1 // Copyright (c) 2012 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 'use strict'; 6 7 /** 8 * A namespace class for image encoding functions. All methods are static. 9 */ 10 function ImageEncoder() {} 11 12 /** 13 * @type {Array.<Object>} 14 */ 15 ImageEncoder.metadataEncoders = {}; 16 17 /** 18 * @param {function(new:ImageEncoder.MetadataEncoder)} constructor 19 * // TODO(JSDOC). 20 * @param {string} mimeType // TODO(JSDOC). 21 */ 22 ImageEncoder.registerMetadataEncoder = function(constructor, mimeType) { 23 ImageEncoder.metadataEncoders[mimeType] = constructor; 24 }; 25 26 /** 27 * Create a metadata encoder. 28 * 29 * The encoder will own and modify a copy of the original metadata. 30 * 31 * @param {Object} metadata Original metadata. 32 * @return {ImageEncoder.MetadataEncoder} Created metadata encoder. 33 */ 34 ImageEncoder.createMetadataEncoder = function(metadata) { 35 var constructor = ImageEncoder.metadataEncoders[metadata.mimeType] || 36 ImageEncoder.MetadataEncoder; 37 return new constructor(metadata); 38 }; 39 40 41 /** 42 * Create a metadata encoder object holding a copy of metadata 43 * modified according to the properties of the supplied image. 44 * 45 * @param {Object} metadata Original metadata. 46 * @param {HTMLCanvasElement} canvas Canvas to use for metadata. 47 * @param {number} quality Encoding quality (defaults to 1). 48 * @return {ImageEncoder.MetadataEncoder} Encoder with encoded metadata. 49 */ 50 ImageEncoder.encodeMetadata = function(metadata, canvas, quality) { 51 var encoder = ImageEncoder.createMetadataEncoder(metadata); 52 encoder.setImageData(canvas); 53 encoder.setThumbnailData(ImageEncoder.createThumbnail(canvas), quality || 1); 54 return encoder; 55 }; 56 57 58 /** 59 * Return a blob with the encoded image with metadata inserted. 60 * @param {HTMLCanvasElement} canvas The canvas with the image to be encoded. 61 * @param {ImageEncoder.MetadataEncoder} metadataEncoder Encoder to use. 62 * @param {number} quality (0..1], Encoding quality, defaults to 0.9. 63 * @return {Blob} encoded data. 64 */ 65 ImageEncoder.getBlob = function(canvas, metadataEncoder, quality) { 66 // Contrary to what one might think 1.0 is not a good default. Opening and 67 // saving an typical photo taken with consumer camera increases its file size 68 // by 50-100%. 69 // Experiments show that 0.9 is much better. It shrinks some photos a bit, 70 // keeps others about the same size, but does not visibly lower the quality. 71 quality = quality || 0.9; 72 73 ImageUtil.trace.resetTimer('dataurl'); 74 // WebKit does not support canvas.toBlob yet so canvas.toDataURL is 75 // the only way to use the Chrome built-in image encoder. 76 var dataURL = 77 canvas.toDataURL(metadataEncoder.getMetadata().mimeType, quality); 78 ImageUtil.trace.reportTimer('dataurl'); 79 80 var encodedImage = ImageEncoder.decodeDataURL(dataURL); 81 82 var encodedMetadata = metadataEncoder.encode(); 83 84 var slices = []; 85 86 // TODO(kaznacheev): refactor |stringToArrayBuffer| and |encode| to return 87 // arrays instead of array buffers. 88 function appendSlice(arrayBuffer) { 89 slices.push(new DataView(arrayBuffer)); 90 } 91 92 ImageUtil.trace.resetTimer('blob'); 93 if (encodedMetadata.byteLength != 0) { 94 var metadataRange = metadataEncoder.findInsertionRange(encodedImage); 95 appendSlice(ImageEncoder.stringToArrayBuffer( 96 encodedImage, 0, metadataRange.from)); 97 98 appendSlice(metadataEncoder.encode()); 99 100 appendSlice(ImageEncoder.stringToArrayBuffer( 101 encodedImage, metadataRange.to, encodedImage.length)); 102 } else { 103 appendSlice(ImageEncoder.stringToArrayBuffer( 104 encodedImage, 0, encodedImage.length)); 105 } 106 var blob = new Blob(slices, {type: metadataEncoder.getMetadata().mimeType}); 107 ImageUtil.trace.reportTimer('blob'); 108 return blob; 109 }; 110 111 /** 112 * Decode a dataURL into a binary string containing the encoded image. 113 * 114 * Why return a string? Calling atob and having the rest of the code deal 115 * with a string is several times faster than decoding base64 in Javascript. 116 * 117 * @param {string} dataURL Data URL to decode. 118 * @return {string} A binary string (char codes are the actual byte values). 119 */ 120 ImageEncoder.decodeDataURL = function(dataURL) { 121 // Skip the prefix ('data:image/<type>;base64,') 122 var base64string = dataURL.substring(dataURL.indexOf(',') + 1); 123 return atob(base64string); 124 }; 125 126 /** 127 * Return a thumbnail for an image. 128 * @param {HTMLCanvasElement} canvas Original image. 129 * @param {number=} opt_shrinkage Thumbnail should be at least this much smaller 130 * than the original image (in each dimension). 131 * @return {HTMLCanvasElement} Thumbnail canvas. 132 */ 133 ImageEncoder.createThumbnail = function(canvas, opt_shrinkage) { 134 var MAX_THUMBNAIL_DIMENSION = 320; 135 136 opt_shrinkage = Math.max(opt_shrinkage || 4, 137 canvas.width / MAX_THUMBNAIL_DIMENSION, 138 canvas.height / MAX_THUMBNAIL_DIMENSION); 139 140 var thumbnailCanvas = canvas.ownerDocument.createElement('canvas'); 141 thumbnailCanvas.width = Math.round(canvas.width / opt_shrinkage); 142 thumbnailCanvas.height = Math.round(canvas.height / opt_shrinkage); 143 144 var context = thumbnailCanvas.getContext('2d'); 145 context.drawImage(canvas, 146 0, 0, canvas.width, canvas.height, 147 0, 0, thumbnailCanvas.width, thumbnailCanvas.height); 148 149 return thumbnailCanvas; 150 }; 151 152 /** 153 * TODO(JSDOC) 154 * @param {string} string // TODO(JSDOC). 155 * @param {number} from // TODO(JSDOC). 156 * @param {number} to // TODO(JSDOC). 157 * @return {ArrayBuffer} // TODO(JSDOC). 158 */ 159 ImageEncoder.stringToArrayBuffer = function(string, from, to) { 160 var size = to - from; 161 var array = new Uint8Array(size); 162 for (var i = 0; i != size; i++) { 163 array[i] = string.charCodeAt(from + i); 164 } 165 return array.buffer; 166 }; 167 168 /** 169 * A base class for a metadata encoder. 170 * 171 * Serves as a default metadata encoder for images that none of the metadata 172 * parsers recognized. 173 * 174 * @param {Object} original_metadata Starting metadata. 175 * @constructor 176 */ 177 ImageEncoder.MetadataEncoder = function(original_metadata) { 178 this.metadata_ = MetadataCache.cloneMetadata(original_metadata) || {}; 179 if (this.metadata_.mimeType != 'image/jpeg') { 180 // Chrome can only encode JPEG and PNG. Force PNG mime type so that we 181 // can save to file and generate a thumbnail. 182 this.metadata_.mimeType = 'image/png'; 183 } 184 }; 185 186 /** 187 * TODO(JSDOC) 188 * @return {Object} // TODO(JSDOC). 189 */ 190 ImageEncoder.MetadataEncoder.prototype.getMetadata = function() { 191 return this.metadata_; 192 }; 193 194 /** 195 * @param {HTMLCanvasElement|Object} canvas Canvas or or anything with 196 * width and height properties. 197 */ 198 ImageEncoder.MetadataEncoder.prototype.setImageData = function(canvas) { 199 this.metadata_.width = canvas.width; 200 this.metadata_.height = canvas.height; 201 }; 202 203 /** 204 * @param {HTMLCanvasElement} canvas Canvas to use as thumbnail. 205 * @param {number} quality Thumbnail quality. 206 */ 207 ImageEncoder.MetadataEncoder.prototype.setThumbnailData = 208 function(canvas, quality) { 209 this.metadata_.thumbnailURL = 210 canvas.toDataURL(this.metadata_.mimeType, quality); 211 }; 212 213 /** 214 * Return a range where the metadata is (or should be) located. 215 * @param {string} encodedImage // TODO(JSDOC). 216 * @return {Object} An object with from and to properties. 217 */ 218 ImageEncoder.MetadataEncoder.prototype. 219 findInsertionRange = function(encodedImage) { return {from: 0, to: 0} }; 220 221 /** 222 * Return serialized metadata ready to write to an image file. 223 * The return type is optimized for passing to Blob.append. 224 * @return {ArrayBuffer} // TODO(JSDOC). 225 */ 226 ImageEncoder.MetadataEncoder.prototype.encode = function() { 227 return new Uint8Array(0).buffer; 228 }; 229
