First version of JPEG and EXIF parser with starter unit tests and updated docs.

image/parsers/README.md

@@ -1,6 +1,6 @@
 General-purpose, event-based parsers for digital images.
 
-Currently only supports GIF.
+Currently only supports GIF and JPEG.
 
 Some nice implementations for HEIF, JPEG, PNG, TIFF here:
 https://github.com/MikeKovarik/exifr/tree/master/src/file-parsers

image/parsers/exif.js

@@ -0,0 +1,234 @@
+import { ByteStream } from '../../io/bytestream.js';
+
+/** @enum {number} */
+export const ExifTagNumber = {
+  // Tags used by IFD0.
+  IMAGE_DESCRIPTION: 0x010e,
+  MAKE: 0x010f,
+  MODEL: 0x0110,
+  ORIENTATION: 0x0112,
+  X_RESOLUTION: 0x011a,
+  Y_RESOLUTION: 0x011b,
+  RESOLUTION_UNIT: 0x0128,
+  SOFTWARE: 0x0131,
+  DATE_TIME: 0x0132,
+  WHITE_POINT: 0x013e,
+  PRIMARY_CHROMATICITIES: 0x013f,
+  Y_CB_CR_COEFFICIENTS: 0x0211,
+  Y_CB_CR_POSITIONING: 0x0213,
+  REFERENCE_BLACK_WHITE: 0x0214,
+  COPYRIGHT: 0x8298,
+  EXIF_OFFSET: 0x8769,
+
+  // Tags used by Exif SubIFD.
+  EXPOSURE_TIME: 0x829a,
+  F_NUMBER: 0x829d,
+  EXPOSURE_PROGRAM: 0x8822,
+  ISO_SPEED_RATINGS: 0x8827,
+  EXIF_VERSION: 0x9000,
+  DATE_TIME_ORIGINAL: 0x9003,
+  DATE_TIME_DIGITIZED: 0x9004,
+  COMPONENT_CONFIGURATION: 0x9101,
+  COMPRESSED_BITS_PER_PIXEL: 0x9102,
+  SHUTTER_SPEED_VALUE: 0x9201,
+  APERTURE_VALUE: 0x9202,
+  BRIGHTNESS_VALUE: 0x9203,
+  EXPOSURE_BIAS_VALUE: 0x9204,
+  MAX_APERTURE_VALUE: 0x9205,
+  SUBJECT_DISTANCE: 0x9206,
+  METERING_MODE: 0x9207,
+  LIGHT_SOURCE: 0x9208,
+  FLASH: 0x9209,
+  FOCAL_LENGTH: 0x920a,
+  MAKER_NOTE: 0x927c,
+  USER_COMMENT: 0x9286,
+  FLASH_PIX_VERSION: 0xa000,
+  COLOR_SPACE: 0xa001,
+  EXIF_IMAGE_WIDTH: 0xa002,
+  EXIF_IMAGE_HEIGHT: 0xa003,
+  RELATED_SOUND_FILE: 0xa004,
+  EXIF_INTEROPERABILITY_OFFSET: 0xa005,
+  FOCAL_PLANE_X_RESOLUTION: 0xa20e,
+  FOCAL_PLANE_Y_RESOLUTION: 0x20f,
+  FOCAL_PLANE_RESOLUTION_UNIT: 0xa210,
+  SENSING_METHOD: 0xa217,
+  FILE_SOURCE: 0xa300,
+  SCENE_TYPE: 0xa301,
+
+  // Tags used by IFD1.
+  IMAGE_WIDTH: 0x0100,
+  IMAGE_LENGTH: 0x0101,
+  BITS_PER_SAMPLE: 0x0102,
+  COMPRESSION: 0x0103,
+  PHOTOMETRIC_INTERPRETATION: 0x0106,
+  STRIP_OFFSETS: 0x0111,
+  SAMPLES_PER_PIXEL: 0x0115,
+  ROWS_PER_STRIP: 0x0116,
+  STRIP_BYTE_COUNTS: 0x0117,
+  // X_RESOLUTION, Y_RESOLUTION
+  PLANAR_CONFIGURATION: 0x011c,
+  // RESOLUTION_UNIT
+  JPEG_IF_OFFSET: 0x0201,
+  JPEG_IF_BYTE_COUNT: 0x0202,
+  // Y_CB_CR_COEFFICIENTS
+  Y_CB_CR_SUB_SAMPLING: 0x0212,
+  // Y_CB_CR_POSITIONING, REFERENCE_BLACK_WHITE
+};
+
+/** @enum {number} */
+export const ExifDataFormat = {
+  UNSIGNED_BYTE: 1,
+  ASCII_STRING: 2,
+  UNSIGNED_SHORT: 3,
+  UNSIGNED_LONG: 4,
+  UNSIGNED_RATIONAL: 5,
+  SIGNED_BYTE: 6,
+  UNDEFINED: 7,
+  SIGNED_SHORT: 8,
+  SIGNED_LONG: 9,
+  SIGNED_RATIONAL: 10,
+  SINGLE_FLOAT: 11,
+  DOUBLE_FLOAT: 12,
+};
+
+/**
+ * @typedef ExifValue
+ * @property {ExifTagNumber} tagNumber The numerical value of the tag.
+ * @property {string=} tagName A string representing the tag number.
+ * @property {ExifDataFormat} dataFormat The data format.
+ * @property {number=} numericalValue Populated for SIGNED/UNSIGNED BYTE/SHORT/LONG/FLOAT.
+ * @property {string=} stringValue Populated only for ASCII_STRING.
+ * @property {number=} numeratorValue Populated only for SIGNED/UNSIGNED RATIONAL.
+ * @property {number=} denominatorValue Populated only for SIGNED/UNSIGNED RATIONAL.
+ * @property {number=} numComponents Populated only for UNDEFINED data format.
+ * @property {number=} offsetValue Populated only for UNDEFINED data format.
+ */
+
+/**
+ * @param {number} tagNumber 
+ * @param {string} type 
+ * @param {number} len 
+ * @param {number} dataVal 
+ */
+function warnBadLength(tagNumber, type, len, dataVal) {
+  const hexTag = tagNumber.toString(16);
+  console.warn(`Tag 0x${hexTag} is ${type} with len=${len} and data=${dataVal}`);
+}
+
+/**
+ * @param {ByteStream} stream
+ * @param {ByteStream} lookAheadStream
+ * @param {boolean} debug
+ * @returns {ExifValue}
+ */
+export function getExifValue(stream, lookAheadStream, DEBUG = false) {
+  const tagNumber = stream.readNumber(2);
+  let tagName = findNameWithValue(ExifTagNumber, tagNumber);
+  if (!tagName) {
+    tagName = `UNKNOWN (0x${tagNumber.toString(16)})`;
+  }
+
+  let dataFormat = stream.readNumber(2);
+
+  // Handle bad types for special tags.
+  if (tagNumber === ExifTagNumber.EXIF_OFFSET) {
+    dataFormat = ExifDataFormat.UNSIGNED_LONG;
+  }
+  
+  const dataFormatName = findNameWithValue(ExifDataFormat, dataFormat);
+  if (!dataFormatName) throw `Invalid data format: ${dataFormat}`;
+
+  /** @type {ExifValue} */
+  const exifValue = {
+    tagNumber,
+    tagName,
+    dataFormat,
+  };
+
+  let len = stream.readNumber(4);
+  switch (dataFormat) {
+    case ExifDataFormat.UNSIGNED_BYTE:
+      if (len !== 1 && DEBUG) {
+        warnBadLength(tagNumber, dataFormatName, len, stream.peekNumber(4));
+      }
+      exifValue.numericalValue = stream.readNumber(1);
+      stream.skip(3);
+      break;
+    case ExifDataFormat.ASCII_STRING:
+      if (len <= 4) {
+        exifValue.stringValue = stream.readString(4);
+      } else {
+        const strOffset = stream.readNumber(4);
+        exifValue.stringValue = lookAheadStream.tee().skip(strOffset).readString(len - 1);
+      }
+      break;
+    case ExifDataFormat.UNSIGNED_SHORT:
+      if (len !== 1 && DEBUG) {
+        warnBadLength(tagNumber, dataFormatName, len, stream.peekNumber(4));
+      }
+      exifValue.numericalValue = stream.readNumber(2);
+      stream.skip(2);
+      break;
+    case ExifDataFormat.UNSIGNED_LONG:
+      if (len !== 1 && DEBUG) {
+        warnBadLength(tagNumber, dataFormatName, len, stream.peekNumber(4));
+      }
+      exifValue.numericalValue = stream.readNumber(4);
+      break;
+    case ExifDataFormat.UNSIGNED_RATIONAL:
+      if (len !== 1 && DEBUG) {
+        warnBadLength(tagNumber, dataFormatName, len, stream.peekNumber(4));
+      }
+
+      const uratStream = lookAheadStream.tee().skip(stream.readNumber(4));
+      exifValue.numeratorValue = uratStream.readNumber(4);
+      exifValue.denominatorValue = uratStream.readNumber(4);
+      break;
+    case ExifDataFormat.SIGNED_BYTE:
+      if (len !== 1 && DEBUG) {
+        warnBadLength(tagNumber, dataFormatName, len, stream.peekSignedNumber(4));
+      }
+      exifValue.numericalValue = stream.readSignedNumber(1);
+      stream.skip(3);
+      break;
+    case ExifDataFormat.UNDEFINED:
+      exifValue.numComponents = len;
+      exifValue.offsetValue = stream.readNumber(4);    
+      break;
+    case ExifDataFormat.SIGNED_SHORT:
+      if (len !== 1 && DEBUG) {
+        warnBadLength(tagNumber, dataFormatName, len, stream.peekSignedNumber(4));
+      }
+      exifValue.numericalValue = stream.readSignedNumber(2);
+      stream.skip(2);
+      break;
+    case ExifDataFormat.SIGNED_LONG:
+      if (len !== 1) {
+        warnBadLength(tagNumber, dataFormatName, len, stream.peekSignedNumber(4));
+      }
+      exifValue.numericalValue = stream.readSignedNumber(4);
+      break;
+    case ExifDataFormat.SIGNED_RATIONAL:
+      if (len !== 1 && DEBUG) {
+        warnBadLength(tagNumber, dataFormatName, len, stream.peekNumber(4));
+      }
+
+      const ratStream = lookAheadStream.tee().skip(stream.readNumber(4));
+      exifValue.numeratorValue = ratStream.readSignedNumber(4);
+      exifValue.denominatorValue = ratStream.readSignedNumber(4);
+      break;
+    default:
+      throw `Bad data format: ${dataFormat}`;
+  }
+  return exifValue;
+}
+
+/**
+ * @param {Object} obj A numeric enum.
+ * @param {number} valToFind The value to find.
+ * @returns {string|null}
+ */
+function findNameWithValue(obj, valToFind) {
+  const entry = Object.entries(obj).find(([k,v]) => v === valToFind);
+  return entry ? entry[0] : null;
+}

image/parsers/jpeg.js

@@ -0,0 +1,612 @@
+/*
+ * jpeg.js
+ *
+ * An event-based parser for JPEG images.
+ *
+ * Licensed under the MIT License
+ *
+ * Copyright(c) 2024 Google Inc.
+ */
+
+import { ByteStream } from '../../io/bytestream.js';
+import { ExifTagNumber, getExifValue } from './exif.js';
+
+/**
+ * @typedef {import('./exif.js').ExifValue} ExifValue
+ */
+
+// https://en.wikipedia.org/wiki/JPEG_File_Interchange_Format
+// https://www.media.mit.edu/pia/Research/deepview/exif.html
+// https://mykb.cipindanci.com/archive/SuperKB/1294/JPEG%20File%20Layout%20and%20Format.htm
+// https://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf
+
+let DEBUG = false;
+
+/** @enum {string} */
+export const JpegParseEventType = {
+  APP0_MARKER: 'app0_marker',
+  APP0_EXTENSION: 'app0_extension',
+  APP1_EXIF: 'app1_exif',
+  DEFINE_QUANTIZATION_TABLE: 'define_quantization_table',
+  DEFINE_HUFFMAN_TABLE: 'define_huffman_table',
+  START_OF_FRAME: 'start_of_frame',
+  START_OF_SCAN: 'start_of_scan',
+}
+
+/** @enum {number} */
+const JpegSegmentType = {
+  SOF0: 0xC0,
+  SOF1: 0xC1,
+  SOF2: 0xC2,
+  DHT: 0xC4,
+  SOI: 0xD8,
+  EOI: 0xD9,
+  SOS: 0xDA,
+  DQT: 0xDB,
+  APP0: 0xE0,
+  APP1: 0xE1,
+};
+
+/**
+ * @param {Uint8Array} bytes An array of bytes of size 2.
+ * @returns {JpegSegmentType} Returns the second byte in bytes.
+ */
+function getJpegMarker(bytes) {
+  if (bytes.byteLength < 2) throw `Bad bytes length: ${bytes.byteLength}`;
+  if (bytes[0] !== 0xFF) throw `Bad marker, first byte=0x${bytes[0].toString(16)}`;
+  return bytes[1];
+}
+
+/** @enum {number} */
+export const JpegDensityUnits = {
+  NO_UNITS: 0,
+  PIXELS_PER_INCH: 1,
+  PIXELS_PER_CM: 2,
+};
+
+/**
+ * @typedef JpegApp0Marker
+ * @property {string} jfifVersion Like '1.02'.
+ * @property {JpegDensityUnits} densityUnits
+ * @property {number} xDensity
+ * @property {number} yDensity
+ * @property {number} xThumbnail
+ * @property {number} yThumbnail
+ * @property {Uint8Array} thumbnailData RGB data. Size is 3 x thumbnailWidth x thumbnailHeight.
+ */
+
+export class JpegApp0MarkerEvent extends Event {
+  /** @param {JpegApp0Marker} */
+  constructor(segment) {
+    super(JpegParseEventType.APP0_MARKER);
+    /** @type {JpegApp0Marker} */
+    this.app0Marker = segment;
+  }
+}
+
+/** @enum {number} */
+export const JpegExtensionThumbnailFormat = {
+  JPEG: 0x10,
+  ONE_BYTE_PER_PIXEL_PALETTIZED: 0x11,
+  THREE_BYTES_PER_PIXEL_RGB: 0x13,
+};
+
+/**
+ * @typedef JpegApp0Extension
+ * @property {JpegExtensionThumbnailFormat} thumbnailFormat
+ * @property {Uint8Array} thumbnailData Raw thumbnail data
+ */
+
+export class JpegApp0ExtensionEvent extends Event {
+  /** @param {JpegApp0Extension} */
+  constructor(segment) {
+    super(JpegParseEventType.APP0_EXTENSION);
+    /** @type {JpegApp0Extension} */
+    this.app0Extension = segment;
+  }
+}
+
+export class JpegApp1ExifEvent extends Event {
+  /** @param {Map<number, ExifValue>} exifValueMap */
+  constructor(exifValueMap) {
+    super(JpegParseEventType.APP1_EXIF);
+    /** @type {Map<number, ExifValue>} */
+    this.exifValueMap = exifValueMap;
+  }
+}
+
+/**
+ * @typedef JpegDefineQuantizationTable
+ * @property {number} tableNumber Table/component number.
+ * @property {number} precision (0=byte, 1=word).
+ * @property {number[]} tableValues 64 numbers representing the quantization table.
+ */
+
+export class JpegDefineQuantizationTableEvent extends Event {
+  /** @param {JpegDefineQuantizationTable} table */
+  constructor(table) {
+    super(JpegParseEventType.DEFINE_QUANTIZATION_TABLE);
+    /** @type {JpegDefineQuantizationTable} */
+    this.quantizationTable = table;
+  }
+}
+
+/** @enum {number} */
+export const JpegHuffmanTableType = {
+  DC: 0,
+  AC: 1,
+};
+
+/**
+ * @typedef JpegDefineHuffmanTable
+ * @property {number} tableNumber Table/component number (0-3).
+ * @property {JpegHuffmanTableType} tableType Either DC or AC.
+ * @property {number[]} numberOfSymbols A 16-byte array specifying the # of symbols of each length.
+ * @property {number[]} symbols
+ */
+
+export class JpegDefineHuffmanTableEvent extends Event {
+  /** @param {JpegDefineHuffmanTable} table */
+  constructor(table) {
+    super(JpegParseEventType.DEFINE_HUFFMAN_TABLE);
+    /** @type {JpegDefineHuffmanTable} */
+    this.huffmanTable = table;
+  }
+}
+
+/** @enum {number} */
+const JpegDctType = {
+  BASELINE: 0,
+  EXTENDED_SEQUENTIAL: 1,
+  PROGRESSIVE: 2,
+};
+
+/** @enum {number} */
+const JpegComponentType = {
+  Y: 1,
+  CB: 2,
+  CR: 3,
+  I: 4,
+  Q: 5,
+};
+
+/**
+ * @typedef JpegComponentDetail
+ * @property {JpegComponentType} componentId
+ * @property {number} verticalSamplingFactor
+ * @property {number} horizontalSamplingFactor
+ * @property {number} quantizationTableNumber
+ */
+
+/**
+ * @typedef JpegStartOfFrame
+ * @property {JpegDctType} dctType
+ * @property {number} dataPrecision
+ * @property {number} imageHeight
+ * @property {number} imageWidth
+ * @property {number} numberOfComponents Usually 1, 3, or 4.
+ * @property {JpegComponentDetail[]} componentDetails
+ */
+
+export class JpegStartOfFrameEvent extends Event {
+  /** @param {JpegStartOfFrame} sof */
+  constructor(sof) {
+    super(JpegParseEventType.START_OF_FRAME);
+    /** @type {JpegStartOfFrame} */
+    this.startOfFrame = sof;
+  }
+}
+
+/**
+ * @typedef JpegStartOfScan
+ * @property {number} componentsInScan
+ * @property {number} componentSelectorY
+ * @property {number} huffmanTableSelectorY
+ * @property {number} componentSelectorCb
+ * @property {number} huffmanTableSelectorCb
+ * @property {number} componentSelectorCr
+ * @property {number} huffmanTableSelectorCr
+ * @property {number} scanStartPositionInBlock
+ * @property {number} scanEndPositionInBlock
+ * @property {number} successiveApproximationBitPosition
+ * @property {Uint8Array} rawImageData
+ */
+
+export class JpegStartOfScanEvent extends Event {
+  constructor(sos) {
+    super(JpegParseEventType.START_OF_SCAN);
+    /** @type {JpegStartOfScan} */
+    this.sos = sos;
+  }
+}
+
+export class JpegParser extends EventTarget {
+  /**
+   * @type {ByteStream}
+   * @private
+   */
+  bstream;
+
+  /**
+   * @type {boolean}
+   * @private
+   */
+  hasApp0MarkerSegment = false;
+
+  /** @param {ArrayBuffer} ab */
+  constructor(ab) {
+    super();
+    this.bstream = new ByteStream(ab);
+  }
+
+  /**
+   * Type-safe way to bind a listener for a JpegApp0MarkerEvent.
+   * @param {function(JpegApp0MarkerEvent): void} listener
+   * @returns {JpegParser} for chaining
+   */
+  onApp0Marker(listener) {
+    super.addEventListener(JpegParseEventType.APP0_MARKER, listener);
+    return this;
+  }
+
+  /**
+   * Type-safe way to bind a listener for a JpegApp0ExtensionEvent.
+   * @param {function(JpegApp0MarkerEvent): void} listener
+   * @returns {JpegParser} for chaining
+   */
+  onApp0Extension(listener) {
+    super.addEventListener(JpegParseEventType.APP0_EXTENSION, listener);
+    return this;
+  }
+
+  /**
+   * Type-safe way to bind a listener for a JpegApp1ExifEvent.
+   * @param {function(JpegApp1ExifEvent): void} listener
+   * @returns {JpegParser} for chaining
+   */
+  onApp1Exif(listener) {
+    super.addEventListener(JpegParseEventType.APP1_EXIF, listener);
+    return this;
+  }
+
+  /**
+   * Type-safe way to bind a listener for a JpegDefineQuantizationTableEvent.
+   * @param {function(JpegDefineQuantizationTableEvent): void} listener
+   * @returns {JpegParser} for chaining
+   */
+  onDefineQuantizationTable(listener) {
+    super.addEventListener(JpegParseEventType.DEFINE_QUANTIZATION_TABLE, listener);
+    return this;
+  }
+
+  /**
+   * Type-safe way to bind a listener for a JpegDefineHuffmanTableEvent.
+   * @param {function(JpegDefineHuffmanTableEvent): void} listener
+   * @returns {JpegParser} for chaining
+   */
+  onDefineHuffmanTable(listener) {
+    super.addEventListener(JpegParseEventType.DEFINE_HUFFMAN_TABLE, listener);
+    return this;
+  }
+
+  /**
+   * Type-safe way to bind a listener for a JpegStartOfFrameEvent.
+   * @param {function(JpegStartOfFrameEvent): void} listener
+   * @returns {JpegParser} for chaining
+   */
+  onStartOfFrame(listener) {
+    super.addEventListener(JpegParseEventType.START_OF_FRAME, listener);
+    return this;
+  }
+
+  /**
+   * Type-safe way to bind a listener for a JpegStartOfScanEvent.
+   * @param {function(JpegStartOfScanEvent): void} listener
+   * @returns {JpegParser} for chaining
+   */
+  onStartOfScan(listener) {
+    super.addEventListener(JpegParseEventType.START_OF_SCAN, listener);
+    return this;
+  }
+
+  /** @returns {Promise<void>} A Promise that resolves when the parsing is complete. */
+  async start() {
+    const segmentType = getJpegMarker(this.bstream.readBytes(2));
+    if (segmentType !== JpegSegmentType.SOI) throw `Did not start with a SOI`;
+
+    let jpegMarker;
+    do {
+      jpegMarker = getJpegMarker(this.bstream.readBytes(2));
+
+      if (jpegMarker === JpegSegmentType.APP0) {
+        this.bstream.setBigEndian();
+        const length = this.bstream.readNumber(2);
+        const skipAheadStream = this.bstream.tee().skip(length - 2);
+
+        const identifier = this.bstream.readString(4);
+        if (identifier === 'JFIF') {
+          if (this.hasApp0MarkerSegment) throw `JFIF found after JFIF`;
+          if (this.bstream.readNumber(1) !== 0) throw 'No null byte terminator for JFIF';
+
+          this.hasApp0MarkerSegment = true;
+          const majorVer = `${this.bstream.readNumber(1)}.`;
+          const minorVer = `${this.bstream.readNumber(1)}`.padStart(2, '0');
+          const densityUnits = this.bstream.readNumber(1);
+          const xDensity = this.bstream.readNumber(2);
+          const yDensity = this.bstream.readNumber(2);
+          const xThumbnail = this.bstream.readNumber(1);
+          const yThumbnail = this.bstream.readNumber(1);
+  
+          /** @type {JpegApp0Marker} */
+          let app0MarkerSegment = {
+            jfifVersion: `${majorVer}${minorVer}`,
+            densityUnits,
+            xDensity,
+            yDensity,
+            xThumbnail,
+            yThumbnail,
+            thumbnailData: this.bstream.readBytes(3 * xThumbnail * yThumbnail),
+          };
+          this.dispatchEvent(new JpegApp0MarkerEvent(app0MarkerSegment));  
+        }
+        else if (identifier === 'JFXX') {
+          if (!this.hasApp0MarkerSegment) throw `JFXX found without JFIF`;
+          if (this.bstream.readNumber(1) !== 0) throw 'No null byte terminator for JFXX';
+
+          const thumbnailFormat = this.bstream.readNumber(1);
+          if (!Object.values(JpegExtensionThumbnailFormat).includes(thumbnailFormat)) {
+            throw `Bad Extension Thumbnail Format: ${thumbnailFormat}`;
+          }
+
+          // The JFXX segment has length (2), 'JFXX' (4), null byte (1), thumbnail format (1)
+          const thumbnailData = this.bstream.readBytes(length - 8);
+
+          /** @type {JpegApp0Extension} */
+          let app0ExtensionSegment = {
+            thumbnailFormat,
+            thumbnailData,
+          };
+          this.dispatchEvent(new JpegApp0ExtensionEvent(app0ExtensionSegment));
+        }
+        else {
+          throw `Bad APP0 identifier: ${identifier}`;
+        }
+
+        this.bstream = skipAheadStream;
+      } // End of APP0
+      else if (jpegMarker === JpegSegmentType.APP1) {
+        this.bstream.setBigEndian();
+        const length = this.bstream.readNumber(2);
+        const skipAheadStream = this.bstream.tee().skip(length - 2);
+
+        const identifier = this.bstream.readString(4);
+        if (identifier !== 'Exif') {
+          // TODO: Handle XMP.
+          // console.log(identifier + this.bstream.readString(length - 2 - 4));
+          this.bstream = skipAheadStream;
+          continue;
+        }
+        if (this.bstream.readNumber(2) !== 0) throw `No null byte termination`;
+
+        const lookAheadStream = this.bstream.tee();
+        const tiffByteAlign = this.bstream.readString(2);
+        if (tiffByteAlign === 'II') {
+          this.bstream.setLittleEndian();
+          lookAheadStream.setLittleEndian();
+        } else if (tiffByteAlign === 'MM') {
+          this.bstream.setBigEndian();
+          lookAheadStream.setBigEndian();
+        } else {
+          throw `Invalid TIFF byte align symbol: ${tiffByteAlign}`;
+        }
+
+        const tiffMarker = this.bstream.readNumber(2);
+        if (tiffMarker !== 0x002A) {
+          throw `Invalid marker, not 0x002a: 0x${tiffMarker.toString(16)}`;
+        }
+
+        /** @type {Map<number, ExifValue} */
+        const exifValueMap = new Map();
+
+        // The offset includes the tiffByteAlign (2), marker (2), and the offset field itself (4).
+        const ifdOffset = this.bstream.readNumber(4) - 8;
+
+        let ifdStream = this.bstream.tee();
+        let nextIfdOffset;
+        while (true) {
+          nextIfdOffset = this.readExifIfd(ifdStream, lookAheadStream, exifValueMap);
+
+          // No more IFDs, so stop the loop.
+          if (nextIfdOffset === 0) break;
+
+          // Else, we have another IFD to read, point the stream at it.
+          ifdStream = lookAheadStream.tee().skip(nextIfdOffset);
+        }
+
+        this.dispatchEvent(new JpegApp1ExifEvent(exifValueMap));
+
+        this.bstream = skipAheadStream;
+      } // End of APP1
+      else if (jpegMarker === JpegSegmentType.DQT) {
+        this.bstream.setBigEndian();
+        const length = this.bstream.readNumber(2);
+
+        const dqtLength = length - 2;
+        let ptr = 0;
+        while (ptr < dqtLength) {
+          // https://gist.github.com/FranckFreiburger/d8e7445245221c5cf38e69a88f22eeeb#file-getjpegquality-js-L76
+          const firstByte = this.bstream.readNumber(1);
+          // Lower 4 bits are the component index.
+          const tableNumber = (firstByte & 0xF);
+          // Upper 4 bits are the precision (0=byte, 1=word).
+          const precision = ((firstByte & 0xF0) >> 4);
+          if (precision !== 0 && precision !== 1) throw `Weird value for DQT precision: ${precision}`;
+
+          const valSize = precision === 0 ? 1 : 2;
+          const tableValues = new Array(64);
+          for (let v = 0; v < 64; ++v) {
+            tableValues[v] = this.bstream.readNumber(valSize);
+          }
+
+          /** @type {JpegDefineQuantizationTable} */
+          const table = {
+            tableNumber,
+            precision,
+            tableValues,
+          };
+          this.dispatchEvent(new JpegDefineQuantizationTableEvent(table));
+
+          ptr += (1 + valSize * 64);
+        }
+      } // End of DQT
+      else if (jpegMarker === JpegSegmentType.DHT) {
+        this.bstream.setBigEndian();
+        const length = this.bstream.readNumber(2);
+        let ptr = 2;
+
+        while (ptr < length) {
+          const firstByte = this.bstream.readNumber(1);
+          const tableNumber = (firstByte & 0xF);
+          const tableType = ((firstByte & 0xF0) >> 4);
+          if (tableNumber > 3) throw `Weird DHT table number = ${tableNumber}`;
+          if (tableType !== 0 && tableType !== 1) throw `Weird DHT table type = ${tableType}`;
+
+          const numberOfSymbols = Array.from(this.bstream.readBytes(16));
+          let numCodes = 0;
+          for (let symbolLength = 1; symbolLength <= 16; ++symbolLength) {
+            const numSymbolsAtLength = numberOfSymbols[symbolLength - 1];
+            numCodes += numSymbolsAtLength;
+          }
+          if (numCodes > 256) throw `Bad # of DHT codes: ${numCodes}`;
+
+          const symbols = Array.from(this.bstream.readBytes(numCodes));
+
+          /** @type {JpegDefineHuffmanTable} */
+          const table = {
+            tableNumber,
+            tableType,
+            numberOfSymbols,
+            symbols,
+          };
+          this.dispatchEvent(new JpegDefineHuffmanTableEvent(table));
+
+          ptr += (1 + 16 + numCodes);
+        }
+        if (ptr !== length) throw `Bad DHT ptr: ${ptr}!`;
+      } // End of DHT
+      else if (jpegMarker === JpegSegmentType.SOF0
+            || jpegMarker === JpegSegmentType.SOF1
+            || jpegMarker === JpegSegmentType.SOF2) {
+        this.bstream.setBigEndian();
+        const length = this.bstream.readNumber(2);
+
+        const dctType = (jpegMarker - JpegSegmentType.SOF0);
+        if (![0, 1, 2].includes(dctType)) throw `Weird DCT type: ${dctType}`;
+
+        const dataPrecision = this.bstream.readNumber(1);
+        const imageHeight = this.bstream.readNumber(2);
+        const imageWidth = this.bstream.readNumber(2);
+        const numberOfComponents = this.bstream.readNumber(1);
+        const componentDetails = [];
+        for (let c = 0; c < numberOfComponents; ++c) {
+          const componentId = this.bstream.readNumber(1);
+          const nextByte = this.bstream.readNumber(1);
+          const verticalSamplingFactor = (nextByte & 0xF);
+          const horizontalSamplingFactor = ((nextByte & 0xF0) >> 4);
+          const quantizationTableNumber = this.bstream.readNumber(1);
+
+          componentDetails.push({
+            componentId,
+            verticalSamplingFactor,
+            horizontalSamplingFactor,
+            quantizationTableNumber,
+          });
+        }
+
+        /** @type {JpegStartOfFrame} */
+        const sof = {
+          dctType,
+          dataPrecision,
+          imageHeight,
+          imageWidth,
+          numberOfComponents,
+          componentDetails,
+        };
+
+        this.dispatchEvent(new JpegStartOfFrameEvent(sof));
+      } // End of SOF0, SOF1, SOF2
+      else if (jpegMarker === JpegSegmentType.SOS) {
+        this.bstream.setBigEndian();
+        const length = this.bstream.readNumber(2);
+        // console.log(`Inside SOS with length = ${length}`);
+        if (length !== 12) throw `Bad length in SOS header: ${length}`;
+
+        /** @type {JpegStartOfScan} */
+        const sos = {
+          componentsInScan: this.bstream.readNumber(1),
+          componentSelectorY: this.bstream.readNumber(1),
+          huffmanTableSelectorY: this.bstream.readNumber(1),
+          componentSelectorCb: this.bstream.readNumber(1),
+          huffmanTableSelectorCb: this.bstream.readNumber(1),
+          componentSelectorCr: this.bstream.readNumber(1),
+          huffmanTableSelectorCr: this.bstream.readNumber(1),
+          scanStartPositionInBlock: this.bstream.readNumber(1),
+          scanEndPositionInBlock: this.bstream.readNumber(1),
+          successiveApproximationBitPosition: this.bstream.readNumber(1),
+        };
+
+        const rawImageDataStream = this.bstream.tee();
+        let numBytes = 0;
+        // Immediately after SOS header is the compressed image data until the EOI marker is seen.
+        // Seek until we find the EOI marker.
+        while (true) {
+          if (this.bstream.readNumber(1) === 0xFF &&
+              this.bstream.peekNumber(1) === JpegSegmentType.EOI) {
+            jpegMarker = this.bstream.readNumber(1);
+            break;
+          } else {
+            numBytes++;
+          }
+        }
+
+        // NOTE: The below will have the null bytes after every 0xFF value.
+        sos.rawImageData = rawImageDataStream.readBytes(numBytes);
+
+        this.dispatchEvent(new JpegStartOfScanEvent(sos));
+      } // End of SOS
+      else {
+        this.bstream.setBigEndian();
+        const length = this.bstream.peekNumber(2);
+        if (DEBUG) console.log(`Unsupported JPEG marker 0xff${jpegMarker.toString(16)} with length ${length}`);
+        this.bstream.skip(length);
+      }
+    } while (jpegMarker !== JpegSegmentType.EOI);
+  }
+
+  /**
+   * Reads an Image File Directory from stream.
+   * @param {ByteStream} stream The stream to extract the Exif value descriptor.
+   * @param {ByteStream} lookAheadStream The lookahead stream if the offset is used.
+   * @param {Map<number, ExifValue} exifValueMap This map to add the Exif values.
+   * @returns {number} The next IFD offset.
+   */
+  readExifIfd(stream, lookAheadStream, exifValueMap) {
+    let exifOffsetStream;
+    const numDirectoryEntries = stream.readNumber(2);
+    for (let entry = 0; entry < numDirectoryEntries; ++entry) {
+      const exifValue = getExifValue(stream, lookAheadStream, DEBUG);
+      const exifTagNumber = exifValue.tagNumber;
+      exifValueMap.set(exifTagNumber, exifValue);
+      if (exifValue.tagNumber === ExifTagNumber.EXIF_OFFSET) {
+        exifOffsetStream = lookAheadStream.tee().skip(exifValue.numericalValue);
+      }
+    } // Loop over Directory Entries.
+
+    if (exifOffsetStream) {
+      this.readExifIfd(exifOffsetStream, lookAheadStream, exifValueMap);
+    }
+
+    const nextIfdOffset = stream.readNumber(4);
+    return nextIfdOffset;
+  }
+}