ImageMagick: Command-line Tools: Compare
Author: ImageMagick Studio LLC
Description: ImageMagick® is a software suite to create, edit, compose, or convert bitmap images. It can read and write images in a variety of formats (over 200) including PNG, JPEG, JPEG-2000, GIF, WebP, Postscript, PDF, and SVG. Use ImageMagick to resize, flip, mirror, rotate, distort, shear and transform images, adjust image colors, apply various special effects, or draw text, lines, polygons, ellipses and Bézier curves.
Keywords: command-line, tools:, compare, ImageMagick, PerlMagick, image processing, image, photo, software, Magick++, OpenMP, convert

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.png
    Image: 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.png
    19.63

Or, if you just want the overall image distortion, use this command:

    -> compare -metric PSNR rose.jpg reconstruct.jpg difference.png
    28.31

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.gif
    85.05 (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