Differ

Differ is a multiplatform image diffing library that's designed to be lightweight and flexible.

Differ's default ImageComparator, SimpleImageComparator, does basic per-pixel Euclidean distance comparisons, returning a result of the number of pixels whose distance is greater than the provided maxDistance. An optional shift value can be applied both horizontally and vertically, to allow the comparator to account for differences which are the result of anti-aliasing errors.

Usage

The simplest comparison simply takes two Images and returns a result that has the number of pixels that are different between the two.

val differ = SimpleImageComparator()
val result = differ.compare(left, right)

if (result.pixelDifferences > 0) {
    println("Images are different.")
} else {
    println("Images are identical.")
}

SimpleImageComparator can be configured to adjust the sensitivity of it's comparison using the maxDistance property, which defines the max distance that two colors can be from each other in 4 dimensional space without triggering a difference.

// This comparator will be more lenient than the default.
val differ = SimpleImageComparator(maxDistance = 0.007)
val result = differ.compare(left, right)

if (result.pixelDifferences > 0) {
    println("Images are different.")
} else {
    println("Images are identical.")
}

Anti-Aliasing Support

If bitmap images are resized, it's common for some pixels to have minor differences not because the underlying image differs, but because of anti-aliasing errors, as pixels are approximated.

SimpleImageComparator allows you to compensate for this by allowing you to define a horizontal and vertical shift, which allows the comparator to take neighboring pixels within the shift window into account when comparing pixels.

// This comparator will use the neighboring 2 pixels, both horizontally and vertically,
//  to account for potential anti-alias issues.
val differ = SimpleImageComparator(hShift = 2, vShift = 2)
val result = differ.compare(left, right)

if (result.pixelDifferences > 0) {
    println("Images are different.")
} else {
    println("Images are identical.")
}

Note that adding a shift window can potentially increase comparison times exponentially, since for every pixel that doesn't immediately match the comparator will compare another 2 pixels for every shift value.

License

Copyright (c) 2022 Dropbox, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.