本帖最后由 bluejd 于 2016-4-9 21:35 编辑
http://vmi.tv/training/useful-stuff/HDR_SURVIVAL_GUIDE
Technical Guide: HDR StandardsThere are 3 main HDR standards worth mentioning with some standards like TechniColour/Philips being derivates of existing standards and nothing more than marketing and branding exercises. These 3 standards are: - Dolby Vision
- SMPTE 2084
- BBC HLG HDR
Dolby VisionDolby is a pioneer of HDR and they were the first to come to the market with a working version of HDR TV and were the first to demonstrate HDR in practice. Some of their developments have been taken by SMPTE into their standard SMPTE ST2084 and Dolby certainly hold a special place in the world of HDR. The beauty of Dolby Vision is that it is a complete end-to-end solution from mastering to delivery to the end user. Dolby has developed special mastering screens capable of displaying up to 4000 nits and they offer full technical support to projects that decide to master in Dolby Vision. Another benefit of Dolby Vision is its capability to adjust its playback to a specific screens. So for example if screen is capable of 1000 nits brightness, playback would be adjusted to that value. But the same stream would be playable on SDR screens through very clever HDR to SDR conversion. That means that a single Dolby Vision master is capable of being displayed on SDR, HDR screens of 1,000 nits, 2,000 nits or 10.000 nits. Today there are only 2 companies that have licensed Dolby Vision technology: LG and Vizio.
SMPTE ST2084 This is a standard that has been accepted by the UHD Alliance for UHD Premium and is currently leading the race as being the most widely accepted way of mastering, distributing and viewing HDR content. In contrast to Dolby Vision, this standard does not require any licensing and TV manufacturers like: Sony, Panasonic and Samsung have all adopted it. SMPTE ST2084 is based around theoretical display capable of displaying 10.000 nits with all real life displays being referenced to this value. With this dynamic range traditional gamma curve is not enough and this standard is using a new type of EOTF (electro optical transfer function) or a PQ curve that looks like the one bellow in comparison to standard rec709 gamma curve.
In order to reiterate one more time the fact that HDR is not made for displaying images brighter but adding a highlight detail to otherwise clipped images in SDR below is a histogram that shows difference between an image being shown in SDR and HDR. As it can be seen SDR and HDR image look almost identical in its main body with only added highlight detail in HDR and this is what gives the picture a 'WOW' factor.
BBC HLG HDRWhile SMPTE ST2084 was created with feature film industry in mind, BBC’s HLG (Hybrid Log Gamma) HDR was developed with a range of TV programming in mind including sports, live entertainment or even cooking shows. In contrast to SMPTE ST2084, HLG does not use reference display but instead has a different curve for different brightness and maximum screen brightness of 5000 nits. What is also interesting about BBC’s approach is that it is taking into account surrounding brightness which affect the curve. Below is an example of how curve is changing depending on surrounding brightness change. ConclusionIt is refreshing to find that HDR is one of those technologies and innovations that won’t need any heavy marketing for it to be accepted by the end user. Once you glance at a HDR TV screen you won’t need any convincing that images look much better, fresh and exciting. If 3D was interesting for its novelty and 4K advantage was not visible to all, HDR is not going to fall into any of these two categories. The question is only, how long will it take before ‘Premium’ becomes the norm.
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发表于 2016-4-9 17:44
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