A lightweight image comparison tool

Table of Contents

• Installation
• Usage
  • Command-Line Usage
  • Object Usage
  • Cropping
  • Perceptual Comparison
  • Logging
  • Block-Out
• Examples
• API-Documentation
• Tests
• Project Focus
• Project Naming
• Contributions
• Contributors
• Third-party libraries
• License

##Image Comparison and Result

##Installation

Install this module with the following command:

npm install blink-diff

Add the module to your package.json dependencies:

npm install --save blink-diff

Add the module to your package.json dev-dependencies:

npm install --save-dev blink-diff

##Usage

The package can be used in two different ways:

• per command line
• through an object

###Command-Line usage

The command-line tool can be found in the bin directory. You can run the application with

blink-diff --output <output>.png <image1>.png <image2>.png

Use image1 and image2 as the images you want to compare. Only PNGs are supported at this point.

The command-line tool exposes a couple of flags and parameters for the comparison:

--verbose           Turn on verbose mode
--debug             Turn on debug mode - leaving all filters and modifications on the result
--threshold p       Number of pixels/percent 'p' below which differences are ignored
--threshold-type t  'pixel' and 'percent' as type of threshold. (default: pixel)
--delta p           Max. distance colors in the 4 dimensional color-space without triggering a difference. (default: 20)
--copyImageA        Copies first image to output as base. (default: true)
--copyImageB        Copies second image to output as base.
--no-copy           Doesn't copy anything to output as base.
--output o          Write difference to the file 'o'
--filter f          Filters f (separated with comma) that will be applied before the comparison.
--no-composition    Turns the composition feature off
--compose-ltr       Compose output image from left to right
--compose-ttb       Compose output image from top to bottom
--hide-shift        Hides shift highlighting (default: false)
--h-shift           Acceptable horizontal shift of pixel. (default: 0)
--v-shift           Acceptable vertical shift of pixel. (default: 0)
--block-out x,y,w,h Block-out area. Can be repeated multiple times.
--version           Print version
--help              This help

###Object usage

The package can also be used directly in code, without going through the command-line.

Example:

var diff = new BlinkDiff({
    imageAPath: 'path/to/first/image', // Use file-path
    imageBPath: 'path/to/second/image',

    thresholdType: BlinkDiff.THRESHOLD_PERCENT,
    threshold: 0.01, // 1% threshold

    imageOutputPath: 'path/to/output/image'
});

diff.run(function (error, result) {
   if (error) {
      throw error;
   } else {
      console.log(diff.hasPassed(result.code) ? 'Passed' : 'Failed');
      console.log('Found ' + result.differences + ' differences.');
   }
});

All the parameters that were available in the command-line tool are also available through the class constructor, however they might use slightly different wording. The class exposes additional parameters that are not available from the command-line:

• imageAPath Defines the path to the first image that should be compared (required; imageAPath or imageA is required - see example below)
• imageA Supplies first image that should be compared (required; imageAPath or imageA is required - see example below) - This can be a PNGImage instance or a Buffer instance with PNG data
• imageBPath Defines the path to the second image that should be compared (required; imageBPath or imageB is required - see example below)
• imageB Supplies second image that should be compared (required; imageBPath or imageB is required - see example below) - This can be a PNGImage instance or a Buffer instance with PNG data
• imageOutputPath Defines the path to the output-file. If you leaves this one off, then this feature is turned-off.
• imageOutputLimit Defines when an image output should be created. This can be for different images, similar or different images, or all comparisons. (default: BlinkDiff.OUTPUT_ALL)
• verbose Verbose output (default: false)
• thresholdType Type of threshold check. This can be BlinkDiff.THRESHOLD_PIXEL and BlinkDiff.THRESHOLD_PERCENT (default: BlinkDiff.THRESHOLD_PIXEL)
• threshold Number of pixels/percent p below which differences are ignored (default: 500) - For percentage thresholds: 1 = 100%, 0.2 = 20%
• delta Distance between the color coordinates in the 4 dimensional color-space that will not trigger a difference. (default: 20)
• outputMaskRed Red intensity for the difference highlighting in the output file (default: 255)
• outputMaskGreen Green intensity for the difference highlighting in the output file (default: 0)
• outputMaskBlue Blue intensity for the difference highlighting in the output file (default: 0)
• outputMaskAlpha Alpha intensity for the difference highlighting in the output file (default: 255)
• outputMaskOpacity Opacity of the pixel for the difference highlighting in the output file (default: 0.7 - slightly transparent)
• outputShiftRed Red intensity for the shift highlighting in the output file (default: 255)
• outputShiftGreen Green intensity for the shift highlighting in the output file (default: 165)
• outputShiftBlue Blue intensity for the shift highlighting in the output file (default: 0)
• outputShiftAlpha Alpha intensity for the shift highlighting in the output file (default: 255)
• outputShiftOpacity Opacity of the pixel for the shift highlighting in the output file (default: 0.7 - slightly transparent)
• outputBackgroundRed Red intensity for the background in the output file (default: 0)
• outputBackgroundGreen Green intensity for the background in the output file (default: 0)
• outputBackgroundBlue Blue intensity for the background in the output file (default: 0)
• outputBackgroundAlpha Alpha intensity for the background in the output file (default: undefined)
• outputBackgroundOpacity Opacity of the pixel for the background in the output file (default: 0.6 - transparent)
• blockOut Object or list of objects with coordinates that should be blocked before testing.
• blockOutRed Red intensity for the block-out in the output file (default: 0) This color will only be visible in the result when debug-mode is turned on.
• blockOutGreen Green intensity for the block-out in the output file (default: 0) This color will only be visible in the result when debug-mode is turned on.
• blockOutBlue Blue intensity for the block-out in the output file (default: 0) This color will only be visible in the result when debug-mode is turned on.
• blockOutAlpha Alpha intensity for the block-out in the output file (default: 255)
• blockOutOpacity Opacity of the pixel for the block-out in the output file (default: 1.0)
• copyImageAToOutput Copies the first image to the output image before the comparison begins. This will make sure that the output image will highlight the differences on the first image. (default)
• copyImageBToOutput Copies the second image to the output image before the comparison begins. This will make sure that the output image will highlight the differences on the second image.
• filter Filters that will be applied before the comparison. Available filters are: blur, grayScale, lightness, luma, luminosity, sepia
• debug When set, then the applied filters will be shown on the output image. (default: false)
• composition Creates as output a composition of all three images (approved, highlight, and build) (default: true)
• composeLeftToRight Creates comparison-composition from left to right, otherwise it lets decide the app on what is best
• composeTopToBottom Creates comparison-composition from top to bottom, otherwise it lets decide the app on what is best
• hShift Horizontal shift for possible antialiasing (default: 2) Set to 0 to turn this off.
• vShift Vertical shift for possible antialiasing (default: 2) Set to 0 to turn this off.
• hideShift Uses the background color for "highlighting" shifts. (default: false)
• cropImageA Cropping for first image (default: no cropping) - Format: { x:, y:, width:, height: }
• cropImageB Cropping for second image (default: no cropping) - Format: { x:, y:, width:, height: }
• perceptual Turn the perceptual comparison mode on. See below for more information.
• gamma Gamma correction for all colors (will be used as base) (default: none) - Any value here will turn the perceptual comparison mode on
• gammaR Gamma correction for red - Any value here will turn the perceptual comparison mode on
• gammaG Gamma correction for green - Any value here will turn the perceptual comparison mode on
• gammaB Gamma correction for blue - Any value here will turn the perceptual comparison mode on

Example:

var firstImage = PNGImage.readImage('path/to/first/image', function (err) {

  if (err) {
    throw err;
  }

  var diff = new BlinkDiff({
      imageA: srcImage, // Use already loaded image for first image
      imageBPath: 'path/to/second/image', // Use file-path to select image

      delta: 50, // Make comparison more tolerant
      
      outputMaskRed: 0,
      outputMaskBlue: 255, // Use blue for highlighting differences
      
      hideShift: true, // Hide anti-aliasing differences - will still determine but not showing it

      imageOutputPath: 'path/to/output/image'
  });

  diff.run(function (error, result) {
    if (error) {
      throw error;
    } else {
      console.log(diff.hasPassed(result.code) ? 'Passed' : 'Failed');
      console.log('Found ' + result.differences + ' differences.');
    }
  });
});

####Cropping Images can be cropped before they are compared by using the cropImageA or cropImageB parameters. Single values can be left off, and the system will calculate the correct dimensions. However, x/y coordinates have priority over width/height as the position are usually more important than the dimensions - image will also be clipped by the system when needed.

####Perceptual Comparison The perceptual comparison mode considers the perception of colors in the human brain. It transforms all the colors into a human perception color-space, which is quite different to the typical physical bound RGB color-space. There, in the perceptual color-space, the distance between colors is according to the human perception and should therefore closer resemble the differences a human would perceive seeing the images.

####Logging

By default, the logger doesn't log events anywhere, but you can change this behavior by overwriting blinkDiff.log:

var blinkDiff = new BlinkDiff({
    ...
});

blinkDiff.log = function (text) {
    // Do whatever you want to do
};

...

####Block-Out Sometimes, it is necessary to block-out some specific areas in an image that should be ignored for comparisons. For example, this can be IDs or even time-labels that change with the time. Adding block-outs to images may decrease false positives and therefore stabilizes these comparisons.

The color of the block-outs can be selected by the API parameters. However, the block-out areas will not be visible by default - they are hidden even though they are used. To make them visible, turn the debug-mode on.

##Examples

There are some examples in the examples folder, in which I used screenshots of YDN to check for visual regressions (and made some manual modifications to the dom to make differences appear ;-)). You can find examples for:

• Color changes in YDN_Color
• Missing DOM elements in YDN_Missing (including some anti-aliasing)
• Multiple differences in YDN_Multi
• Disrupted sorting in YDN_Sort
• Swapped items in YDN_Swap (including block-out areas)
• Text capitalization in YDN_Upper

All screenshots were compared to YDN.png, a previously approved screenshot without a regression. Each of the regressions has the screenshot and the output result, highlighting the differences.

##API-Documentation

Generate the documentation with following command:

npm run docs

The documentation will be generated in the docs folder of the module root.

##Tests

Run the tests with the following command:

npm run test

The code-coverage will be written to the coverage folder in the module root.

##Project Focus There are three types of image comparisons:

• Pixel-by-pixel - Used to compare low-frequency images like screenshots from web-sites, making sure that small styling differences trigger
• Perceptual - Used to compare image creation applications, for example rendering engines and photo manipulation applications that are taking the human perception into account, ignoring differences a human probably would not see
• Context - Used to see if parts of images are missing or are severely distorted, but accepts smaller and/or perceptual differences

Blink-Diff was initially created to compare screenshots. These images are generally low-frequency, meaning larger areas with the same color and less gradients than in photos. The pixel-by-pixel comparison was chosen as it will trigger for differences that a human might not be able to see. We believe that a bug is still a bug even if a human won't see it - a regression might have happened that wasn't intended. A perceptual comparison would not trigger small differences, possibly missing problems that could get worse down the road. Pixel-by-pixel comparisons have the reputation of triggering too often, adding manual labor, checking images by hand. Blink-Diff was created to keep this in mind and was optimized to reduce false-positives by taking sub-pixeling and anti-aliasing into account. Additional features like thresholds and the pythagorean distance calculation in the four dimensional color-space makes sure that this won't happen too often. Additionally, filters can be applied to the images, for example to compare luminosity of pixels and not the saturation thereof. Blink-Diff also supports partially the perceptual comparison that can be turned on when supplying perceptual=true. Then, the colors will be compared in accordance with the human perception and not according to the physical world. High-frequency filters, however, are not yet supported.

##Project Naming The name comes from the Blink comparator that was used in Astronomy to recognize differences in multiple photos, taking a picture of the same area in the sky over consecutive days, months, or years. Most notably, it was used to discover Pluto.

##Contributions Feel free to create an issue or create a pull-request if you have an idea on how to improve blink-diff. We are pretty relaxed on the contribution rules; add tests for your pull-requests when possible, but it is also ok if there are none - we'll add them for you. We are trying to improve blink-diff as much as possible, and this can only be done by contributions from the community.

Also, even if you simply gave us an idea for a feature and did not actually write the code, we will still add you as the Contributor down below since it probably wouldn't be there without you. So, keep them coming!

##Contributors

• sarbbottam
• koola
• jeffposnick
• a-nwhitmont
• azu
• bradex

##Third-party libraries

The following third-party libraries are used by this module:

###Dependencies

• promise: https://github.com/then/promise
• pngjs-image: https://github.com/yahoo/pngjs-image

###Dev-Dependencies

• chai: http://chaijs.com
• coveralls: https://github.com/cainus/node-coveralls
• codeclimate-test-reporter: https://github.com/codeclimate/javascript-test-reporter
• istanbul: https://github.com/gotwarlost/istanbul
• mocha: https://github.com/visionmedia/mocha
• sinon: http://sinonjs.org
• sinon-chai: https://github.com/domenic/sinon-chai
• yuidocjs: https://github.com/yui/yuidoc

##License

The MIT License

Copyright 2014-2015 Yahoo Inc.