原文出自https://www.insightmedia.info/should-hdr-displays-follow-the-pq-curve/
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Manyin the group believed that since the Perceptual Quantizer (PQ) curve is adisplay-referred Electro-Optic Transfer Function (EOTF) the answer is yes – oneshould follow the PQ curve as well as possible. It turns out the answer is amore nebulous: it depends.
For example, if you are using a reference monitor for content color grading,then you want to follow the PQ curve. For a consumer display, it may or may notfollow the PQ curve. It depends on the philosophy and sophistication of thedisplay company, it would seem. Therefore, measuring the ability of the displayto follow the PQ is a good thing for reference monitors, but may not be auseful metric for consumer displays.
很多人认为pq既然是个显示设备相关的eotf，那么理所当然的就得严格遵循pq曲线，不，实际上pq曲线的应用是很灵活的
例如，如果你正使用一部彩监进行内容颜色分级（color grading），那就应该遵守pq曲线。对于消费级显示设备，它未必需要遵循pq曲线，它取决于显示设备商家的制造理念与品味，对于彩监而言，是否遵循pq是其好坏的一个指标，但对于消费级显示设备，它没啥意义。
What is the PQ Curve?（什么是pq 曲线）
The Perceptual Quantizer curve was developed and approved by SMPTE (ST-2084).As a display-referred EOTF, that means a specific code value in the content issupposed to produce a specific luminance level on the display.
Pq曲线也叫st2084，它是SMPTE
(电影电视工程师协会)制定的一个标准，它规定了输出给显示设备1个特定的数值，显示器就输出一个特定亮度
The PQ curve does not have a linear relationship between code values and lightintensity. That is because quantization for our eyes would not be efficient. Wedo not sense light linearly – humans are more sensitive to subtle light changesin dimmer light than bright light. The figure below shows the relationshipbetween code values and luminance for the PQ EOTF. Note the around 75% of thecode values are devoted to the less than 1,000 cd/m² level.
Pq曲线所描述的数值与亮度的关系并非线性的，这是因为人眼对于低亮度的敏感性要强于高亮度，如下图所示（个人理解:x表示灰阶等级，y表示实际亮度），75%的数值分布在1000 cd/m² 以下
ManyHDR capturing sources such as cameras with RAW recorders can acquire data overthe full luminance range of PQ: from 0.0001 cd/m² to 10,000 cd/m². However, nocommercially available displays can produce luminance over this range. As aresult, some processing of content data must be done to output on the display.This process is called color grading and compresses the full range of luminanceand color values to that of the mastering display. Further compression mayresult when this content is delivered to a display whose range of colors andluminance is less than the mastering monitor.
Hdr的视频采集设备通常是以raw形式存储素材的，它们能记录从0.0001 cd/m² 到10,000 cd/m²全亮度范围(即full range)，但目前没有任何市售显示设备能牛x到显示全部亮度范围，因此需要做颜色分级和亮度压缩以匹配母显示器(mastering display)，如果这些内容将来需要在比母显示设备显示能力还要差一些的设备上播放的话，色彩和亮度还会做进一步压缩。
The one exception to this rule we have seen was demonstrated recently at CES2018. Sony showed an HDR display that was capable of producing 10,000 cd/m² ina very small area. In this case, no compression of the PQ-encoded data wasneeded as the display was capable of matching what the content called for.
当然，万事都有例外，索尼曾在2018年ces上向我们展现了一部能在很小区域显示10,000 cd/m² 的hdr设备，对于这部机器而言，pq曲线无需任何压缩，它能将播放内容很好的显现给世人。
ToneMapping Options（色调映射选项）
When the capability of the display does not match the performance of themastering monitor, some tone mapping is needed.
For a mastering monitor, pixels that have luminance values above the peak valueof the display are “clipped” to the peak luminance of the display. For example,if a scene has some bright clouds with captured luminance ranges that varyabove 1000 cd/m², these all get clipped to the 1000 cd/m² level, thuslosing that subtle detail in the display. Exactly how this is done is acreative decision during color grading and can be called creative intent andthese subtle highlights are lost (although in some cases values above theclipping point remain in the content).
当显示设备的能力比母显示器要低的时候，就需要进行色调映射
对于一部母显示设备，如果像素的亮度值超过了其峰值亮度，那它们就会被修剪至峰值亮度。例如某个画面的一片云彩亮度值都超过了1000 cd/m²，那它们会全都修剪至1000 cd/m²，结果就是损失了一部分微小的细节。具体如何制作，那就是创作者所要考虑的，我们称这种操作为创作者意图，当然，这部分细节未必都丢失，只是我们看不到而已（也就是btb和wtw部分）
Using consumer displays, one can apply tone mapping and gamut mapping. Tonemapping is needed when the consumer display has a reduced luminance rangecompared to the mastering display. Tone mapping remaps luminance from thesource values to the lower values the display can support. Gamut mapping refersto the remapping of color values that exceed the capabilities of the display tothe native gamut of the display. These two processes are tied together and aresometimes referred to simply as tone mapping or color volume remapping.
对于消费级显示设备，它能做的是色调映射与色域映射，色调映射的作用是将亮度值映射到显示设备能支持的范围，而色域映射则是将超过原生色域的颜色映射到显示器能显示的范围，这两个过程一般是紧密联系的，通常我们简称为色调映射或者色积重映射。
Reference Monitors
Reference monitors are used to grade the content for distribution. These aresupposed to be highly accurate monitors but they are not capable of displayingthe full range of luminance in the PQ standard, nor the full range of colors inthe BT.2020 color gamut. Today, there are two practical reference monitors at1000 and 4000 cd/m² with each supporting the DCI-P3 color gamut, which issmaller than the BT.2020 color gamut. In the world of HDR for consumers,BT.2020 is mandatory. This does not mean that the display has to have 2020primaries, but it should follow BT.2020 colorimetry.
参考级显示设备一般是用来制作消费级影视内容的，它们拥有一流的色准但既也无法显示全部的pq标准亮度范围，也无法显示全部的bt2020色域。当前，2个典型的参考级显示器标准是1000 和4000 cd/m²，这两种标准都支持dci-p3色域，虽然消费级hdr规定要支持bt2020但并不代表显示设备得显示全2020色域，但其色彩显示必须遵循2020标准。
So what does the mastering monitor do? For pixel luminance up to thecapabilities of the display (1000 or 4000 cd/m²), these values follow the PQcurve and are displayed as accurately as possible. Ideally, red, green and bluecomponents also follow the PQ curve so that their combination, a gray level,also follows the PQ curve.
如果像素的亮度值在是峰值显示范围内，那将严格遵循pq曲线显示，理想的状况下，红，绿，蓝三原色也严格遵循pq曲线，因此其组合而成的灰阶也遵循pq曲线。
For pixel values above the peak luminance of the display, a reference monitorshould “clip” those values back to the maximum capabilities of the display.Tone mapping changes the luminance of the pixel to the maximum value of thedisplay, but not the chromaticity of the pixel. Such a change just in luminancemay change our perception of the pixel (a bright yellow pixel looks yellow buta dim yellow pixel looks brown).
当像素亮度值大于峰值亮度时，参考级显示器会做“修剪”（clip）。色调映射会改变像素亮度至峰值亮度，但保留像素色度值，其结果就是像素的色彩显现改变了，比如亮黄色看起来就变成了黄色，暗黄色看起来像棕色。
Clipping can also mean adjusting the chromaticity of the pixel. Forchromaticity points outside of the native color gamut (P3 in this case),clipping generally means extending a line from the chromaticity point back tothe D65 white point and finding where it intersects the native gamut (seefigure). Again, this may change our perception of this color (less saturated ora hue shift).
修剪过程也可能会改变像素的色度，当某个像素的色度超出原生色彩范围时（当前以p3为例），修剪过程是这样的，以该像素点在色域图中的位置为起点，划一条直线通过d65点，该直线与p3色域的边界线的交点就是修剪后的色度值，其结果就是色彩变淡了或者色度改变了。
Mastering displays operate this way because they define the original intentionof the content. Tone mapping and gamut remapping need to take place in anydisplay showing the content with compromised specs compared to the masteringdisplay.
母显示器可以为所欲为，因为它们直接可以反映创作者意图，但是那些规格比母显示设备差的消费级显示设备，色调映射与色域映射就是个必然过程了
From a metrology point of view, it is verydesirable to measure the red, green, blue light output vs. code value to seehow accurately it follows the PQ curve up to the point of maximum luminance ofthe display. Any code values above the nominal peak level of the content shouldall map to the same maximum luminance level.
从计量学观点来看，很有必要分别对红，绿，蓝三色光的输出与编码值进行比对以确定在到达设备峰值亮度前与pq曲线的吻合度。任何大于峰值亮度的编码值都应被映射成相同亮度值。（貌似现在的评测都只做灰度曲线测试）。
Consumer Displays（消费级显示设备）
Consumer displays playing back HDR content offer a wide range of luminancelevels and color gamut coverage. Some have capabilities that are similar to the1000 cd/m², P3 gamut reference monitor while others fall below this capability.Such consumer displays don’t usually clip, but employ tone mapping and gamutmapping to create as pleasing a picture as possible. 消费级显示设备在回放HDR内容时候在亮度以及色域两个特性上差异很大。有些设备能提供接近1000 cd/m²以及P3色域性能，有些设备却没有，这样的显示器并不会做修剪，但会利用色调映射与色域映射来尽可能显示出令人愉悦的画面。
Conventional wisdom has been that youfollow the PQ curve up to a certain luminance level and then start to applytone mapping and gamut mapping. But at the ICDM meeting, Florian Friedrich ofFF Pictures led the discussion to provide more clarity into this thinking. 普遍的做法是在一定亮度之前遵循pq曲线，超出的亮度采用色调映射。不过在ICDM（Industrial Conference on Data Mining）会议上，FF影业的Florian Friedrich发起的讨论则让这种想法变得更为清晰。
According to Friedrich, conventional wisdomwould say that you follow the PQ curve to perhaps 500 cd/m², then remap codevalues from 500 to 1000 cd/m² (for content mastered at 1000 cd/m²) to the500-700 cd/m² range using a smooth rollover curve for TVs with a peak luminancecapability of 700 nits. 按Florian Friedrich的说法，你可以遵循PQ曲线到500 cd/m²，然后重编码500~1000（假设视频内容是1000 cd/m²标准来制作的）的亮度值至500~700至更为平滑的曲线以便峰值亮度为700nit的显示设备可以正常显示。 
According to Friedrich, conventional wisdomwould say that you follow the PQ curve to perhaps 500 cd/m², then remap codevalues from 500 to 1000 cd/m² (for content mastered at 1000 cd/m²) to the500-700 cd/m² range using a smooth rollover curve for TVs with a peak luminancecapability of 700 nits.按Florian Friedrich的说法，你可以遵循PQ曲线到500 cd/m²，然后重编码500~1000（假设视频内容是1000 cd/m²标准来制作的）的亮度值至500~700至更为平滑的曲线以便峰值亮度为700nit的显示设备可以正常显示。
This is a form of compression. For example,if adjacent pixels varied by five code values in the content, they might becompressed to a difference of two code values for display. The difference isstill there, but is not perceivable by the eye. If the deliveredcontent was mastered at 4000 cd/m², the compression is even higher.这也是一种压缩方式，例如，如果两两相邻的5个像素有5个数值，它们可能被压缩成只有2组不同数值，虽然区别还在，但可能人眼就分别不出有什么不同了，如果视频内容是4000标准制作的，那么这种压缩会更显著。
Tone and gamut mapping is still needed tomap higher luminance values to the capabilities of the display, but this isdone in a way to preserve creative intent. For example, tone and gamut mappingmay be done to try to preserve the brightness of the pixel but may introducecolor shifts that make detection of subtle shades hard to see. Alternatively,tone and gamut mapping can work to preserve these subtle gradations bymaintaining hue subtleties in the remapping process, but may sacrifice theoverall luminance a bit.色调映射与色域映射过程依然需要以保证高亮度值能匹配显示器的性能，但它们的处理过程会尽量保留主创者的意图。例如，色调映射与色域映射会保留像素亮度值，但导致颜色偏移，从而使得一些色彩过渡变得不可见。当然，也能保留这些颜色的色度以再现色彩过渡，但这会牺牲整体亮度。
Dynamic metadata allows the HDR display tochange its tone and gamut mapping on a scene-by-scene or frame-by-frame basis.Samsung’s new HDR10+ scheme provides metadata to optimize the picture qualityof the HDR display – particularly when the display has luminance below 1000cd/m² and/or color performance less than DCI-P3.动态数据源允许HDR显示设备依据场景或者帧来改变色调映射与色域映射。三星的HDR10+方案就是利用动态数据源来提升HDR显示效果，尤其针对那些显示性能在1000以下且色域低于DCI-P3的（个人觉得Dolby vision也是用来干这事的，毕竟现在市面上亮度1000（10%窗口）色域100% p3的HDR电视基本没有）。
Below is a graphic showing an example of amodified EOTF for a particular scene. In the “linear range”, the displayfollows the PQ curve up to the knee point (some luminance level). Above that,compression is applied but it may not be a simple roll off type curve. Instead,it may take a more complex shape that is better at preserving face or skydetails, for example. In these ranges, the display may return to following the PQcurve.下图显示了某个特定场景修改EOTF的例子，在线性区域（linear range），显示设备遵循PQ曲线至拐点（knee point，或者说某个亮度值），之后开始压缩，但并非采用单一的曲线，而是采用混合曲线进行压缩，比如当画面是人脸或者天空细节时，就遵循PQ曲线。
From a metrology point of view, one can measure how well theconsumer display follows the PQ curve, but reporting a high deviation may bemisleading. For example, users will assume that a large deviation is not good,when in fact such deviations may have been done intentionally to offer a betterpicture for the consumer.从计量学的标准来看，你可以测量一台显示设备与PQ曲线的吻合度，但如果它与标准偏差很大就认为设备很差，这种想法并不可取。例如，有些时候用户认为偏差越大越不好，但实际上这种偏差是有意为之以产生更好的画面效果。
In the opinion of Friedrich, an ideal display should follow the PQcurve as accurately as possible. Displays with significant compromises in peakbrightness or dynamic range shall apply tone mapping, where a pleasing image ismore important than following the PQ curve.Friedrich认为，理想的显示设备要尽可能的严格遵循PQ曲线，对于那些压缩峰值亮度或者动态范围的应该要进行色调映射，毕竟，好的画面显现要比PQ曲线重要得多。
After the meeting, Friedrich did some experiments using a tonemapping pattern he developed. After optimizing the HDR performance offront-projection systems under different conditions he came to the conclusionthat the important logarithmic steps in the PQ signal (1000, 100, 10, 1, 0.1,0.01) should be maintained with decent contrast and differentiation. If asystem can’t maintain those factors of 10 well, then limitations in thesefactors are more pleasing than following the PQ curve accurately. “Aminimum factor of four within the available dynamic range of the display iswhat I found out should be maintained for a pleasing image with decentcontrast,” he said. In other words, the differentiation in between black,shadows, mid tones and highlights should be preserved with decent contrast anddifferentiation in between these steps. Friedrich has also developed somePlug-Ins for color grading and editing systems such as Davinci Resolve andAdobe Premiere in order to help content creators deal with soft clipping ofblacks and highlights, metrics in HDR signals and conversions in betweendifferent SDR and HDR versions. – CC会议结束后Friedrich以他自己开发的色调映射算法做了一些实验，采用的是一套优化过的投影系统，实验分几个不同场景进行。最后他得出了一个结论，在整个PQ曲线中最重要的环节是(1000，100，10，1，0.1，0.01)这几个区域，这些区域需要保留良好的对比度以及区分度（个人理解是0.01~0.1，0.1~1，1~10，10~100，100~1000）。如果一个系统不能把这些区域分为10个子区间，那么它的子区间数就比PQ曲线更能决定画面的好坏。“显示设备至少需要提供4个子区间才能提供良好的对比度以产生出好的画面。Friedrich同时也开发一些达芬奇和ADOBE PREMIERE的插件用于色彩分级和编辑，它们能更好的帮助创作者们进行软修剪黑位和高光，HDR量化或者HDR与SDR之间的转换。
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