Editing Multi-exposure HDR capture (section)

== Process ==
High-dynamic-range photographs are generally composites of multiple standard dynamic range images, often captured using [[Bracketing#Exposure bracketing|exposure bracketing]]. Afterwards, [[photo manipulation]] software  [[Exposure fusion|merges the input files]] into a single HDR image, which is then also [[tone mapping|tone mapped]] in accordance with the limitations of the planned output or display.

=== Capturing multiple images (exposure bracketing) ===
[[File:Einfluss der Zeit auf die Belichtung.jpg|thumb|right|Exposure bracketing by varying the [[shutter speed]] from {{frac|500}} to 30 seconds]]
{{Main|Bracketing#Exposure bracketing}}
Any camera that allows manual exposure control can perform multi-exposure HDR image capture, although one equipped with [[Autobracketing|automatic exposure bracketing (AEB)]] facilitates the process. Some cameras have an AEB feature that spans a far greater dynamic range than others, from ±0.6 in simpler cameras to ±18&nbsp;EV in top professional cameras, {{as of|lc=y|2020|post=.}}<ref>{{cite web |title=Auto Exposure Bracketing Settings by Camera Model |url= http://hdr-photography.com/aeb.html |work=HDR Photography Resources |date=February 28, 2016<!--Last-updated date, bottom of page.--> |access-date=June 12, 2020}}</ref>

The exposure value (EV) refers to the amount of light applied to the light-sensitive detector, whether film or digital sensor such as a [[Charge-coupled device|CCD]]. An increase or decrease of one stop is defined as a doubling or halving of the amount of light captured. Revealing detail in the darkest of shadows requires an increased EV, while preserving detail in very bright situations requires very low EVs.

EV is controlled using one of two photographic controls: varying either the size of the [[aperture]] or the exposure time. A set of images with multiple EVs intended for HDR processing should be captured only by altering the exposure time; altering the aperture size also would affect the [[depth of field]] and so the resultant multiple images would be quite different, preventing their final combination into a single HDR image.

Multi-exposure HDR photography generally is limited to still scenes because any movement between successive images will impede or prevent success in combining them afterward. Also, because the photographer must capture three or more images to obtain the desired [[luminance]] range, taking such a full set of images takes extra time. Photographers have developed calculation methods and techniques to partially overcome these problems, but the use of a sturdy tripod is advised to minimize framing differences between exposures.

=== Merging the images into an HDR image ===
[[File:Dynamic Range Increase.jpg|thumb|Highlight areas from the window (upper right) are extracted from an underexposed image (upper left) and composited with a scene-averaged exposure (bottom left) to produce a HDR image (bottom right).]]
{{See also|Exposure fusion}}
Tonal information and details from shadow areas can be recovered from images that are deliberately overexposed (i.e., with positive EV compared to the correct scene exposure), while similar tonal information from highlight areas can be recovered from images that are deliberately underexposed (negative EV). The process of selecting and extracting shadow and highlight information from these over/underexposed images and then combining them with image(s) that are exposed correctly for the overall scene is known as [[exposure fusion]]. Exposure fusion can be performed manually, relying on the HDR operator's judgment, experience, and training, but usually, fusion is performed automatically by software.

=== Storing ===
{{See also|High dynamic range#Storage}}
Information stored in high-dynamic-range images typically corresponds to the physical values of [[luminance]] or [[radiance]] that can be observed in the real world. This is different from traditional [[digital images]], which represent colors as they should appear on a monitor or a paper print. Therefore, HDR image formats are often called ''scene-referred'', in contrast to traditional digital images, which are ''device-referred'' or ''output-referred''. Furthermore, traditional images are usually encoded for the human [[visual system]] (maximizing the visual information stored in the fixed number of bits), which is usually called ''gamma encoding'' or ''[[gamma correction]]''. The values stored for HDR images are often gamma compressed using mathematical functions such as [[power law]]s [[logarithm]]s, or [[floating point]] linear values, since [[Fixed-point arithmetic|fixed-point]] linear encodings are increasingly inefficient over higher dynamic ranges.<ref name="gregward">{{cite web|last=Ward|first=Greg|title=High Dynamic Range Image Encodings|url=http://www.anyhere.com/gward/hdrenc/hdr_encodings.html|work=Anyhere.com|publisher=Anyhere Software}}</ref><ref>{{cite web|title=The Radiance Picture File Format|url=http://radsite.lbl.gov/radiance/refer/Notes/picture_format.html|url-status=dead|archive-url=https://web.archive.org/web/20190128023610/http://radsite.lbl.gov/radiance/refer/Notes/picture_format.html|archive-date=January 28, 2019|access-date=June 12, 2020|work=RadSite.LBL.gov|publisher=[[Lawrence Berkeley National Laboratory]]}}</ref><ref>{{cite book|last=Fernando|first=Randima|url=http://http.developer.nvidia.com/GPUGems/gpugems_ch26.html|title=GPU Gems|date=2004|publisher=Addison-Wesley|isbn=0-321-22832-4|location=Boston|chapter=26.5 Linear Pixel Values|archive-url=https://web.archive.org/web/20100412001848/http://http.developer.nvidia.com/GPUGems/gpugems_ch26.html|archive-date=April 12, 2010|url-status=dead|via=Developer.Nvidia.com}}</ref>

HDR images often do not use fixed ranges per color [[Channel (digital image)|channel]], other than traditional images, to represent many more colors over a much wider dynamic range (multiple channels). For that purpose, they do not use integer values to represent the single color channels (e.g., 0–255 in an 8 bit per pixel interval for red, green and blue) but instead use a floating point representation. Common values are 16-bit ([[half precision]]) or 32-bit [[Floating point|floating-point]] numbers to represent HDR pixels. However, when the appropriate [[transfer function]] is used, HDR pixels for some applications can be represented with a [[color depth]] that has as few as 10 to 12&nbsp;bits ({{#expr:2^10}} to {{#expr:2^12}} values) for luminance and 8&nbsp;bits ({{#expr:2^8}} values) for [[chrominance]] without introducing any visible quantization [[Visual artifact|artifact]]s.<ref name="gregward" /><ref>{{cite web|last1=Mantiuk|first1=Rafal|last2=Krawczyk|first2=Grzegorz|last3=Myszkowski|first3=Karol|last4=Seidel|first4=Hans-Peter|title=Perception-motivated High Dynamic Range Video Encoding|url=http://resources.mpi-inf.mpg.de/hdrvideo/|work=Resources.MPI-Inf.MPG.de|publisher=[[Max Planck Institute for Informatics]]}}</ref>

=== Tone mapping ===
{{Main article|Tone mapping}}

Tone mapping reduces the dynamic range, or contrast ratio, of an entire image while retaining localized contrast. Although it is a distinct operation, tone mapping is often applied to HDR files by the same software package.

Tone mapping is often needed because the dynamic range that can be displayed is often lower than the dynamic range of the captured or processed image.<ref name=":2">{{cite book|last=Darmont|first=Arnaud|url=http://spie.org/x648.html?product_id=903927|title=High Dynamic Range Imaging: Sensors and Architectures|date=2012|publisher=SPIE press|isbn=978-0-81948-830-5|edition=First}}</ref> [[High-dynamic-range video|HDR displays]] can display image at a higher dynamic range than [[Standard-dynamic-range video|SDR displays]], reducing the need for tone mapping.

=== Types of HDR ===
HDR can be done via several methods:

* DOL: Digital overlap<ref name=":0" />
* BME: Binned multiplexed exposure<ref name=":0" />
* SME: Spatially multiplexed exposure<ref name=":0" />
* QBC: Quad Bayer Coding<ref>{{Cite web |author=ccs_hello |date=25 October 2017 |title=SONY Exmor HDR sensor's terms: DOL, BME, SME, QBC – Electronically Assisted Astronomy (EAA) – Cloudy Nights |url=https://www.cloudynights.com/topic/596377-sony-exmor-hdr-sensors-terms-dol-bme-sme-qbc/ |access-date=6 April 2022 |work=Cloudy Nights}}</ref>{{Unreliable source?|date=April 2022}}

=== Examples ===
This is an example of four standard dynamic range images that are combined to produce three resulting [[Tone mapping|tone mapped]] images:

<gallery heights="120" widths="160" mode="nolines" perrow="6" caption="Exposed images:">
Image:StLouisArchMultExpEV-4.72.JPG|–4 stops
Image:StLouisArchMultExpEV-1.82.JPG|–2 stops
Image:StLouisArchMultExpEV+1.51.JPG|+2 stops
Image:StLouisArchMultExpEV+4.09.JPG|+4 stops
</gallery>
<gallery heights="120" widths="160" mode="nolines" perrow="6" caption="Results after processing:">
File:StLouisArchMultExpCDR.jpg|Simple contrast reduction
File:StLouisArchMultExpToneMapped.jpg|Local tone mapping
File:StLouisArchMultExpEV SNS-HDR.jpg|alt=Natural tone mapping|Natural tone mapping
</gallery>

This is an example of a scene with a very wide dynamic range:

<gallery heights="120" widths="160" mode="nolines" perrow="6" caption="Exposed images:">
Image:HDRI Sample Scene Window - 01.jpg|–6 stops
Image:HDRI Sample Scene Window - 02.jpg|–5 stops
Image:HDRI Sample Scene Window - 03.jpg|–4 stops
Image:HDRI Sample Scene Window - 04.jpg|–3 stops
Image:HDRI Sample Scene Window - 05.jpg|–2 stops
Image:HDRI Sample Scene Window - 06.jpg|–1 stops
Image:HDRI Sample Scene Window - 07.jpg|{{0}}0 stops
Image:HDRI Sample Scene Window - 08.jpg|+1 stops
Image:HDRI Sample Scene Window - 09.jpg|+2 stops
Image:HDRI Sample Scene Window - 10.jpg|+3 stops
Image:HDRI Sample Scene Window - 11.jpg|+4 stops
Image:HDRI Sample Scene Window - 12.jpg|+5 stops
</gallery>
<gallery heights="120" widths="160" mode="nolines" perrow="6" caption="Results after processing:">
Image:HDRI Sample Scene Window.jpg|Natural tone mapping
</gallery>