Blog : ImageMagick Compare Command-line Tool

ImageMagick Compare Command-line Tool
[Example Usage • Option Summary]
Use the compare program to mathematically and visually annotate the difference between an image and its reconstruction. See Command Line Processing for advice on how to structure your compare command or see below for example usages of the command.
Example Usage
We list a few examples of the compare command here to illustrate its usefulness and ease of use. To get started, lets compare an image to one thats been sharpened:
$ convert rose.jpg -sharpen 0x1 reconstruct.jpg $ compare rose.jpg reconstruct.jpg difference.png $ compare -compose src rose.jpg reconstruct.jpg difference.png
   
The red areas of the difference image emphasizes (highlight) pixels that are affected by the image sharpening, whereas white de-emphasizes (lowlight) pixels that are untouched by the sharpening process.
In addition to the visual interpretation of the difference in an image and its reconstruction, we report a mathematical measure of the difference:
$ compare -verbose -metric mae rose.jpg reconstruct.jpg difference.pngImage: rose.jpg
Channel distortion: MAE
red: 2282.91 (0.034835)
green: 1853.99 (0.0282901)
blue: 2008.67 (0.0306503)
all: 1536.39 (0.0234439)
Or, if you just want the red channel distortion, use this command:
$ compare -channel red -metric PSNR rose.jpg reconstruct.jpg difference.png19.63
Or, if you just want the overall image distortion, use this command:
$ compare -metric PSNR rose.jpg reconstruct.jpg difference.png28.31
If the reconstructed image is a subimage of the image, the compare program returns the best match offset. In addition, it returns a similarity image such that an exact match location is completely white and if none of the pixels match, black, otherwise some gray level in-between:
$ compare -metric RMSE -subimage-search logo.png wizard.jpg similarity.gif85.05 (0.00129778) @ 353,157
You can find additional examples of using compare in Graphics from the Command Line. Further discussion is available in More Graphics from the Command Line and Examples of ImageMagick Usage.
The compare program returns 2 on error otherwise 0 if the images are similar or 1 if they are dissimilar.
Option Summary
The compare command recognizes these options. Click on an option to get more details about how that option works.
Option   Description
-alpha
on, activate, off, deactivate, set, opaque, copy", transparent, extract, background, or shape the alpha channel
-authenticate value
decrypt image with this password
-channel type
apply option to select image channels
-colorspace type
set image colorspace
-compose operator
set image composite operator
-decipher filename
convert cipher pixels to plain
-debug events
display copious debugging information
-define format:option
define one or more image format options
-density geometry
horizontal and vertical density of the image
-depth value
image depth
-dissimilarity-threshold value
maximum distortion for (sub)image match (default 0.2)
-encipher filename
convert plain pixels to cipher pixels
-extract geometry
extract area from image
-fuzz distance
colors within this distance are considered equal
-help
print program options
-highlight-color color
emphasize pixel differences with this color
-identify
identify the format and characteristics of the image
-interlace type
type of image interlacing scheme
-limit type value
pixel cache resource limit
-log format
format of debugging information
-lowlight-color color
de-emphasize pixel differences with this color
-metric type
measure differences between images with this metric
-profile filename
add, delete, or apply an image profile
-quality value
JPEG/MIFF/PNG compression level
-quantize colorspace
reduce image colors in this colorspace
-quiet
suppress all warning messages
-regard-warnings
pay attention to warning messages.
-respect-parentheses
settings remain in effect until parenthesis boundary.
-sampling-factor geometry
horizontal and vertical sampling factor
-seed value
seed a new sequence of pseudo-random numbers
-set attribute value
set an image attribute
-similarity-threshold value
minimum distortion for (sub)image match (default 0.0)
-size geometry
width and height of image
-subimage-search
search for subimage
-synchronize
synchronize image to storage device
-taint
mark the image as modified
-transparent-color color
transparent color
-verbose
print detailed information about the image
-version
print version information
-virtual-pixel method
access method for pixels outside the boundaries of the image