Inspired by a forum post over at CreativeIreland, I thought I’d sit down and create a simple AIR application that could read the metadata contained in image files. Unfortunately, reading image metadata turned out to be not as simple as I had hoped. Thankfully, my friend Google turned me on to the wonderful and lightweight java class by Marco Schmidt named ImageInfo.java, which I promptly ported to actionscript. Well, only partially ported, to be precise. I only included the file types supported by Flash (.jpg, .gif, and .png) and didn’t bother with comments or multiple pictures (for animated .gif files). If anyone wants to port the whole shebang, knock yourself out. If you do, post a comment and let me know the result. In any case, the following is what I came up with:
ImageInfo.as

package com.onebyonedesign.utils { import flash.utils.ByteArray; /** * ImageInfo.as * * 2/17/2008 10:37 AM * * Retries metadata from .png, .jpg, and .gif files. * * Based on java class, ImageInfo.java, by Marco Schmidt * (http://schmidt.devlib.org/image-info/) * * @author Devon O. * @version .1 * */ public class ImageInfo { private var _stream:ByteArray; // image properties private var _width:int; private var _height:int; private var _bitsPerPixel:int; private var _progressive:Boolean; private var _physicalWidth:String; private var _physicalHeight:String; private var _physicalHeightDpi:int; private var _physicalWidthDpi:int; private var _fileType:String; private var _mimeType:String; // private var _format:int; private const FILE_TYPES:Array = ["JPEG", "GIF", "PNG"]; private const MIME_TYPES:Array = ["image/jpeg", "image/gif", "image/png"]; private const FORMAT_JPEG:int = 0; private const FORMAT_GIF:int = 1; private const FORMAT_PNG:int = 2; /** * ImageInfo checks an URLStream to see if it is a valid image file. * If it is, information from the file's metadata can be retrieved from the ImageInfo instance. * * @example Typical usage: *  * package { * * import com.onebyonedesign.utils.ImageInfo; * import flash.display.Sprite; * import flash.events.Event; * import flash.net.URLRequest; * import flash.net.URLStream; * import flash.utils.ByteArray; * * public class Test extends Sprite { * * private var request = new URLRequest("someImage.jpg"); * * public function Test():void { * var imageStream:URLStream = new URLStream(); * imageStream.addEventListener(Event.COMPLETE, onImageLoad); * imageStream.load(request); * } * * private function onImageLoad(e:Event):void { * var stream:URLStream = e.currentTarget as URLStream; * var ba:ByteArray = new ByteArray(); * stream.readBytes(ba); * var info:ImageInfo = new ImageInfo(); * if (info.checkType(ba)) { * trace ("file = " + request.url); * trace ("horizontal DPI = " + info.physicalWidthDpi); * trace ("vertical DPI = " + info.physicalHeightDpi); * trace ("pixel width = " + info.width); * trace ("pixel height = " + info.height); * trace ("bit depth = " + info.bitsPerPixel); * trace ("progressive image = " + info.progressive); * trace ("actual width = " + info.physicalWidth + " cm."); * trace ("actual height = " + info.physicalHeight + " cm."); * trace ("file type = " + info.fileType); * trace ("mime type = " + info.mimeType); * } else { * trace (request.url + " is not a valid image file."); * } * } * } * } * * * Typical Output: * * file = someImage.jpg * horizontal DPI = 240 * vertical DPI = 240 * pixel width = 480 * pixel height = 640 * bit depth = 24 * progressive image = false * actual width = 5.08 cm. * actual height = 6.77 cm. * file type = JPEG * mime type = image/jpeg */ public function ImageInfo():void { }; /** * * @param ByteArray of image file * @return True for valid .jpg, .png, and .gif images - false otherwise. */ public function checkType(bytes:ByteArray):Boolean { _stream = bytes; var b1:int = read() & 0xFF; var b2:int = read() & 0xFF; if (b1 == 0x47 && b2 == 0x49) { return checkGif(); } if (b1 == 0xFF && b2 == 0xD8) { return checkJPG(); } if (b1 == 0x89 && b2 == 0x50) { return checkPng(); } return false; } private function checkGif():Boolean { var GIF_MAGIC_87A:ByteArray = new ByteArray() GIF_MAGIC_87A.writeByte(0x46); GIF_MAGIC_87A.writeByte(0x38); GIF_MAGIC_87A.writeByte(0x37); GIF_MAGIC_87A.writeByte(0x61); var GIF_MAGIC_89A:ByteArray = new ByteArray(); GIF_MAGIC_89A.writeByte(0x46); GIF_MAGIC_89A.writeByte(0x38); GIF_MAGIC_89A.writeByte(0x39); GIF_MAGIC_89A.writeByte(0x61); var a:ByteArray = new ByteArray(); if (read(a, 0, 11) != 11) { return false; } if ((!equals(a, 0, GIF_MAGIC_89A, 0, 4)) && (!equals(a, 0, GIF_MAGIC_87A, 0, 4))) { return false; } _format = FORMAT_GIF; _width = getShortLittleEndian(a, 4); _height = getShortLittleEndian(a, 6); var flags:int = a[8] & 0xFF; _bitsPerPixel = ((flags >> 4) & 0x07) + 1; _progressive = (flags & 0x02) != 0; return true; } private function checkPng():Boolean { var PNG_MAGIC:ByteArray = new ByteArray(); PNG_MAGIC.writeByte(0x4E); PNG_MAGIC.writeByte(0x47); PNG_MAGIC.writeByte(0x0D); PNG_MAGIC.writeByte(0x0A); PNG_MAGIC.writeByte(0x1A); PNG_MAGIC.writeByte(0x0A); var a:ByteArray = new ByteArray(); if (read(a, 0, 27) != 27) { return false; } if (!equals(a, 0, PNG_MAGIC, 0, 6)) { return false; } _format = FORMAT_PNG; _width = getIntBigEndian(a, 14); _height = getIntBigEndian(a, 18); _bitsPerPixel = a[22]; var colorType:int = a[23]; if (colorType == 2 || colorType == 6) { _bitsPerPixel *= 3; } _progressive = (a[26]) != 0; return true; } private function checkJPG():Boolean { var APP0_ID:ByteArray = new ByteArray(); APP0_ID.writeByte(0x4A); APP0_ID.writeByte(0x46); APP0_ID.writeByte(0x49); APP0_ID.writeByte(0x46); APP0_ID.writeByte(0x00); var data:ByteArray = new ByteArray(); while (true) { if (read(data, 0, 4) != 4) { return false; } var marker:Number = getShortBigEndian(data, 0); var size:Number = getShortBigEndian(data, 2); if ((marker & 0xff00) != 0xff00) { return false; } if (marker == 0xFFE0) { if (size < 14) { skip(size - 2); continue; } if (read(data, 0, 12) != 12) { return false; } if (equals(APP0_ID, 0, data, 0, 5)) { if (data[7] == 1) { setPhysicalWidthDpi(getShortBigEndian(data, 8)); setPhysicalHeightDpi(getShortBigEndian(data, 10)); } else if (data[7] == 2) { var x:int = getShortBigEndian(data, 8); var y:int = getShortBigEndian(data, 10); setPhysicalWidthDpi(int(x * 2.54)); setPhysicalHeightDpi(int(y * 2.54)); } } skip(size - 14); } else if (marker >= 0xFFC0 && marker <= 0xFFCF && marker != 0xFFC4 && marker != 0xFFC8) { if (read(data, 0, 6) != 6) { return false; } _format = FORMAT_JPEG; _bitsPerPixel = (data[0] & 0xFF) * (data[5] & 0xFF); _progressive = marker == 0xffc2 || marker == 0xffc6 || marker == 0xffca || marker == 0xffce; _width = getShortBigEndian(data, 3); _height = getShortBigEndian(data, 1); var horzPixelsPerCM:Number = _physicalWidthDpi / 2.54; var vertPixelsPerCM:Number = _physicalHeightDpi / 2.54 _physicalWidth = (_width / horzPixelsPerCM).toFixed(2); _physicalHeight = (_height / vertPixelsPerCM).toFixed(2); return true; } else { skip(size - 2); } } return false; } private function getShortBigEndian(ba:ByteArray, offset:int):Number { return ba[offset] << 8 | ba[offset + 1]; } private function getShortLittleEndian(ba:ByteArray, offset:int):int { return ba[offset] | ba[offset + 1] << 8; } private function getIntBigEndian(ba:ByteArray, offset:int):int { return ba[offset] << 24 | ba[offset + 1] << 16 | ba[offset + 2] << 8 | ba[offset + 3]; } private function skip(numBytes:int):void { _stream.position += numBytes; } private function equals(ba1:ByteArray, offs1:int, ba2:ByteArray, offs2:int, num:int):Boolean { while (num-- > 0) { if (ba1[offs1++] != ba2[offs2++]) { return false; } } return true; } private function read(... args):int { switch (args.length) { case 0 : return _stream.readByte(); break; case 1 : _stream.readBytes(args[0]); return args[0].length; break; case 3 : _stream.readBytes(args[0], args[1], args[2]); return args[2]; break; default : throw new ArgumentError("Argument Error at ImageInfo.read(). Expected 0, 1, or 3. Received " + args.length); return null; } } private function setPhysicalHeightDpi(newValue:int):void { _physicalWidthDpi = newValue; } private function setPhysicalWidthDpi(newValue:int):void { _physicalHeightDpi = newValue; } // Read Only Properties of image // vertical and horizontal DPI public function get physicalHeightDpi():int { return _physicalHeightDpi; } public function get physicalWidthDpi():int { return _physicalWidthDpi; } // bit depth public function get bitsPerPixel():int { return _bitsPerPixel; } // width and height in pixels public function get height():int { return _height; } public function get width():int { return _width; } // progressive or not public function get progressive():Boolean { return _progressive; } // file and mimetype public function get fileType():String { return FILE_TYPES[_format]; } public function get mimeType():String { return MIME_TYPES[_format]; } // height and width in centimeters public function get physicalWidth():String { return _physicalWidth; } public function get physicalHeight():String { return _physicalHeight; } } }

Usage is a breeze. Just load the image file into an URLStream instance, read the bytes into a ByteArray, then pass that ByteArray to the checkType() method (there’s some example code included in the .as file to play around with).
Keep in mind, I ported this class to be used in the comfort and safety of a desktop application. Anyone, choosing to use the Loader.loadBytes() method in an online application may want to check out this blog post first.
That said, the OBO Image Info AIR file is now complete. It’s a very simple thing. Just drag a supported image file into a box and a new window will pop up containing that image’s metadata and a thumbnail preview. Click on the preview and check out the image in full scale.
A couple caveats: The application requires the AIR beta 3 runtime, and, as mentioned, it only supports .jpg, .gif, and .png.
Here’s a screenshot.
And you can download the zipped .air file here, if interested.