How to use the command `magick compare` (with examples)

The magick compare command is a tool used within the ImageMagick suite for comparing two images. It generates a difference image, providing a visual representation of how the images differ. The intensity of the differences can help in identifying specific areas that have changed, making it a vital tool for developers and designers looking to track changes, identify alterations, or test image outputs.

Use case 1: Basic Image Comparison

Code:

magick compare image1.png image2.png difference.png

Motivation:

The basic comparison command is useful when you simply need to see the differences between two images. This might be in situations where a new version of an image should closely resemble the old version, like in UI design or automated testing of visual outputs. Using this command can quickly pinpoint where the changes have occurred.

Explanation:

• magick compare: This initiates the comparison process using ImageMagick.
• image1.png: The first image to compare.
• image2.png: The second image to compare.
• difference.png: The output image file where the differences between image1.png and image2.png are stored.

Example Output:

If image1.png is identical to image2.png, the difference.png will be a blank image or uniform in color. If there are differences, they will appear in difference.png as highlighted areas.

Use case 2: Generate a Metrics Report

Code:

magick compare -metric RMSE image1.png image2.png difference.png

Motivation:

In addition to visualizing the differences, it is often crucial to have a quantitative measure of dissimilarity. This can be especially important when determining the impact of compression, edge detection, or transformation algorithms. This command provides a numerical result alongside the visual comparison, allowing easier assessment of image similarity.

Explanation:

• -metric RMSE: This option specifies that the Root Mean Square Error (RMSE) metric will be used for the comparison. RMSE is a measure of differences between values predicted by a model or estimator and the values observed.
• The rest of the command (image1.png image2.png difference.png) remains the same as the basic comparison.

Example Output:

In addition to difference.png, the command outputs an RMSE value in the command line. This could be something like 1234.56 which indicates the numerical difference in pixel values between the two images.

Use case 3: Ignore Color Differences

Code:

magick compare -fuzz 10% -metric AE image1.png image2.png difference.png

Motivation:

Sometimes, minor variations in color are acceptable and don’t indicate meaningful differences. This might occur due to lighting changes, slight variations in rendering, or non-visible color shifts in grayscale images. This command allows exclusions of small color variations from the comparison.

Explanation:

• -fuzz 10%: Specifies the percentage of color change that can be ignored. Here, 10% of color variance is considered negligible.
• -metric AE: The Absolute Error count metric is used here, helping to count the number of differing pixels without considering the magnitude of those differences.

Example Output:

You’ll receive a difference.png highlighting only the significant differences, along with an AE value like 789, indicating the number of pixels that exceeded the 10% fuzz threshold.

Conclusion:

The magick compare command offers a flexible and powerful means to compare images, useful in numerous fields such as graphic design, software testing, and digital media forensics. Whether you’re evaluating the accuracy of image transformations or detecting unauthorized alterations, leveraging its comprehensive functionality helps satisfy both visual checks and numerical analysis needs.