Provided by: dcmtk_3.6.6-5_amd64 
NAME
dcmicmp - Compare DICOM images and compute difference metrics
SYNOPSIS
dcmicmp [options] dcmfile-in-1 dcmfile-in-2
DESCRIPTION
The dcmicmp utility reads two DICOM images, an original 'reference image' and a post-
processed 'test image', to which some kind of processing such as a lossy image
compression, followed by decompression, has been applied. This tool requires that both
images have the same resolution, the same number of frames and are either both color or
monochrome. Compressed images are not supported.
The dcmicmp utility then compares both images and computes and prints metrics that
describe how similar or different both images are:
• the maximum absolute error is the largest difference between an pixel value in the
reference image and the corresponding pixel value in the test image.
• the mean absolute error (MAE) is the average difference between original pixel value and
test image pixel value
• the root mean square error (RMSE) is computed by adding the squares of all difference
values, then dividing by the number of values added, and then taking the square root.
• The peak signal to noise ratio (PSNR) considers the reference image as a signal and the
differences between reference and test image as noise. PSNR is the maximum signal
strength (i.e. maximum pixel value in the reference image) divided by the RMSE,
expressed on a logarithmic scale in dB.
• The signal to noise ratio (PSNR) also considers the reference image as a signal and the
differences between reference and test image as noise. SNR is the average signal
strength divided by the RMSE, expressed on a logarithmic scale in dB.
All metrics are computed as defined in R.C. Gonzalez and R.E. Woods, 'Digital Image
Processing,' Prentice Hall 2008.
PARAMETERS
dcmfile-in-1 Reference DICOM image file for comparison
dcmfile-in-2 Test DICOM image file for comparison
OPTIONS
general options
-h --help
print this help text and exit
--version
print version information and exit
--arguments
print expanded command line arguments
-q --quiet
quiet mode, print no warnings and errors
-v --verbose
verbose mode, print processing details
-d --debug
debug mode, print debug information
-ll --log-level [l]evel: string constant
(fatal, error, warn, info, debug, trace)
use level l for the logger
-lc --log-config [f]ilename: string
use config file f for the logger
input options
input file format:
+f --read-file
read file format or data set (default)
+fo --read-file-only
read file format only
-f --read-dataset
read data set without file meta information
input transfer syntax:
-t= --read-xfer-auto
use TS recognition (default)
-td --read-xfer-detect
ignore TS specified in the file meta header
-te --read-xfer-little
read with explicit VR little endian TS
-tb --read-xfer-big
read with explicit VR big endian TS
-ti --read-xfer-implicit
read with implicit VR little endian TS
image processing options
modality LUT transformation:
+M --use-modality
use modality LUT transformation (default)
-M --no-modality
ignore stored modality LUT transformation
VOI LUT transformation:
-W --no-windowing
no VOI windowing (default)
+Wi --use-window [n]umber: integer
use the n-th VOI window from image file
+Wl --use-voi-lut [n]umber: integer
use the n-th VOI look up table from image file
+Wm --min-max-window
compute VOI window using min-max algorithm
on both images separately
+Wn --min-max-window-n
compute VOI window using min-max algorithm
on both images separately, ignoring extremes
+Wr --min-max-ref
compute VOI window using min-max algorithm
and use same window for the test image
+Wq --min-max-n-ref
compute VOI window using min-max algorithm,
ignoring extreme values
and use same window for the test image
+Ww --set-window [c]enter [w]idth: float
compute VOI window using center c and width w
+Wfl --linear-function
set VOI LUT function to LINEAR
+Wfs --sigmoid-function
set VOI LUT function to SIGMOID
presentation LUT transformation:
+Pid --identity-shape
set presentation LUT shape to IDENTITY
+Piv --inverse-shape
set presentation LUT shape to INVERSE
+Pod --lin-od-shape
set presentation LUT shape to LIN OD
image comparison metrics options
+ce --check-error [l]imit: integer
check if max absolute error <= limit
# Return exit code EXITCODE_LIMIT_EXCEEDED_MAX_ERROR if the computed
# maximum absolute error is larger than the given limit.
+cm --check-mae [l]imit: float
check if mean absolute error <= limit
# Return exit code EXITCODE_LIMIT_EXCEEDED_MAE if the computed
# mean absolute error is larger than the given limit.
+cr --check-rmse [l]imit: float
check if root mean square error <= limit
# Return exit code EXITCODE_LIMIT_EXCEEDED_RMSE if the computed
# root mean square error is larger than the given limit.
+cp --check-psnr [l]imit: float
check if PSNR >= limit
# Return exit code EXITCODE_LIMIT_EXCEEDED_PSNR if the computed
# peak signal to noise ratio is smaller than the given limit
# (for PSNR, higher values mean better image quality)
+cs --check-snr [l]imit: float
check if SNR >= limit
# Return exit code EXITCODE_LIMIT_EXCEEDED_PSNR if the computed
# signal to noise ratio is smaller than the given limit
# (for SNR, higher values mean better image quality)
output options
+sd --save-diff [f]ilename: string
write secondary capture difference image
# Create a Multiframe Secondary Capture image that contains a
# difference image between reference and test image. For monochrome
# images, one difference frame is created for each frame in the reference
# image. For color images, three monochrome frames are created for each
# frame in the reference image, corresponding to the differences in the
# red, green and blue color plane. The difference image will have
# BitsStored 8 or 16, depending on the properties of the reference image.
+a --amplify [f]actor: float
multiply diff image pixel values by f
# This option can be used to amplify the grayscale values in the
# difference image by multiplying each value with the given factor.
# Alternatively, a DICOM VOI LUT window may be used when visualizing
# the difference image.
NOTES
grayscale display pipeline
Monochrome DICOM images require that a multi-stage display pipeline is executed in order
to convert the raw pixel values to the so-called presentation values (p-values) that are
sent to the (possibly calibrated) display. When comparing the similarity of images before
and after post-processing, it can be relevant to activate some stages of this display
pipeline before calculating the difference image and metrics. The image processing options
allow the caller to either activate or deactivate the Modality LUT, VOI LUT and
Presentation LUT transformations. In any case, the same transformation is applied to both
images, although possibly with different parameters if for example the 'first VOI LUT
window' stored in each image is applied. This assumes that the post-processing algorithm
(e.g. compression algorithm) has adapted the values of such windows during compression
such that the image display after applying the window is as close as possible to the
reference. For images with more than 8 bits/sample it may be important to known which VOI
LUT transformation will be applied by the user when viewing the image, because this may
affect the perceived image quality. Therefore, absolute Window parameters can also be
given with the --set-window option, which will then be applied to both images.
suitability of images for diagnostic purposes
The user should also note that the metrics computed by this tool cannot predict or
estimate the suitability of lossy compressed image for diagnostic purposes. Much more
complex image processing and an understanding of the image content (e.g. body part) would
be needed for this purpose. The metrics computed provide an estimation of the level of
distortion caused by the post-processing - no more and no less.
TRANSFER SYNTAXES
dcmicmp supports the following transfer syntaxes for input:
LittleEndianImplicitTransferSyntax 1.2.840.10008.1.2
LittleEndianExplicitTransferSyntax 1.2.840.10008.1.2.1
DeflatedExplicitVRLittleEndianTransferSyntax 1.2.840.10008.1.2.1.99 (*)
BigEndianExplicitTransferSyntax 1.2.840.10008.1.2.2
The difference image file is always written in Little Endian Implicit Transfer Syntax.
(*) if compiled with zlib support enabled
LOGGING
The level of logging output of the various command line tools and underlying libraries can
be specified by the user. By default, only errors and warnings are written to the standard
error stream. Using option --verbose also informational messages like processing details
are reported. Option --debug can be used to get more details on the internal activity,
e.g. for debugging purposes. Other logging levels can be selected using option --log-
level. In --quiet mode only fatal errors are reported. In such very severe error events,
the application will usually terminate. For more details on the different logging levels,
see documentation of module 'oflog'.
In case the logging output should be written to file (optionally with logfile rotation),
to syslog (Unix) or the event log (Windows) option --log-config can be used. This
configuration file also allows for directing only certain messages to a particular output
stream and for filtering certain messages based on the module or application where they
are generated. An example configuration file is provided in <etcdir>/logger.cfg.
COMMAND LINE
All command line tools use the following notation for parameters: square brackets enclose
optional values (0-1), three trailing dots indicate that multiple values are allowed
(1-n), a combination of both means 0 to n values.
Command line options are distinguished from parameters by a leading '+' or '-' sign,
respectively. Usually, order and position of command line options are arbitrary (i.e. they
can appear anywhere). However, if options are mutually exclusive the rightmost appearance
is used. This behavior conforms to the standard evaluation rules of common Unix shells.
In addition, one or more command files can be specified using an '@' sign as a prefix to
the filename (e.g. @command.txt). Such a command argument is replaced by the content of
the corresponding text file (multiple whitespaces are treated as a single separator unless
they appear between two quotation marks) prior to any further evaluation. Please note that
a command file cannot contain another command file.
EXIT CODES
The dcmicmp utility uses the following exit codes when terminating. This enables the user
to check for the reason why the application terminated.
general
EXITCODE_NO_ERROR 0
EXITCODE_COMMANDLINE_SYNTAX_ERROR 1
input/output file errors
EXITCODE_INVALID_INPUT_FILE 22
EXITCODE_CANNOT_WRITE_OUTPUT_FILE 40
image processing errors
EXITCODE_INITIALIZE_DIFF_IMAGE 80
EXITCODE_DISPLAY_PIPELINE 81
EXITCODE_IMAGE_COMPARISON 82
error codes for exceeded limits
EXITCODE_LIMIT_EXCEEDED_MAX_ERROR 90
EXITCODE_LIMIT_EXCEEDED_MAE 91
EXITCODE_LIMIT_EXCEEDED_RMSE 92
EXITCODE_LIMIT_EXCEEDED_PSNR 93
EXITCODE_LIMIT_EXCEEDED_SNR 94
ENVIRONMENT
The dcmicmp utility will attempt to load DICOM data dictionaries specified in the
DCMDICTPATH environment variable. By default, i.e. if the DCMDICTPATH environment variable
is not set, the file <datadir>/dicom.dic will be loaded unless the dictionary is built
into the application (default for Windows).
The default behavior should be preferred and the DCMDICTPATH environment variable only
used when alternative data dictionaries are required. The DCMDICTPATH environment variable
has the same format as the Unix shell PATH variable in that a colon (':') separates
entries. On Windows systems, a semicolon (';') is used as a separator. The data dictionary
code will attempt to load each file specified in the DCMDICTPATH environment variable. It
is an error if no data dictionary can be loaded.
SEE ALSO
dcm2pnm(1)
COPYRIGHT
Copyright (C) 2018 by OFFIS e.V., Escherweg 2, 26121 Oldenburg, Germany.