hdrmerge is a scientific HDR merging tool: its goal is to create images that are accurate linear measurements of the radiance received by a camera capable of producing RAW output. It does not do any fancy noicy removal or other types of postprocessing and instead tries to be simple, understandable and hackable.
You will need a recent C++ compiler with support for C++11 and OpenMP. g++4.8 and msvc 12.0 has been tested so far. This program depends on: libxml2 (for Exiv2), libjpeg, OpenEXR, libexiv2 and Boost.
Run
cmake .
followed by
make
to start the compilation.
Syntax: ./hdrmerge [options] <RAW file format string / list of multiple files>
Motivation:
hdrmerge is a scientific HDR merging tool: its goal is to create images that
are accurate linear measurements of the radiance received by the camera.
It does not do any fancy noicy removal or other types of postprocessing
and instead tries to be simple, understandable and hackable.
Summary:
This program takes an exposure series of DNG/CR2/.. RAW files and merges it
into a high dynamic-range EXR image. Given a printf-style format expression
for the input file names, the program automatically figures out both the
number of images and their exposure times. Any metadata (e.g. lens data)
present in the input RAW files is also copied over into the output EXR file.
The program automatically checks for common mistakes like duplicate exposures,
leaving autofocus or auto-ISO turned on by accident, and it can do useful
operations like cropping, resampling, and removing vignetting. Used with
just a single image, it works a lot like a hypothetical 'dcraw' in floating
point mode. OpenMP is used wherever possible to accelerate image processing.
Note that this program makes the assumption that the input frames are well-
aligned so that no alignment correction is necessary.
The order of operations is as follows (all steps except 1 and 10 are
optional; brackets indicate steps that disabled by default):
1. Load RAWs -> 2. HDR Merge -> 3. Demosaic -> 4. Transform colors ->
5. [White balance] -> 6. [Scale] -> 7. [Remove vignetting] -> 8. [Crop] ->
9. [Resample] -> 10. [Flip/rotate] -> 11. Write OpenEXR
The following sections contain additional information on some of these steps.
Step 1: Load RAWs
hdrmerge uses the RawSpeed library to support a wide range of RAW formats.
For simplicity, HDR processing is currently restricted to sensors having a
standard RGB Bayer grid. From time to time, it may be necessary to update
the RawSpeed source code to support new camera models. To do this, run the
'rawspeed/update_rawspeed.sh' shell script and recompile.
Step 2: Merge
Exposures are merged based on a simple Poisson noise model. In other words,
the exposures are simply summed together and divided by the total exposure.
time. To avoid problems with over- and under-exposure, each pixel is
furthermore weighted such that only well-exposed pixels contribute to this
summation.
For this procedure, it is crucial that hdrmerge knows the correct exposure
time for each image. Many cameras today use exposure values that are really
fractional powers of two rather than common rounded values (i.e. 1/32 as
opposed to 1/30 sec). hdrmerge will try to retrieve the true exposure value
from the EXIF tag. Unfortunately, some cameras "lie" in their EXIF tags
and use yet another set of exposure times, which can seriously throw off
the HDR merging process. If your camera does this, pass the parameter
--fitexptimes to manually estimate the actual exposure times from the
input set of images.
A subtle issue that one should be aware of is that even professional-grade
lenses from the big two SLR manufactorers tend to have rather inaccurate
apertures. Take a photo sequence of a still scene at identical camera
settings, and you will notice that there is a perceptible amount of flicker
when turning it into a movie. This is because the aperture radius in each
shot may vary by a random amount that could be as large as 5%. This is not
not much of an issue if you're just doing video capture or still
photography, hence lens manufacturers don't correct for it. But it can
cause significant headaches in long capture sessions, where it introduces
random intensity scale factors from image to image. There are two
workarounds: 1. shoot wide open, or 2. use a trick used by time-lapse
photographers that is referred to as 'lens twist' or 'aperture twist'
(search for these keywords online to find videos that demonstrate
how it works).
Step 3: Demosaic
This program uses Adaptive Homogeneity-Directed demosaicing (AHD) to
interpolate colors over the image. Importantly, demosaicing is done *after*
HDR merging, on the resulting floating point-valued Bayer grid.
Step 7: Vignetting correction
To remove vignetting from your photographs, take a single well-exposed
picture of a uniformly colored object. Ideally, take a picture through
the opening of an integrating sphere, if you have one. Then run hdrmerge
on this picture using the --vcal parameter. This fits a radial polynomial
of the form 1 + ax^2 + bx^4 + cx^6 to the image and prints out the
coefficients. These can then be passed using the --vcorr parameter
Step 9: Resample
This program can do high quality Lanczos resampling to get lower resolution
output if desired. This can sometimes cause ringing on high frequency edges,
in which case a tent filter may be preferable (selectable via --rfilter).
Command line options:
--help Print information on how to use this program
--config arg Load the configuration file 'arg' as an additional
source of command line parameters. Should contain
one parameter per line in key=value format. The
command line takes precedence when an argument is
specified multiple times.
--saturation arg Saturation threshold of the sensor: the ratio of
the sensor's theoretical dynamic range, at which
saturation occurs in practice (in [0,1]).
Estimated automatically if not specified.
--fitexptimes On some cameras, the exposure times in the EXIF
tags can't be trusted. Use this parameter to
estimate them automatically for the current image
sequence
--exptimes arg Override the EXIF exposure times with a manually
specified sequence of the format
'time1,time2,time3,..'
--nodemosaic If specified, the raw Bayer grid is exported as a
grayscale EXR file
--colormode arg (=sRGB) Output color space (one of 'native'/'sRGB'/'XYZ')
--sensor2xyz arg Matrix that transforms from the sensor color space
to XYZ tristimulus values
--scale arg Optional scale factor that is applied to the image
--crop arg Crop to a rectangular area. 'arg' should be
specified in the form x,y,width,height
--resample arg Resample the image to a different resolution.
'arg' can be a pair of integers like 1188x790 or
the max. resolution (maintaining the aspect ratio)
--rfilter arg (=lanczos) Resampling filter used by the --resample option
(available choices: 'tent' or 'lanczos')
--wbalpatch arg White balance the image using a grey patch
occupying the region 'arg' (specified as
x,y,width,height). Prints output suitable for
--wbal
--wbal arg White balance the image using floating point
multipliers 'arg' specified as r,g,b
--vcal Calibrate vignetting correction given a uniformly
illuminated image
--vcorr arg Apply the vignetting correction computed using
--vcal
--flip arg Flip the output image along the specified axes
(one of 'x', 'y', or 'xy')
--rotate arg (=0) Rotate the output image by 90, 180 or 270 degrees
--format arg (=half) Choose the desired output file format -- one of
'half' (OpenEXR, 16 bit HDR / half precision),
'single' (OpenEXR, 32 bit / single precision),
'jpeg' (libjpeg, 8 bit LDR for convenience)
--output arg (=output.exr) Name of the output file in OpenEXR format. When
only a single RAW file is processed, its name is
used by default (with the ending replaced by
.exr/.jpeg
Note that all options can also be specified permanently by creating a text
file named 'hdrmerge.cfg' in the current directory. It should contain options
in key=value format.
Examples:
Create an OpenEXR file from files specified in printf format.
$ hdrmerge --output scene.exr scene_%02i.cr2
As above, but explicitly specify the files (in any order):
$ hdrmerge --output scene.exr scene_001.cr2 scene_002.cr2 scene_003.cr2
hdrmerge is licensed under the GNU General Public License (Version 3), which can be retrieved at the following address: http://www.gnu.org/licenses/gpl-3.0.txt