实时HDR视频的混合Log Gamma工作流

对我来说, high dynamic range (HDR) video is a much more exciting development than 3D or resolutions greater than 4K. 当拍摄得当时, 分级, 并在HDR显示器上观看, viewing so much detail in the highlights of HDR video is a visual treat.

The best part about HDR content is that the workflow doesn’t need to be overly complicated, thanks to the implementation of the Hybrid Log Gamma (HLG) that was codeveloped by the BBC in the U.K. 和日本的NHK. HLG is available on the Sony FS5, AX700, Z150, PXW-Z90, and HXR-NX80; and on the Panasonic GH5.

HLG isn’t perfect, nor is it the best gamma curve for all productions. 使用SLOG3拍摄的HDR视频, 专业级配, and exported twice—once for HDR and once for SDR—can look better. But the benefit of using HLG is that you can use one video signal to support both HDR and SDR 生活，而无需对lut进行评分或使用lut. 目前, 超过400个HDTV和UHDTV型号支持HLG播放, 连同选定的投影仪, 谷歌的安卓电视平台, 和YouTube. 迟早的事, you might be asked to produce a 生活 event with an HDR viewing option, 这个关于HLG工作流程的入门是一个很好的开始.

什么是HLG?

As a gamma curve, the HLG curve is very similar to the broadcast standard Rec. 709曲线从0-70% (下面的图1). HLG gets very aggressive with content that falls above 70% in order to be able to record a larger dynamic range. The current Sony implementation allows for 10–11 stops of exposure, compared to 6 stops in Rec. 709.

图1. HLG和Rec. 709年曲线 资料来源:GrandDrake, http://commons.wikimedia.org/w/index.php?curid=48210963

One aspect of filming and viewing HDR content that initially surprised me—but in fact makes perfect sense—is that regardless of whether you are showing content in HDR or SDR, your main content has the same brightness on the de生活ry end (图2). This is especially true for your talent; you would not expose them in the highlights. This creates challenges when filming with different LOG and Gamma curves because you need to expose differently on the acquisition end of the workflow, 取决于LOG或Gamma曲线.

图2. 比较相同镜头的SDR和HDR版本

设置曝光，分辨率，位深和位率

在一般情况下, 使用HLG拍摄时, 你应该把曝光的比平时暗一点, in order to make sure that you don’t push your talent into the highlight end of the curve where there isn’t as much information recorded per stop. You also won’t be able to do much grading to HLG footage because the slope of the curve changes dramatically from the lows and mids to the highlights. This shouldn’t be a big issue for camera operators and technical directors who are used to producing 生活 content, as they are accustomed to having to nail the exposure for 生活 content already.

当涉及到分辨率和色彩位深度时, shooting with HLG is no different than shooting with any LOG or Gamma curve. 如果你的摄像机只支持8位彩色的4K UHD, then your footage won’t grade as well as HD footage filmed in 10-bit color. 也, 大多数HDR显示器都能够支持10位颜色, 所以如果你输出8位颜色, 您将每个红色的播放限制为256个值, 蓝色的, 和绿色, 而不是1,024值，10位.

用HLG拍摄时, it’s especially important not to get stingy with the codec and bitrate choice on the recording end. 事实上, you may want to add an external recorder with a DNxHD or ProRes Codec as opposed to the long GOP interframe in-camera codecs your camera supports. 只是在选择视频输出时要非常小心, as the HDMI and HD-SDI outputs on cameras like the Sony FS5 have different maximum resolutions and color bit depth.

选择色彩空间

选择色彩空间可能会有点令人困惑. 当使用HLG拍摄时，您可以选择Rec. 709和Rec. 2020的颜色. HDR电视更喜欢2020年, 而标清显示器更喜欢709, so one of your workflows is going to have hue and saturation levels slightly off.

If you’re shooting with HLG exclusively for SDR de生活ry and you want to protect your highlights, 使用矩形. 709的颜色. If your shoot requires 生活 HDR de生活ry, then you might want to 使用矩形. 2020年的颜色，让SDR看起来稍微偏离. If this doesn’t sound like a perfect workflow, that’s because it isn’t. 这是一种混合工作流程，伴随着各种权衡. 最后, if you’re de生活ring SDR 生活 and HDR for later on-demand viewing, 你可能想和Rec一起拍. 709颜色和做一些HDR版本的光分级.

Alternatively, you might want to consider one of the other LOG or Gamma curves and apply 生活 LUTs. You can also perform a proper color grade for each of your HDR and SDR outputs. 在一天结束的时候, most 生活 producers will need to test each workflow and decide for themselves which one best suits their needs.