# Image Similarity Comparison

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 RMSE rose.jpg reconstruct.jpg difference.png
    Image: rose.jpg
     Channel distortion: RMSE
       red: 2833.91 (0.0432427)
        green: 2401.45 (0.0366438)
        blue: 2560.43 (0.0390696)
        all: 2604.73 (0.0397456)

Or, if you just want the red channel distortion, use this command:

    $ compare -channel red -metric PSNR rose.jpg reconstruct.jpg difference.png
    27.2817 (0.227348)

If your image contains an alpha channel (transparency) you need to explicitly include it in the comparison:

    $ compare -channel rgba -metric PSNR rose.jpg reconstruct.jpg difference.png
    28.0142 (0.233452)

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

    $ compare -metric PSNR rose.jpg reconstruct.jpg difference.png
    28.0142 (0.233452)

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 NCC -subimage-search logo.png hat.png similarity.pfm
    0 (0) @ 417,44 [0.0]
    $ identify -define identify:locate=maximum -define identify:limit=1 similarity.pfm
    Channel maximum locations:
      Gray: 65532.9 (0.999967) 417,44

The value in the brackets is the normalized simularity metric.

Note, similarity search respects the virtual page offset. To ensure expected results, make sure your image page offset is +0+0.

The subimage search feature in ImageMagick can be quite slow due to its iterative nature. However, if your release of ImageMagick includes FFTW (Fastest Fourier Transform in the West) support and your metric is NCC, you can expect an order of magnitude speed increase in processing time.

If the `-subimage-search` option is not enabled, ImageMagick performs a direct pixel-by-pixel comparison between the two images, starting from their page offset, typically the top-left corners. In cases where the images differ in size, the smaller image is aligned with the larger one. The extra areas in the smaller image are treated as virtual pixels. These virtual pixels can be processed according to the chosen virtual pixel method, which influences how unmatched regions are handled in the comparison. Consequently, the comparison metrics may be affected by these additional areas, potentially impacting the overall similarity assessment. To only compare authentic pixels, use `-define compare:virtual-pixels=false`.

You can find additional examples of using `compare` in Examples of ImageMagick Usage.

Two images are considered similar if their difference according to the specified metric and fuzz value is 0, with the exception of the normalized cross correlation metric (NCC), where two images are considered similar when their normalized cross correlation is 1. The default metric is NCC.

The compare program returns 2 on error, 0 if the images are similar, or a value between 0 and 1 if they are not similar.

## Option Summary

The `compare` command recognizes these options. Click on an option to get more details about how that option works.

Option | Description
--- | ---
-adjoin | join images into a single multi-image file
-alpha | on, activate, off, deactivate, set, opaque, copy", transparent, extract, background, or shape the alpha channel
-authenticate value | decrypt image with this password
-background color | background color
-brightness-contrast geometry | improve brightness / contrast of the image
-channel type | apply option to select image channels
-colorspace type | set image colorspace
-compose operator | set image composite operator
-crop geometry | crop the image
-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)
-distort type coefficients | distort image
-encipher filename | convert plain pixels to cipher pixels
-extract geometry | extract area from image
-fuzz distance | colors within this distance are considered equal
-gravity type | horizontal and vertical text placement
-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
-level value | adjust the level of image contrast
-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. The default metric is RMSE.
-negate | replace each pixel with its complementary color
-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
-read-mask filename | associate a read mask with the image
-regard-warnings | pay attention to warning messages.
-repage geometry | size and location of an image canvas
-resize geometry | resize the image
-respect-parentheses | settings remain in effect until parenthesis boundary.
-rotate degrees | apply Paeth rotation to the image
-sampling-factor geometry | horizontal and vertical sampling factor
-seed value | seed a new sequence of pseudo-random numbers
-separate | separate an image channel into a grayscale image
-set attribute value | set an image attribute
-sigmoidal-contrast geometry | increase the contrast without saturating highlights or shadows
-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
-trim | trim image edges
-verbose | print detailed information about the image
-version | print version information
-virtual-pixel method | access method for pixels outside the boundaries of the image
-read-mask filename | associate a write mask with the image

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