feat: add comment tag encoding (#87)

index.d.ts

@@ -10,7 +10,7 @@ type UintArrRet = RawImageData<Uint8Array>;
 type ImageData = BufferRet | UintArrRet;
 type BufferLike = Buffer | Uint8Array | ArrayLike<number> | Iterable<number> | ArrayBuffer;
 
-export declare function encode(imgData: RawImageData<BufferLike>, quality?: number): BufferRet;
+export declare function encode(imgData: RawImageData<BufferLike> & {comments?: string[]}, quality?: number): BufferRet
 
 export declare function decode(
   jpegData: BufferLike,

lib/encoder.js

@@ -535,6 +535,20 @@ function JPEGEncoder(quality) {
 				writeByte(std_ac_chrominance_values[p]);
 			}
 		}
+
+		function writeCOM(comments)
+		{
+			if (typeof comments === "undefined" || comments.constructor !== Array) return;
+			comments.forEach(e => {
+				if (typeof e !== "string") return;
+				writeWord(0xFFFE); // marker
+				var l = e.length;
+				writeWord(l + 2); // length itself as well
+				var i;
+				for (i = 0; i < l; i++)
+					writeByte(e.charCodeAt(i));
+			});
+		}
 
 		function writeSOS()
 		{
@@ -625,6 +639,7 @@ function JPEGEncoder(quality) {
 			// Add JPEG headers
 			writeWord(0xFFD8); // SOI
 			writeAPP0();
+			writeCOM(image.comments);
 			writeAPP1(image.exifBuffer);
 			writeDQT();
 			writeSOF0(image.width,image.height);
@@ -782,7 +797,7 @@ function encode(imgData, qu) {
   return {
     data: data,
     width: imgData.width,
-    height: imgData.height
+    height: imgData.height,
   };
 }
 

test/index.js

@@ -179,6 +179,32 @@ it('should be able to create a JPEG from an array', function () {
   expect(jpegImageData.data).toEqual(expected);
 });
 
+it('should be able to create a JPEG from an array with comment', function () {
+  var width = 320,
+    height = 180;
+  var comments = ["First comment", "Second comment"];
+  var frameData = new Buffer(width * height * 4);
+  var i = 0;
+  while (i < frameData.length) {
+    frameData[i++] = 0xff; // red
+    frameData[i++] = 0x00; // green
+    frameData[i++] = 0x00; // blue
+    frameData[i++] = 0xff; // alpha - ignored in JPEGs
+  }
+  var rawImageData = {
+    data: frameData,
+    width: width,
+    height: height,
+    comments: comments,
+  };
+  var jpegImageData = jpeg.encode(rawImageData, 50); 
+  expect(jpegImageData.width).toEqual(width);
+  expect(jpegImageData.height).toEqual(height);
+  var expected = fixture('redbox_comment.jpg');
+  expect(jpegImageData.data).toEqual(expected);
+  expect(jpeg.decode(jpegImageData.data).comments).toEqual(['First comment', 'Second comment']);
+});
+
 it('should be able to decode a JPEG into a typed array', function () {
   var jpegData = fixture('grumpycat.jpg');
   var rawImageData = jpeg.decode(jpegData, {useTArray: true});