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Blackest Blacks: Ten Things to Know 关于 Producing HDR

高动态范围, 或HDR, gives video 生产商 an expanded range of colors and brightness to display their videos. From a standards perspective, it’s the journey from 矩形. 709视频到矩形. 2020, as you can see in the excellent simulation shown in 图1. From a brightness perspective, it’s the journey from 300–500 nits to 1,000–4,000年尼特. 从市场营销的角度来看, it’s the promise of the blackest black viewers have ever seen on their screens.

虽然从高清到4K的过渡很简单, the transition from standard dynamic range (SDR) to HDR is much more profound, 它需要许多新的工具和工作流程的变化. 在本文中, I’ll identify the 10 things you need to know about HDR and introduce you to some HDR 生产商 and technologists, and to technologies these 生产商 leveraged along the way.

1. 其实很简单.

HDR听起来很复杂，在技术层面上确实如此. Abstractly, 然而, it involves just five simple concepts.

第一个, to acquire the expanded brightness and color palette needed f或HDR display, you have to capture and maintain your video in 10-bit or higher formats. 第二个, you’ll need to color grade your video to fully use the expanded palette. 第三, you’ll have to choose and support one or more HDR technologies to reach the broadest number of viewers. 第四, 对于其中的一些技术, you’ll need to manage color and other metadata through the production workflow to optimize display on your endpoints. 最后, although you’ll be using the same codecs and adaptive bitrate (ABR) formats as before, you’ll have to change a few encoding settings to ensure compatibility with your selected HDR TVs and other devices.

我并不想过分简化，你应该知道这一点 VIVE生活方式网络, which debuted a live HDR service at the NAB show in March, advised that it took “months of R&D”来完成其工作流程. 然而, we’ll cover these five steps in the discussion that follows, which should leave you with a high-level overview of the issues you’ll face when attempting to produce and distribute HDR video.

图1. 矩形. 709 vs. 矩形. 2020 vs. P3 color spaces (from SMPTE presentation entitled SMPTE ST 2094 and Dynamic Metadata by Lars Borg, 首席科学家, Adobe)

2. All that said, you probably don’t care in the short term.

嘿, 我很感谢你读这篇文章, 但除非你的目标客户有HDR显示器, 制作HDR内容是没有意义的. 今天, 而且是为了短期的未来, the only significant concentrations of HDR displays are in the living room. 如果你正在制作优质内容, you may want to shoot long-tail content in an HDR-compatible format, but if you’re distributing primarily to computers and 移动 devices, 要达到临界质量还需要几年的时间. 如果你不制作优质内容, it will be years before you should consider producing garden-variety training, 新闻, 市场营销, 或其他HDR格式的企业视频.

另一方面, 如果你是一家瞄准OTT的付费发行商, you should be well on your way with HDR plans and workflows. 根据IHS Markit的数据, about 4 million HDR TVs shipped in 2016, and 30 million units are expected by 2020. 因为HDR内容来自Netflix, VUDU, 以及大多数其他优质内容分销商, 它正在成为高端OTT的赌注.

3. 有两个基本问题.

第一个, your production software is inadequate, at least in the short term. 其次，您的显示硬件不足. 为什么? Because of the expanded color and brightness discussed earlier. 多年来，我们一直在和矩形合作. 709 color gamut shown in 图1, the smallest square in the RGB color spectrum. We shot, captured, encoded, and displayed our videos in 矩形. 709. 这是大多数windows显示器的色域, and while 2015 and later Macs expanded to the Digital Cinema (P3) format, even that is still much less than the colors represented by 矩形. 2020. 试图彩色级视频片段绑定为一个矩形. 在矩形中显示2020. 709 or even a P3 color space is like trying to paint with colors that you can’t see.

The same holds true for brightness, which is measured in nits. SDR displays and computer monitors display at between 300 and 500 nits, 而HDR显示目标1,000尼特或更高. 再一次。, you can’t adjust your brightest brights without using hardware and software that shows an accurate preview. 出于这些原因, 在工作流的某个点上, you’ll need to grade your color and brightness on hardware that displays it and in software that can drive the display.

不过，您不必从头开始. Many hardware and software components of your existing workflow will continue to work as before. 例如, Adobe Premiere Pro can’t display HDR at this point (though it’s coming soon), 但可以通过hdr相关的元数据. So you can still edit in Premiere Pro (or Final Cut Pro X), but you probably will need a color grading program to optimize the colors and contrast in your video.

4. You can probably use your existing camera (for VOD anyway).

There are two requirements necessary to shoot video that can be produced as HDR. 第一个, 相机必须拍摄HDR视频, 也就是说动态范围在11到15档之间, 广色域, 4 k的决议, 最好是每秒60帧. 第二个, it has to store that 信息 in a raw or logarithmic format that preserves the full dynamic range of the sensors. 这意味着许多老年人, 然而，高质量的数字电影摄像机, 比如Red One或Epic, 还能用来拍摄HDR来制作VOD吗.

例如, VIVE Networks is using a Red Epic for VOD productions. 我们稍后会讨论, 然而, VIVE had to use a much newer and much more expensive Sony HDC-4300 for live and live-to-tape productions.

5. 您需要选择一个或多个HDR标准.

Your job as content producer is to make your content look as good as possible and to enable it to play on as many end points as possible. 表1 presents the three most prominent options available f或HDR and some of their key characteristics. 让我们找出这三个，然后深入研究表格.

hdr table1

表1. 比较前三名HDR技术

短暂的, Dolby Vision is a standards-based system developed by Dolby, whose primary business model is licensing revenue from TV set, 移动, 还有其他玩家. Note that Dolby Vision display hardware is backward-compatible with HDR10, 所以每个启用杜比视界的设备都可以播放HDR10.

HDR10 is an open standard supported by many companies and industry groups. 最后, Hybrid Log-Gamma (HLG) was jointly developed by the BBC and Japanese state broadcaster NHK, which needed a single format that could display on HDR and SDR 4K sets, and could be used for live and traditional broadcast channels.

The first four lines in 表1 involve quality, starting with metadata. 在杜比视界和HDR10, the metadata carries color and brightness 信息 to optimize the picture on the display. 与杜比, this 信息 is dynamically updated throughout the video; with HDR10 (but not HDR10+, 下面讨论), 这是静态, 整部电影只拍一次.

图2 说明了不同之处. On the left, one tonal mapping value is applied to clips of varying brightness. As a result, the two darkest clips have poor contrast, though the brightest clip looks perfect. On the right, multiple tonal mapping values are applied to match the content. 两个颜色较深的片段对比度要好得多, 当最亮的剪辑, 掌握与静态解相同的值, 看起来一样. 以这种方式, 动态色调映射, 通过动态元数据启用, 应该产生卓越的体验吗.

blackest2

图2. 静态vs. 动态色调映射(改编自Borg的演示)

如表所示，HLG不使用元数据. 相反，它部署了一个 混合动力 method of storing the color and brightness 信息 that separates the SDR data from the HDR data. SDR使用传统的 γ 曲线与SDR显示兼容, while the HDR data is encoded using a logarithmic curve that lets compatible HDR displays stretch the signal over the additional brightness and contrast in the display. This allows one bit stream to serve both SDR and HDR sets, and you can see how the name was derived from the italicized words in the paragraph. Later in this feature, you’ll read more about the creative challenges this technique imposes.

在元数据, 杜比视界最多可以处理12位, which may help avoid color banding and similar issues over the two other systems that process at 10 bits. Although the contrast of most current HDR displays peak at around 1,000年尼特, 你可以掌握杜比视界高达4,000年尼特, which should make it more future-proof for more capable TVs in the future. 最后, 作为许可证的一部分, Dolby Vision-equipped displays use a consistent color mapping engine that should ensure that Dolby Vision-mastered content provides better color and tone accuracy.

As stated above, backward compatibility with older 4K sets without HDR was a major driver for HLG. Dolby Vision can support backward compatibility via a dual-layer system, in which the base layer contains a legacy SDR bit-stream and an optional enhancement layer carries a supplementary signal and the Dolby Vision metadata. 用于交付到已知的支持杜比视觉的系统, you can also produce a single layer that contains both video and metadata, 大多数生产商使用的是哪种方法. HDR10 does not have a configuration that supports SDR devices.

截至发稿时, 三星和亚马逊发布了HDR10+, 也就是带有动态色调映射的HDR10. This takes HDR from static mapping on the left in 图2 to dynamic mapping on the right; a very significant improvement. This feature will be available on 2017 TVs and on older HDR10-compatible TVs via a firmware update.

图3 shows pre-NAB support for the various manufacturers and publishers for the technologies discussed above. 全面覆盖, you’re going to have to select two or possibly three technologies and create the bit-streams necessary to support them. 希望这个名单不会继续增长.

图3. HDR support by manufacturer and publisher (from Ultra HD 4K 新闻)

除了上面的特性表分析之外, several considerations will likely dictate which standards you decide to support. 例如, 杜比视界也被部署在电影院, 所以如果你在使用那里的技术, you have a substantial investment in expertise and equipment, and the Dolby workflow easily supports distribution to all display points. 这使得杜比支持自然.

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