If there’s one thing filmmaking has no shortage of, it’s acronyms. The deeper you delve into the technical aspects of the industry, the more prevalent (and convoluted) these acronyms become. Likewise, as technology improves, more join the ranks. One of the latest is HLG, which stands for Hybrid Log Gamma.
Introduced back in 2014, Japanese broadcasting company NHK and the BBC developed HLG in a joint effort to solve a problem. Broadcasters needed a more compatible signal for a wide variety of television sets. It’s a simple enough idea, but as you develop video related projects, it’s important to know more of the specifics and whether or not you want to incorporate HLG into your workflow.
HLG and HDR
Hybrid Log Gamma is an open-source standard HDR (High Dynamic Range) video format. Simply put, dynamic range is the measurement between a maximum and a minimum value. In terms of video, this measurement has to do with an image’s luminance — the brightness range between black and white.
Many televisions offer HDR options as one of their selling points these days. However, unlike resolution parameters (1080p, 4K, etc), HDR isn’t as easily quantified. Luminosity is a spectrum. So, the range can vary between how it’s captured on camera and how it’s displayed when broadcast.
Imagine a grayscale with only five color tones to get from black to white. This range is obviously small, resulting in big color jumps between tones. There’s little room for color nuance. With High Dynamic Range, the scale increases dramatically. That means HDR delivers more nuance within the range while also expanding the range into deeper blacks and brighter whites. The result is additional color depth and contrast that make images pop like never before.
The HDR landscape
HLG is the latest HDR standard to compete alongside the commonly-used HDR10 and the more recent (and exclusive) Dolby Vision. While they all seek to serve a similar purpose in delivering HDR content to viewers, they have their differences.
Dolby Vision wants to set itself apart as the top tier HDR option. This standard offers more bit-depth with 12-bits over HDR10’s 10-bit. Ultimately, it works off similar principles and systems. Being an exclusive format, however, means manufacturers have to secure a license to use Dolby Vision in their devices. As such, HDR10 tends to be the default for most. However, the next version, HDR10+, is hoping to compete with Dolby Vision’s superior clarity.
Not every device can playback all of these formats. Fortunately, this isn’t like the format wars of previous eras. Consumers don’t have to choose between Betamax and VHS or Blu-Ray and HD DVD. Just about any television made within the last couple years will allow for playback of at least one HDR format.
What makes HLG different?
So what sets Hybrid Log Gamma apart from these other HDR formats? The biggest difference is how HLG data is transferred through devices. The current HDR formats send the signal information along with metadata. This tells the television the specific color settings to display.
HLG doesn’t need metadata, however. It instead features a non-linear Electro-Optical Transfer Function (EOTF). Simply put, this incorporates the normal Gamma curve for light coding that Standard Dynamic Range uses. Then, it incorporates a logarithmic curve to deliver the HDR aspects of a signal.
In more basic terms, by using the curve to send its signal instead of metadata, HLG is able to put SDR and HDR into a single signal. Devices unable to read the HDR info just ignore the extra data. This gives HLG a form of backward compatibility since it can be read by both SDR or HDR sets. While this doesn’t have any impact on physical media, it’s a game-changer for broadcast and streaming — what it was created for.
As is the case with any new format (DVD, Blu-ray, HDTV) universal adoption is a slow process. Despite being around for a few years, this is the case with HDR in general. There are still a plethora of standard range devices in households. For broadcasting shows and events, stations obviously can’t ignore a significant portion of their audience.
Because other HDR formats utilize metadata to tell devices how to display images, broadcasters have to send a separate SDR signal in order to ensure everyone can watch. This effectively doubles their broadcasting bandwidth. HLG doesn’t have this issue. It can be broadcast completely within a single signal. For broadcasting and streaming, HLG offers the best of both worlds without being as data-intensive. This equates to HLG being a far less expensive option.
Being the newest format, it’s also the one with the least device support. Don’t fear, however. Design support could easily be integrated on a device via a firmware update. Potentially, widespread adoption of HLG is just a quick download away.
Producing in HLG
Aside from making sure you’re not leaving a big part of your audience behind, HLG has a more specific benefit for video producers crafting HDR content. Every creator wants to stay on top of the latest and greatest in technology, but doing so can get expensive.
First of all, you’ll need to make sure the camera you’re using has HLG HDR capabilities. There are a few cameras on the market supporting HLG as an “instant HDR” setting — most recently, the more consumer-friendly (though still around $2000) Sony a7R III and Panasonic GH5S.
Generally speaking, when you record HDR footage (without HLG), it’s not as simple as just importing it to your editing program and getting to work. Proper grading and mastering require specific HDR reference monitors. These can cost anywhere from $5,000-25,000 on the low end. There are ways to rig something up using an off the shelf television set, but it wouldn’t be as reliable or effective for the long term.
Because HLG works off a logarithmic curve, however, you can record a video and immediately play it back using a regular TV or display. This provides the “instant HDR” workflow for your post-production process while still allowing you to take advantage of the higher dynamic ranges in your picture.
HLG is a more consumer-friendly HDR option for video creators, but it’s not without drawbacks. Since it doesn’t use the same logs (S-Log and V-Log) as other HDR formats, color grading can be a bit more difficult. The metadata tends to change or become lost altogether. For broadcast purposes or YouTube, this isn’t as big of a deal since the encoding isn’t the same as for physical transfers. Depending on your project, it’s definitely something to keep in mind.
Don’t get left behind
Though it’s only been around for a few years, it’s clear that HDR isn’t going anywhere and will become a new standard for video content. If you want to keep your projects on the cutting edge, you’ll have to find a way to adapt.
While there still are some downsides and kinks, HLG seems like an excellent entry point for creators. Its hybrid nature makes it a perfect bridge between the old standards and the new, while also being a significantly cheaper option when it comes to post-production.