npm package: @foxandgeese/blink-diff
Description: A lightweight image comparison tool
Latest version: 2.0.0

Blink-Diff
==========

A lightweight image comparison tool

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**Table of Contents**
* [Installation](#installation)
* [Usage](#usage)
    * [Command-Line Usage](#command-line-usage)
    * [Object Usage](#object-usage)
    * [Cropping](#cropping)
    * [Perceptual Comparison](#perceptual-comparison)
    * [Logging](#logging)
    * [Block-Out](#block-out)
* [Examples](#examples)
* [API-Documentation](#api-documentation)
* [Tests](#tests)
* [Project Focus](#project-focus)
* [Project Naming](#project-naming)
* [Contributions](#contributions)
* [Contributors](#contributers)
* [Third-party libraries](#third-party-libraries)
* [License](#license)


##Image Comparison and Result
![Composition](https://raw.githubusercontent.com/yahoo/blink-diff/master/images/composition.png)

##Installation

Install this module with the following command:
```shell
npm install blink-diff
```

Add the module to your ```package.json``` dependencies:
```shell
npm install --save blink-diff
```
Add the module to your ```package.json``` dev-dependencies:
```shell
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

```shell
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:**
```javascript
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:<int>, y:<int>, width:<int>, height:<int> }
* ```cropImageB``` Cropping for second image (default: no cropping) - Format: { x:<int>, y:<int>, width:<int>, height:<int> }
* ```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:**
```javascript
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```:

```javascript
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:
```shell
npm run docs
```
The documentation will be generated in the ```docs``` folder of the module root.

##Tests

Run the tests with the following command:
```shell
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](http://en.wikipedia.org/wiki/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](https://github.com/sarbbottam)
* [koola](https://github.com/koola)
* [jeffposnick](https://github.com/jeffposnick)
* [a-nwhitmont](https://github.com/a-nwhitmont)
* [azu](https://github.com/azu)
* [bradex](https://github.com/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/foxandgeese/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.