grafi-invert.js

;(function () {
  /**
    ## ImageData object constructor
    Every return from grafi method is formatted to an ImageData object.
    This constructor is used when `window` is not available.
    (for example you are using grafi in node)
   */
  function GrafiImageData (pixelData, width, height) {
    this.width = width
    this.height = height
    this.data = pixelData
  }

  /**
    ## Color Depth Checker
    To maintain simplicity of code, grafi only accepts ImageData in RGBA
    Length of pixelData must be 4 times as much as available pixels (width * height).
   */
  function checkColorDepth (dataset, width, height) {
    var colorDepth
    if (dataset.width && dataset.height) {
      // When ImageData object was passed as dataset
      colorDepth = dataset.data.length / (dataset.width * dataset.height)
    } else {
      // When just an array was passed as dataset
      colorDepth = dataset.length / (width * height)
    }

    if (colorDepth !== 4) {
      throw new Error('data and size of the image does now match')
    }
  }

  /**
    ## formatter
    Internal function used to format pixel data into ImageData object

    ### Parameters
      - pixelData `Uint8ClampedArray`: pixel representation of the image
      - width `Number`: width of the image
      - hight `Number`: height of the image

    ### Example
        formatter(new Uint8ClampedArray[400], 10, 10)
        // ImageData { data: Uint8ClampedArray[400], width: 10, height: 10, }
   */
  function formatter (pixelData, width, height) {
    // check the size of data matches
    checkColorDepth(pixelData, width, height)

    if (!(pixelData instanceof Uint8ClampedArray)) {
      throw new Error('pixel data passed is not an Uint8ClampedArray')
    }

    // If window is available create ImageData using browser API,
    // otherwise call ImageData constructor
    if (typeof window === 'object') {
      return new window.ImageData(pixelData, width, height)
    }
    return new GrafiImageData(pixelData, width, height)
  }
  /**
    ## invert method
    inverts color of an given image

    ### Parameters
      - imageData `Object`: ImageData object

    ### Example
        var input = { data: Uint8ClampedArray[400], width: 10, height: 10, }
        grafi.invert(input)
   */
  function invert (imgData) {
    checkColorDepth(imgData)
    var dataLength = imgData.data.length
    var newPixeldata = new Uint8ClampedArray(dataLength)
    var i
    for (i = 0; i < dataLength; i++) {
      // the image has Alpha channel, skip invert every 4th one
      if ((i + 1) % 4 === 0) {
        newPixeldata[i] = imgData.data[i]
        continue
      }
      newPixeldata[i] = 255 - imgData.data[i]
    }
    return formatter(newPixeldata, imgData.width, imgData.height)
  }

  var grafi = {}
  grafi.invert = invert

  if (typeof module === 'object' && module.exports) {
    module.exports = grafi
  } else {
    this.grafi = grafi
  }
}())