High dynamic range
High dynamic range (HDR) is a dynamic range higher than usual. The term is often used in discussing display devices, photography, 3D rendering, and sound recording including digital imaging and digital audio production. The term may apply to an analog or digitized signal, or to the means of recording, processing, and reproducing such signals.
High-dynamic-range imaging (HDRI) is the acquisition, creation, storage, distribution or display of images and videos that have a higher dynamic range than traditional images and videos. This can be done with the HDR photography technique, with the use of cameras that natively have a high dynamic range or with computers (for example with the use of HDR rendering). The resulting image can be saved in an traditional image and video format or in a high dynamic range format. It can also be used for traditional SDR displays or for HDR displays.
Capture and creation
In photography and videography, high dynamic range (HDR) is a set of techniques used to increase the dynamic range of captured photos and videos. It typically consists of capturing multiple frames of the same scene but with different exposures and then combining them into one, resulting into a dynamic range higher than those of individually captured frames.
Modern CMOS image sensors can often capture a high dynamic range from a single exposure. The wide dynamic range of the captured image is non-linearly compressed into a smaller dynamic range electronic representation. However, with proper processing, the information from a single exposure can be used to create an HDR image.
Such HDR imaging is used in extreme dynamic range applications like welding or automotive work. In security cameras the term used instead of HDR is "wide dynamic range". Because of the nonlinearity of some sensors image artifacts can be common. Some other cameras designed for use in security applications can automatically provide two or more images for each frame, with changing exposure. For example, a sensor for 30fps video will give out 60fps with the odd frames at a short exposure time and the even frames at a longer exposure time. Some of the sensors on modern phones and cameras may even combine the two images on-chip so that a wider dynamic range without in-pixel compression is directly available to the user for display or processing.
High-dynamic-range rendering (HDRR) is the real-time rendering and display of virtual environments using a dynamic range of 65,535:1 or higher (used in computer, gaming, and entertainment technology).
High-dynamic-range formats for image and video files are able to store more dynamic range than traditional 8-bit gamma formats. These formats include:
- Storage formats such as raw image formats, formats that use a linear transfer function with high bit-depth or a logarithmic transfer function.
- HDR formats for HDR displays such as HDR10, HDR10+, Dolby Vision and hybrid log–gamma (HLG).
Distribution and display
On January 4, 2016, the Ultra HD Alliance announced their certification requirements for a HDR display. The HDR display must have either a peak brightness of over 1000 cd/m2 and a black level less than 0.05 cd/m2 (a contrast ratio of at least 20,000:1) or a peak brightness of over 540 cd/m2 and a black level less than 0.0005 cd/m2 (a contrast ratio of at least 1,080,000:1). The two options allow for different types of HDR displays such as LCD and OLED.
Some options to use HDR transfer functions that better match the human visual system other than a conventional gamma curve include the HLG and perceptual quantizer (PQ). HLG and PQ require a bit depth of 10-bits per sample.
XDR (audio) is used to provide higher-quality audio when using microphone sound systems or recording onto cassette tapes.
Dynamic range compression is a set of techniques used in audio recording and communication to put high-dynamic-range material through channels or media of lower dynamic range. Optionally, dynamic range expansion is used to restore the original high dynamic range on playback.
In radio, high dynamic range is important especially when there are potentially interfering signals. Measures such as spurious-free dynamic range are used to quantify the dynamic range of various system components such as frequency synthesizers. HDR concepts are important in both conventional and software-defined radio design.
Realtime HDR vision
In the 1970s and 1980s, Steve Mann invented the Generation-1 and Generation-2 "Digital Eye Glass", as a vision aid to help people see better, with some versions being built into welding helmets for HDR vision     See also, IEEE Technology and Society Magazine 31(3) and the supplemental material entitled "GlassEyes". 
- Rec. 2100 – ITU-R Recommendation for HDR
- Ultra HD Forum – Organization that has created standards for HDR
- Robertson, Mark A.; Borman, Sean; Stevenson, Robert L. (April 2003). "Estimation-theoretic approach to dynamic range enhancement using multiple exposures". Journal of Electronic Imaging. 12 (2): 220, right column, line 26219–228. doi:10.1117/1.1557695.
The first report of digitally combining multiple pictures of the same scene to improve dynamic range appears to be Mann
- Frédéric Dufaux, Patrick Le Callet, Rafal Mantiuk, Marta Mrak (2016). High Dynamic Range Video – From Acquisition to Display and Applications. doi:10.1016/C2014-0-03232-7. ISBN 978-0-08-100412-8.CS1 maint: multiple names: authors list (link)
- "Compositing Multiple Pictures of the Same Scene", by Steve Mann, in IS&T's 46th Annual Conference, Cambridge, Massachusetts, May 9–14, 1993
- Reinhard, Erik; Ward, Greg; Pattanaik, Sumanta; Debevec, Paul (2005). High dynamic range imaging: acquisition, display, and image-based lighting. Amsterdam: Elsevier/Morgan Kaufmann. p. 7. ISBN 978-0-12-585263-0.
Images that store a depiction of the scene in a range of intensities commensurate with the scene are what we call HDR, or "radiance maps". On the other hand, we call images suitable for display with current display technology LDR.
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- "A magical welding helmet that lets you see the world in HDR–in real-time". Archived from the original on 2016-03-28. Retrieved 2018-03-24.
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- "High Dynamic Range (HDR) on Intel Graphics" (PDF). Intel Corporation. November 2017.