16-11-15【摄影机】高动态范围（HDR）常见问题解答（第一期）

高动态范围（HDR）常见问题解答（第一期）

高动态范围（HDR）是2016年见诸各大媒体的热词，也是现今每一位内容创作者脑中不断思考的话题，不过关于这个概念仍存在很多迷惑和误解。这一系列HDR常见问题解答，阐述了相关标准，希望能够帮助大家拨开迷雾。

本期内容：

1. HDR介绍

什么是HDR？

什么是SDR？

电影和电视的HDR和平面摄影中的HDR是同一个概念吗？

什么是HDR录制？

什么是HDR显示？

目前存在哪些HDR标准？

2. HDR录制

为什么要拍摄HDR？它的优势在哪里？

如何使用ARRI摄影机录制HDR？

HDR在录制运动场景时会不会有问题？

ARRI的摄影机都支持HDR？

采用ARRI摄影机录制HDR有什么独有的优势？

ARRI的色彩处理深度是多少？

为什么ARRI要将16 bit线性图像存储为12 bit对数格式？

如何在现场监看ARRI摄影机录制的HDR？

在现场监看HDR需要使用专用的LUT吗？

如何为HDR录制设定曝光/打光？

假如现场没有HDR监看，需要留意哪些易犯的错误？

拍摄HDR对镜头有什么特殊要求吗？

3. 后期制作

有没有一个HDR标准可以遵循？

HDR工作流程大概是什么样子？

我可以在ACES工作流程中生成HDR母带吗？

我需要事先对HDR或SDR进行调色吗？

是否需要用特定的软件来剪辑/后期输出HDR电影？

有没有可能将SDR母带向上转换为HDR？

有没有可能将HDR母带向下转换为SDR？

1. 介绍

什么是HDR？

HDR是“高动态范围（High Dynamic Range）”的首字母缩写，它代表着对比度高于“普通”画面。

什么是SDR？

SDR是“标准动态范围（Standard Dynamic Range）”的首字母缩写，目前我们在电影院和电视屏幕上看到的都是标准动态范围对比度的普通画面。

电影和电视的HDR和平面摄影中的HDR是同一个概念吗？

在平面摄影中，HDR代表通过不同快门速度多次曝光在单一画面中呈现更高动态范围的拍摄和修片技法。平面摄影的HDR输出目标是SDR显示设备，生成的画面有一种“超现实”的观感。电影和电视领域的HDR属于UHD标准的范畴，换句话说，它更指代一种技术的提升，能够记录和显示亮部与暗部更多的细节。

什么是HDR录制？

有计划的拍摄。假如你的任务是拍摄一档需要HDR输出的节目，那么务必确认你所采用的影像传感器/图像处理管线从一开始就拥有足够大的宽容度。HDR图像的丰富和逼真程度是在录制阶段就决定了的。

什么是HDR显示？

这是一个还未明确定义的概念，只要能够呈现比“普通”对比度更高的对比度都可视为HDR显示。满足这一要求的显示设备要么基于主动式背光设计，要么就是采用了OLED技术。我们在后面的部分为大家列举了一些消费级HDR显示器/电视机（参见下文“UHD Premium”）和母带级HDR显示器（参见下文“ITU Rec. 2100”）的参数标准。

目前存在哪些HDR标准？

在HDR领域存在多个不同的关联概念，我们将在下一期解释以下这些名词：

•Dolby Vision

•HDR10

•PQ

•SMPTE ST-2084

•Rec 2020

•Rec 2100

2. 录制

为什么要拍摄HDR？它的优势在哪里？

任何想要以“UHD Premium”名义（参见下文“UHD联盟”）推广的UHD内容都必须按照HDR的标准制作母带。Fox Studio已经宣布他们所有的UHD内容都是HDR。

一般情况下，HDR被描述为“更生动鲜活的色彩以及层次更加丰富的黑白”。请试想一下往向窗外的情形：HDR能够帮助我们重现真实生活中丰富、艳丽的色彩，最终带来栩栩如生的观赏体验，并为2D影像增添了空间的立体感。

如何使用ARRI摄影机录制HDR？

没有什么特殊要求，只要以Log C伽马录制ProRes格式或是采用ARRIRAW格式，并且注意曝光就可以了。如果还需要现场监看HDR，请参见下文中的“监看HDR”。

HDR在录制运动场景时会不会有问题？

ARRI摄影机所有的图像数据都是单次曝光，不是平面摄影领域常用的多次曝光再合成手法，因此避免了相关的图像瑕疵问题。然而，HDR可能出现一个不良效应，那就是运动抖动，对比度越高越明显。

ARRI的摄影机都支持HDR？

自从2010年发布第一代ALEXA开始，ARRI的数字摄影机产品线曝光宽容度就全部满足HDR图像的录制要求。当前几大主要的内容厂商，包括Amazon和HBO都在采用ARRI摄影机来创作HDR节目。

采用ARRI摄影机录制HDR有什么独有的优势？

ARRI的ALEXA系列和AMIRA摄影机的动态范围是目前最宽广的，它们为依靠单一机型创作HDR和SDR内容提供了一体化解决方案。

ARRI的色彩处理深度是多少？

机内处理是每通道16 bit，ARRIRAW文件的处理深度是16 bit或者更高。许多软件系统的内部处理深度是32 bit。

为什么ARRI要将16 bit线性图像存储为12 bit Log格式？

16 bit线性编码既低效也无必要。图像转换为12 bit无论是信息数据还是品质都不会有损失。以对数分布来重现数字图像是一种有效的bit分布方式，因为每一档都具备基本相同的编码值，它还模拟了肉眼对光线的递增反应模式，存储在高光与暗部区域的分段增量是相同的。

如何在现场监看ARRI摄影机录制的HDR？

ARRI提供有画面风格文件和3D LUT，通过它们将Log C画面转换为SMPTE ST-2084 (PQ)编码的HDR图像。这些画面风格文件可用在以下摄影机上：ALEXA SXT和SUP不低于5.0版的AMIRA以及ALEXA Mini。至于其它的ALEXA机型则必须在机外处理LUT：先输出Log C画面，再使用外置的LUT盒进行转换。有一些HDR显示器已经可以加载3D LUT。

在现场监看HDR需要使用专用的LUT吗？

是的，你需要使用一个3D LUT将Log C画面转换成HDR图像。

如何为HDR录制设定曝光/打光？

相较于你平时的工作方式并没有特殊的变化。相反，你甚至可以试试将曝光范围进一步扩大。不过需要记住一点，ALEXA或AMIRA摄影机录制的动态范围比SDR显示器能够呈现的更大，为后期制作提供了更充裕的曝光校正空间，但是到了HDR领域，由于HDR自身的动态范围增大，这个校正空间就被压缩了，因此，如果出现高光削峰和暗部损失，在HDR画面中会变得更加醒目。

假如现场没有HDR监看，需要留意哪些易犯的错误？

由于这种工作流程太新，关于这个问题无法从实战中总结出太多经验提供给各位。一个可能出现的问题是现场的自然光或画面中的光源。在HDR后期制作中，当调整图像到某种极端程度时，这些光源也许会过于明亮而造成分散观众注意力的干扰，导致削弱了创作者的表达意图。

拍摄HDR对镜头有什么特殊要求吗？

试图记录大对比场景时，散射光和眩光会成为你的敌人，因此镜头镀膜的品质尤为关键。组件繁多的长焦镜头也许会更难驾驭，镜头前端如果使用了外置滤镜也会助长眩光。一些创作者已经提到，有时镜头眩光对HDR图像的干扰过大。

3. 后期制作

有没有一个HDR标准可以遵循？

目前HDR视频有两种发行格式可以采用：HDR10和Dolby Vision。而HDR内容的制作方式是多样化的。

HDR工作流程大概是什么样子？

HDR工作流程和目前的工作流程没有太大不同。先将Log C（或ARRIRAW）素材导入色彩校正系统，然后应用一个3D LUT转换给监视器输出。不同的HDR使用的这个3D LUT是不同的，ARRI提供有这类3D LUT文件，不过各制作项目可以自由创建自己的画面风格，在机内使用或者用于母带制作。

我可以在ACES工作流程中生成HDR母带吗？

可以，尽管官方发布日期待定，但ACES系统将会包含基于参考渲染变换的转换为HDR输出的功能。

我需要事先对HDR或SDR进行调色吗？

不需要，但是这两种素材的色彩必须分开处理。如果先从HDR开始，那么就要对SDR再做调整，反之亦然。

是否需要用特定的软件来剪辑/后期输出HDR电影？

不需要特定的软件。任何能够处理Log C数据和3D LUT的系统都能用来生成HDR母带。只有创建特殊的发行格式时才有可能需要用到特定的软件。

有没有可能将SDR母带向上转换为HDR？

可以，有一些调色工具提供这种功能，但是Log C素材（或胶片扫描数据）经过母带处理后转换品质可能不会尽如人意。

有没有可能将HDR母带向下转换为SDR？

可以，但是如之前已经提到的，你必须在色彩校正环节再做调整。

下期将会为您讲解有关HDR的显示与发行，以及相关术语的详细解释，敬请关注。

请注意：

ARRI为AMIRA、ALEXA Mini和ALEXA SXT摄影机的现场监看应用以及利用3D-LUT 辅助ARRI摄影机拍摄的HDR内容进行后期制作都提供了HDR画面风格文件。下载这些文件请到以下网址：

http://www.arri.com/?eID=registration&file_uid=16651

有关HDR的更多信息，您可点击查看以下链接：

ARRI摄影机：始于2010年的HDR最佳机型

AMIRA以UHD和HDR捕捉焰火

High Dynamic Range FAQ (Part I)

High Dynamic Range (HDR), the media-buzz-word of 2016, is on every content creator’s mind nowadays – as there is much confusion. This collection of frequently asked questions on the subject of HDR names all relevant standards and tries to clear out the big mysteries surrounding that topic.

This part will cover the following topics:

1. Introduction

What is HDR?

What is SDR?

Is HDR for film and TV the same as HDR in photography?

What is HDR acquisition?

What is an HDR display?

What HDR standards exist?

2. Acquisition

Why should I shoot for HDR? What are the benefits?

How do I record HDR with ARRI cameras?

Are there motion artefacts with HDR?

Are ARRI cameras HDR compatible?

What are the advantages of using an ARRI camera for HDR capture?

What is the bit depth of ARRI’s color processing?

Why does ARRI store 16 bit linear images in a 12 bit log format?

How do you monitor HDR on set with ARRI cameras?

Do I need a special LUT for HDR monitoring on set?

How do I expose/light for HDR?

Are there any pitfalls when shooting for HDR with no HDR monitor on set?

Are there any special lens requirements when shooting for HDR?

3. Postproduction

Is there one HDR standard?

Can I generate an HDR master in an ACES workflow?

Do I grade HDR or SDR first?

Do I need special software to edit/postproduce an HDR movie?

Is it possible to up-convert an SDR master to HDR?

Is it possible to down-convert an HDR master to SDR?

1. Introduction

What is HDR?

HDR is short for High Dynamic Range, a term that describes images with a higher than “normal” contrast ratio.

What is SDR?

Standard Dynamic Range (SDR) is the regular contrast ratio we are currently used to seeing on cinema and TV screens.

Is HDR for film and TV the same as HDR in photography?

In stills photography the term HDR is used to describe a technique for displaying greater dynamic range in a single image by combining several exposures photographed at different shutter speeds. The result (rendered for an SDR display) creates a “hyper-realistic” looking image. The term HDR in the context of discussions about UHD standards for film and TV, on the other hand, refers to an expansion of the technological options to capture and display brighter whites and darker blacks.

What is HDR acquisition?

Shooting with foresight. When you’re on a job capturing footage for a show designated to have HDR deliverables, make sure you are using a sensor/image-processing pipeline that is capable of capturing enough latitude in the first place. The richness and fidelity of an HDR image is defined in the acquisition phase.

What is an HDR display?

A non-specific term for displays that can produce a higher than “normal” contrast ratio. Such displays are either based on active backlight or OLED technology. There are specifications for consumer HDR displays/TVs (see UHD Premium) and for mastering HDR displays (see ITU Rec. 2100).

What HDR standards exist?

There are various terms associated with the HDR subject area. We explain the following terms later on in the FAQ:

Dolby Vision

HDR10

PQ

SMPTE ST-2084

Rec 2020

Rec 2100

2. Acquisition

Why should I shoot for HDR? What are the benefits?

Any UHD content that wants to be advertised as “UHD Premium” (see UHD Alliance) needs to be mastered in HDR. Fox Studio announced that all their UHD content will be in HDR.

Generally HDR is being described as “more lifelike colors with deeper blacks and higher whites”. Think of looking out of your window: HDR aims to recreate the richness of colors and color-intensities that we see in real life. This ultimately leads to a more immersive viewing experience and adds a sense of depth to 2D imagery.

How do I record HDR with ARRI cameras?

Nothing out of the ordinary is needed, just record ProRes clips with Log C gamma or ARRIRAW and expose carefully. For on-set monitoring see “Monitoring HDR”.

Are there motion artefacts with HDR?

ARRI cameras capture all image data at once rather than mixing different exposures, as it is common in stills photography, so this avoids associated artefacts. However, a possible side effect of HDR is motion judder, which would become more pronounced because of the higher contrast.

Are ARRI cameras HDR compatible?

Since the introduction of ALEXA in 2010, ARRI’s digital camera lineup has offered enough latitude for HDR imagery. Currently several major content producers, including Amazon and HBO, are using ARRI cameras to create HDR shows.

What are the advantages of using an ARRI camera for HDR capture?

ARRI’s ALEXA and AMIRA cameras capture the widest dynamic range currently possible, offering an “all-in-one” solution for originating HDR and SDR content with one camera. 

What is the bit depth of ARRI’s color processing?

Images in the camera are processed in 16 bits per channel. ARRIRAW files are processed in 16 bits or more. Many software systems work internally with 32 bits.

Why does ARRI store 16 bit linear images in a 12 bit log format?

16 bit linear encoding is neither efficient nor necessary. The images can be converted to non-linear 12 bit without any loss of information or quality. Representing digital images with a logarithmic scale is an efficient allocation of the bits, because a relatively equal amount of code values is assigned to each stop. It also mirrors the incremental way the human eye responds to light, storing the same fractional increments in the highlights as in the shadows.

How do you monitor HDR on set with ARRI cameras?

ARRI provides look files and 3D LUTs that convert the Log C image into an HDR image using SMPTE ST-2084 (PQ) encoding. The look files can be used in the following cameras: ALEXA SXT and – from SUP 5.0 – AMIRA and ALEXA Mini. With other ALEXA models the LUT processing has to take place out of the camera: a Log C image is outputted and converted using an external LUT box. Some HDR monitors already allow a 3D LUT to be loaded.

Do I need a special LUT for HDR monitoring on set?

Yes, you should use a 3D LUT that converts the Log C image into an HDR image.  

How do I expose/light for HDR?

No special change in your style of working is required; in fact you can try to push your exposure range even a bit more. But keep in mind that while an ALEXA or AMIRA camera captures much more dynamic range than can be displayed on an SDR monitor, providing latitude for correcting the exposure in post, this possibility is reduced with HDR because the dynamic range of the HDR master is larger. The clipping of highlights or loss of shadow detail may therefore become more visible in an HDR image.

Are there any pitfalls when shooting for HDR with no HDR monitor on set?

There is not much experience-based feedback on this, as the workflows are so new. One possible issue is practicals, or light sources within the frame. In an HDR image they may become too bright and distracting when pushing the image to extremes in post. This could divert the audience’s attention from where it should be and undermine the creative intention of a scene.

Are there any special lens requirements when shooting for HDR?

When you want to capture a scene with a large contrast range, stray light or flare is your enemy. The quality of the lens coating is therefore very important. Long telephoto lenses with many elements are probably more difficult to use. External filters in front of the lens can also increase flare. Some people have noted that lens flares can become too distracting in HDR images.

3. Postproduction

Is there one HDR standard?

Currently there are two distribution formats for HDR video: HDR10 and Dolby Vision. The HDR content, however, can be produced in several ways.

What does an HDR workflow look like?

An HDR workflow is not much different from current workflows. The Log C (or ARRIRAW) footage is loaded into a color correction system. A 3D LUT is then used to convert the images for the monitor output. This 3D LUT needs to be changed for different kinds of HDR. ARRI provides such 3D LUTs, but every production is free to design their own look and to use it in-camera, as well as in the mastering process.

Can I generate an HDR master in an ACES workflow?

Yes, the ACES system will include transforms for HDR output based on the reference rendering transform, though the official release date is TBC.

Do I grade HDR or SDR first?

You don’t have to do HDR or SDR grading first, but you have to do a separate grading for both of them. If you start off with HDR you have to do adjustments for SDR, and vice versa.

Do I need special software to edit/postproduce an HDR movie?

You do not need any special software. Any system that can work with Log C data and 3D LUTs can be used to generate an HDR master. You may need special software to create specific distribution formats.

Is it possible to up-convert an SDR master to HDR?

Yes, some color grading tools offer such features. The quality may not be as good as if the Log C footage (or film scans) had been re-mastered.

Is it possible to down-convert an HDR master to SDR?

Yes, but as already mentioned some questions before, you have to do adjustments in color correction. 

Next Part will introduce you HDR display and distribution, as well as explanations on the terms. Please follow our updates.

Please note:

ARRI provides HDR look files for AMIRA, ALEXA Mini and the ALEXA SXT camera for on-set viewing as well as 3D-LUTs for HDR post-production for images captured with ARRI cameras. You can find those files by copying the following link:

http://www.arri.com/?eID=registration&file_uid=16651

For more information of HDR, please click the following link：

ARRI Cameras：The Best For HDR Since 2010

AMIRA Captures Fireworks in UHD and HDR