Sampling colors from png file.

colorSampler.js

'use strict';

// npm install png-js       https://github.com/devongovett/png.js
// npm install image-size   https://www.npmjs.com/package/image-size

const options = process.argv.slice(2);

if (options.length < 3) {
  throw 'Required arguments: filename, cell width, cell height. Optional 4th argument: row spacing.';
}

const imageFilename = options[0];
const cellWidth = parseInt(options[1], 10); //39
const cellHeight = parseInt(options[2], 10); //6
const rowSpacing = parseInt(options.length === 4 ? options[3] : 0, 10); //14
const imgDims = require('image-size')(imageFilename);
const cellCount = Math.floor(imgDims.width / cellWidth);
const cellDims = {
  width: cellWidth,
  height: cellHeight,
  widthHalf: Math.floor(cellWidth / 2),
  heightHalf: Math.floor(cellHeight / 2)
};

require('png-js').decode(imageFilename, readColors);

/** @description Converts colors to dictionary and writes fo tile. */
function readColors(data) {
  console.log(`Reading ${imageFilename}: ${imgDims.width}x${imgDims.height}, ${data.length / 4} pixels`);
  const colors = [];
  let height = 0;
  while (height < imgDims.height) {
    colors.push(readRow(data, height));
    height += cellDims.height + rowSpacing;
  }
  writeColors(colors);
}

/** @description Converts colors to dictionary and writes fo file. */
function writeColors(colors) {
  const outputFilename = imageFilename.split('.')[0] + '.json';
  const dictionary = colors.reduce(function(dictionary, colorRow, index) {
    dictionary[index] = colorRow;
    return dictionary;
  }, {});
  console.log(`Writing ${colors.length} rows to ${outputFilename}.`);
  require('fs').writeFile(outputFilename, JSON.stringify(dictionary, null, ' '));
}

/** @description Reads row of colors. */
function readRow(data, height) {
  const colorRow = [];
  let width = 0;
  for (let i = 0; i < cellCount; i++) {
    colorRow.push(readCell(data, height, width));
    width += cellDims.width;
  }
  return colorRow;
}

/** @description Reads centerish area of the cell. */
function readCell(data, height, width) {
  const index = (width + cellDims.widthHalf + (height + cellDims.heightHalf) * imgDims.width) * 4;
  return '#' + rgbItemToHex(data[index]) + rgbItemToHex(data[index + 1]) + rgbItemToHex(data[index + 2]);
}

/** @description Convert numeric value to hex representation. */
function rgbItemToHex(component) {
  const hexColor = component.toString(16);
  return (hexColor.length === 1 ? '0' + hexColor : hexColor).toUpperCase();
}