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TECHNICAL

HIGH DYNAMIC RANGE

DIGITAL PHOTOGRAPHY
Clipped highlights and shadows are a common problem for digital camera users. Recent
research in computer graphics provides a solution, in which a high dynamic range image is
constructed from a sequence of different exposures. Guy J Brown FRPS explains

An average exposure of
the interior scene (left)
shows clipping in both
the highlights and
shadows. As a result,
the window areas are
grossly overexposed and

A
there is a loss of shadow t the heart of every digital camera is a sensor that
detail, as indicated by converts incident light into a digital image. A
the histogram (lower number of factors affect the fidelity with which
left). Above: a high the original scene is recorded in digital form. Of these,
dynamic range image the resolution of the sensor has received the most
constructed from a attention from camera manufacturers, with digital
sequence of bracketed cameras boasting ever-higher megapixel counts.
exposures retains detail However, relatively little attention has been paid to the
throughout the tonal range of values that each pixel may represent. Jpeg
range. The images recorded by most digital cameras represent the
corresponding colour at each pixel with a single byte (8 bits) for each of
histogram (right) is not the red, green and blue colour channels. Although this
clipped. allows more than 1.6 million different colours to be
represented, each colour channel of a pixel can only take

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STEFAN LORANT

one of 256 possible values. This means that scenes Above: Construction of underlying principle is that each exposure in the
containing very bright and very dark areas cannot be a HDR image from five sequence will contain some pixels that are properly
effectively recorded, leading to the familiar problem of bracketed exposures. exposed, and others that are under- or over-exposed.
clipped highlights and/or shadows. A common example Simply scaling the values By combining the correctly exposed pixels from each
of such a scene is a window-lit interior, as shown on the of the HDR image for frame and excluding the clipped pixels, a HDR image
previous page, below. Note that the corresponding output to a screen or can be constructed.
histogram is clipped at both ends; some shadow detail is printer is not sufficient; If the response of the camera to light were perfectly
recorded as pure black, and some highlight as pure white. the resulting image is linear, then each pixel could be divided by its exposure
The situation can be improved to some extent by using too dark and lacks local time and averaged (ignoring clipped pixels) to give a HDR
the camera’s Raw image format, which typically uses 10 contrast. Tone mapping image. In practice, however, image sensors do not measure
or 12 bits per colour channel. However, there are still gives a much better light in a perfectly linear manner, and camera
many circumstances in which the limited dynamic range rendition of the scene. manufacturers apply processing to boost contrast, suppress
of a digital camera (typically no more than 8.5 stops) is noise and give a more ‘film-like’ response. Since the
exceeded by high contrast scenes. response function of the camera is unknown (and indeed is
Recent research in computer graphics offers a considered proprietary information by the camera
solution to this problem, in the form of high dynamic manufacturer) it must be deduced by the HDR software.
range (HDR) imaging. In comparison, the images This may require a separate calibration stage, or an
recorded by conventional digital cameras are regarded approximate response curve can be used.
as low dynamic range (LDR). A HDR image of the HDR software may also compensate for other factors that
window-lit interior is shown at the top of the previous affect the correspondence between frames of the exposure
page; note that detail recorded in the highlight and sequence, such as lens flare, ghosting caused by moving
shadow areas is much improved. objects and misalignment due to movement of the camera.
Digital cameras that are able to directly capture the Using this approach, HDR image capture therefore
full dynamic range of a high-contrast scene are involves no more than taking a sequence of bracketed
currently in development, though only a small number exposures with a conventional digital camera.
of commercial solutions are currently available, and However, for good results, the following practical
they are either very expensive or tailored to specific considerations should be taken into account: since the
applications, such as surveillance video capture. technique requires multiple exposures of the same
An alternative to direct HDR capture is to record the scene, it is most suited to scenes that do not contain
scene using a sequence of bracketed LDR exposures, significant motion. The best result will be obtained when
which are then combined using software. The the camera is supported by a tripod. Hand-holding the

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TECHNICAL
camera is possible – but not recommended – since most From a photographer’s point of view, tone mapping
HDR software provides the facility to place misaligned algorithms may be regarded as analogous to methods of
images in register. If you have no option but to handhold the contrast control used during wet printing. Global tone
camera, use automatic bracketing and continuous drive. mapping algorithms are akin to selecting an appropriate
Only vary the exposure time when bracketing. Use manual grade of printing paper. Similarly, local tone mapping
or aperture priority mode, and keep the ISO constant. White may be regarded as a means of automatically performing
balance should also not be changed during the sequence of dodging and burning of local areas.
exposures. As usual when using a digital camera, optimum When evaluating HDR software, it is the
quality will be obtained by storing images in Raw, and then characteristics of the tone mapping algorithm that will
converting them to a lossless file format such as Tiff. Also, be of most concern to the digital photographer. This
some HDR software (such as PhotoMatix) is able to point is illustrated in the following two sections,
construct a HDR image directly from Raw files. which compare the HDR tools provided by Photoshop
It is advisable to shoot an odd number of bracketed CS2 with a specialist application called PhotoMatix.
exposures, such that the middle frame has an average The following descriptions refer to versions of the
exposure for the scene. To meter for the middle exposure, software running under Mac OS X (versions running
either use the camera’s matrix metering, or use an average under Microsoft Windows are also available).
of spot meter readings taken from the highlights and Support for HDR images was introduced in Photoshop
shadows. Starting from the middle exposure, overexpose CS2. Select File > Automate > Merge to HDR and
and underexpose in steps of two stops, until the camera’s choose the images to combine. An option is provided for
histogram display indicates that the shadows and aligning the images, if required. No separate calibration
highlights are not clipped. In practice most scenes can be stage is needed, since Photoshop computes the camera
recorded with three or five exposures. response from the source images.
The flow diagram on the previous page illustrates the Controls are then displayed for setting the bit depth
process of constructing a HDR image from five exposures. and white point of the resulting image. If the 32-bits-
In the longest exposure (+4 stops), highlights are severely per-channel option is chosen, then Photoshop keeps
clipped but the shadows are well recorded. Similarly, the the result as a HDR image, and the selected white
shortest exposure (-4 stops) records full highlight detail, point only affects the image preview. A limited
but clips the shadows. By combining the unclipped pixels number of functions are then available for further
over the whole sequence of images, the full dynamic range manipulation of the image (for example, it can be
of the scene can be accurately recorded. sharpened but may contain only one layer). It is likely
Once a HDR image is produced, it must be encoded that full support for 32-bit images will be provided in
and stored using an appropriate file format. It should later versions of Photoshop.
be noted that there are fundamental differences If the 8- or 16-bits-per-channel option is selected,
between HDR and LDR image encodings. Each pixel then the image values are clipped according to the
in a LDR image is usually represented by a 24-bit selected white point, and tone mapping controls are
number that describes its position in a colour space, displayed. There are four possible options here:
such as Adobe RGB. In contrast, the pixel values in a Exposure and gamma – provides controls for
HDR image are directly related to the radiance in the adjustment of the overall image brightness and
scene that was photographed. contrast; Highlight compression – compresses the
A number of file formats have been developed that highlight values of the HDR image into a suitable
allow HDR images to be encoded in an efficient range; Equalise histogram – reduces the dynamic
manner. The most common of these is Radiance range while attempting to preserve contrast; and Local
RGBE format, which has a .hdr extension. RGBE adaptation – a local tone mapping algorithm which
encoding requires 32 bits per pixel, but allows a far also provides for overall adjustment by a tone curve.
greater dynamic range to be represented than Controls are provided for setting the size of the
conventional 24-bit RGB encodings. neighbourhood that is taken into account when
A far wider range of illumination is captured by a HDR adjusting the value of each pixel.
image than can be reproduced by a display device, or in The first three of these tone mapping algorithms
print. Therefore, the problem arises of how to display a operate globally, and can be computed rapidly. The
HDR image in a manner that is visually acceptable. equalise histogram algorithm works quite well, but the
Simply scaling the contrast range to fit the output device others rarely give acceptable results. The local
is not sufficient – the resulting image is too dark and adaptation algorithm requires much more computation
bears little resemblance to the original scene. Display of time, but usually gives the most pleasing image.
HDR images therefore involves a tone mapping In PhotoMatix meanwhile, HDR images are generated
problem: how to preserve detail in the HDR image while by selecting HDR > Generate and choosing the images to
reducing its dynamic range to fit the output device. Since combine. Options are provided to use a standard response
the human visual system solves a similar problem, much curve or calculate the camera’s response curve from the
inspiration for tone mapping algorithms has come from images (the standard curve usually works well). The
the study of visual perception. images can also be aligned if required. This can either be
Broadly, tone mapping algorithms can be classified done automatically, or by a semi-manual method, in
as global or local. Global algorithms compress the which the user specifies control points that mark the
dynamic range of a HDR image by operating on each same object in different frames.
pixel independently, whereas local algorithms adjust The HDR image is then computed and displayed
the value of each pixel depending on the value of its with a viewer that allows small sections of the image to
neighbouring pixels. The latter more closely model be magnified and viewed with an automatic exposure
the behaviour of the human visual system, and tend to adjustment. A histogram can also be displayed.
give the more visually pleasing result. The HDR image is then tone mapped by selecting

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HDR > Tone Mapping. PhotoMatix provides two tone

mapping algorithms, a global algorithm called the
‘tone compressor’ and a local algorithm called the
‘details enhancer’. The tone compressor provides
controls to determine the overall brightness and
contrast of the image, and to set the white and black
points. It runs quickly and produces reasonable
results. However, most will prefer the look of the
details enhancer algorithm. This requires more
computation time since it works locally, but often
produces beautiful results. The details enhancer also
provides more control, allowing the degree of local
contrast enhancement to be determined. As before,
there are controls for setting the white and black points
and overall image brightness, and the colour
saturation can also be adjusted.
PhotoMatix also provides other functions to combine
shadow and highlight detail from multiple exposures
without actually computing a HDR image. The simplest of
these is Combine > Average which averages the
exposures. Three other ‘highlights and shadows’ options
are provided under the Combine menu, all of which tend
to produce more natural results than tone-mapping, and
require little or no user intervention. On the downside, they
are not as effective at preserving local contrast and may
produce rather flat looking images.
PhotoMatix has a comprehensive batch processing
facility, that allows the algorithms described above to
be applied to a directory containing a large number of
images (eg by combining every three images into a
HDR image and then tone mapping the result and
saving it). The batch processing facility allows HDR
images to be computed directly from the Raw files of
most camera manufacturers.
The image quality obtained from HDR software is
mainly determined by the tone mapping algorithm, and
PhotoMatix scores highly over Photoshop CS2 in this
regard. The PhotoMatix details enhancer does a better
job of rendering shadow detail than the comparable local
adaptation algorithm in Photoshop, and produces images
with a distinctive ethereal appearance.
PhotoMatix is polished software, backed by a helpful
technical support team, and is highly recommended. A
Result of tone mapping further point is that HDRSoft, which markets
the same HDR image PhotoMatrix, also markets the tone-mapping part of
using ‘details enhancer’ PhotoMatix in the form of a Photoshop plug-in. This
in PhotoMatix (above) allows its tone-mapping algorithm to be incorporated
and ‘local adaption’ in into a Photoshop workflow.
Photoshop CS2 (left). In the near future, it is likely that HDR imaging
will become the norm rather than the exception.
Adobe (Photoshop Some camera manufacturers (notably Fujifilm)
CS2) www.adobe.com have already introduced sensors with extended
dynamic range. However, until HDR cameras
HDRsoft (PhotoMatix) become commonplace, the software solutions
www.hdrSoft.com reviewed here offer an effective means of handing
high contrast scenes. In addition, tone-mapped
HDR images have an unusual tonality that can be
exploited for pictorial effect.
Guy J Brown FRPS
guy@guyjbrown.com

The following text is recommended for those seeking

further reading on the subject: High dynamic range
imaging: Acquisition, display and image-based lighting
by Erik Reinhard, Greg Ward, Sumata Pattanaik and
Paul Debevec. Morgan Kaufmann Publishers (ISBN 0-
12-585263-0).

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