palette-to-grayscale

#!/usr/bin/env node

var fs = require('fs')
var PNG = require('pngjs').PNG
var createClassLookup = require('./lib/lookup-class')

if (require.main === module) {
  var classData = JSON.parse(fs.readFileSync(process.argv[2]))
  convertIndexToGrayLevel(process.stdout, process.stdin, classData)
}

module.exports = convertIndexToGrayLevel

// Most image libraries don't give easy access to the PNG palette index for
// each pixel, giving you instead the rgb values. In our colored label images,
// it's the index into the PNG palette that tells us which class a pixel
// belongs to.  This converts such a colored image to a grayscale one where
// the gray value of each pixel is the class number.  Harder to inspect
// visually, but easier to process programmatically
function convertIndexToGrayLevel (output, input, classData) {
  var getClass = createClassLookup(classData)

  input.pipe(
    new PNG()
    .on('parsed', function () {
      for (var y = 0; y < this.height; y++) {
        for (var x = 0; x < this.width; x++) {
          var r = (this.width * y + x) << 2
          var g = r + 1
          var b = r + 2
          var value = getClass(this.data[r], this.data[g], this.data[b])
          this.data[r] = this.data[g] = this.data[b] = value
        }
      }
      this.pack().pipe(output)
    })
  )
}