow:
Many high-end productions will shoot in Log (to get the most data), grade the Log footage to create an HDR master (often in PQ or HLG), and then create a separate SDR Rec. 709 master from that same Log footage (or an SDR "trim pass" from the HDR master). This offers the best of both worlds: maximum capture flexibility and optimized deliverables for all target platforms.