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Filename : ssim.ts
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/**
 * @preserve
 * Copyright 2015-2018 Igor Bezkrovnyi
 * All rights reserved. (MIT Licensed)
 *
 * ssim.ts - part of Image Quantization Library
 */
import { PointContainer } from '../utils/pointContainer';
import { Y } from '../constants/bt709';

// based on https://github.com/rhys-e/structural-similarity
// http://en.wikipedia.org/wiki/Structural_similarity
const K1 = 0.01;
const K2 = 0.03;

export function ssim(image1: PointContainer, image2: PointContainer) {
  if (
    image1.getHeight() !== image2.getHeight() ||
    image1.getWidth() !== image2.getWidth()
  ) {
    throw new Error('Images have different sizes!');
  }

  const bitsPerComponent = 8;
  const L = (1 << bitsPerComponent) - 1;
  const c1 = (K1 * L) ** 2;
  const c2 = (K2 * L) ** 2;

  let numWindows = 0;
  let mssim = 0.0;

  // calculate ssim for each window
  iterate(
    image1,
    image2,
    (lumaValues1, lumaValues2, averageLumaValue1, averageLumaValue2) => {
      // calculate variance and covariance
      let sigxy = 0.0;
      let sigsqx = 0.0;
      let sigsqy = 0.0;

      for (let i = 0; i < lumaValues1.length; i++) {
        sigsqx += (lumaValues1[i] - averageLumaValue1) ** 2;
        sigsqy += (lumaValues2[i] - averageLumaValue2) ** 2;

        sigxy +=
          (lumaValues1[i] - averageLumaValue1) *
          (lumaValues2[i] - averageLumaValue2);
      }

      const numPixelsInWin = lumaValues1.length - 1;
      sigsqx /= numPixelsInWin;
      sigsqy /= numPixelsInWin;
      sigxy /= numPixelsInWin;

      // perform ssim calculation on window
      const numerator =
        (2 * averageLumaValue1 * averageLumaValue2 + c1) * (2 * sigxy + c2);
      const denominator =
        (averageLumaValue1 ** 2 + averageLumaValue2 ** 2 + c1) *
        (sigsqx + sigsqy + c2);
      const ssim = numerator / denominator;

      mssim += ssim;
      numWindows++;
    },
  );
  return mssim / numWindows;
}

function iterate(
  image1: PointContainer,
  image2: PointContainer,
  callback: (
    lumaValues1: number[],
    lumaValues2: number[],
    averageLumaValue1: number,
    averageLumaValue2: number,
  ) => void,
) {
  const windowSize = 8;
  const width = image1.getWidth();
  const height = image1.getHeight();

  for (let y = 0; y < height; y += windowSize) {
    for (let x = 0; x < width; x += windowSize) {
      // avoid out-of-width/height
      const windowWidth = Math.min(windowSize, width - x);
      const windowHeight = Math.min(windowSize, height - y);

      const lumaValues1 = calculateLumaValuesForWindow(
        image1,
        x,
        y,
        windowWidth,
        windowHeight,
      );
      const lumaValues2 = calculateLumaValuesForWindow(
        image2,
        x,
        y,
        windowWidth,
        windowHeight,
      );
      const averageLuma1 = calculateAverageLuma(lumaValues1);
      const averageLuma2 = calculateAverageLuma(lumaValues2);

      callback(lumaValues1, lumaValues2, averageLuma1, averageLuma2);
    }
  }
}

function calculateLumaValuesForWindow(
  image: PointContainer,
  x: number,
  y: number,
  width: number,
  height: number,
) {
  const pointArray = image.getPointArray();
  const lumaValues = [];

  let counter = 0;

  for (let j = y; j < y + height; j++) {
    const offset = j * image.getWidth();
    for (let i = x; i < x + width; i++) {
      const point = pointArray[offset + i];
      lumaValues[counter] =
        point.r * Y.RED + point.g * Y.GREEN + point.b * Y.BLUE;
      counter++;
    }
  }

  return lumaValues;
}

function calculateAverageLuma(lumaValues: number[]) {
  let sumLuma = 0.0;
  for (const luma of lumaValues) {
    sumLuma += luma;
  }

  return sumLuma / lumaValues.length;
}