Allow EXIF metadata to be set/update #650

lib/output.js

@@ -148,9 +148,22 @@ function toBuffer (options, callback) {
  *   .toFile('output-with-metadata.jpg')
  *   .then(info => { ... });
  *
+ * @example
+ * // Set "IFD0-Copyright" in output EXIF metadata
+ * await sharp(input)
+ *   .withMetadata({
+ *     exif: {
+ *       IFD0: {
+ *         Copyright: 'Wernham Hogg'
+ *       }
+ *     }
+ *   })
+ *   .toBuffer();
+ *
  * @param {Object} [options]
  * @param {number} [options.orientation] value between 1 and 8, used to update the EXIF `Orientation` tag.
  * @param {string} [options.icc] filesystem path to output ICC profile, defaults to sRGB.
+ * @param {Object<Object>} [options.exif={}] Object keyed by IFD0, IFD1 etc. of key/value string pairs to write as EXIF data.
  * @returns {Sharp}
  * @throws {Error} Invalid parameters
  */
@@ -171,6 +184,25 @@ function withMetadata (options) {
         throw is.invalidParameterError('icc', 'string filesystem path to ICC profile', options.icc);
       }
     }
+    if (is.defined(options.exif)) {
+      if (is.object(options.exif)) {
+        for (const [ifd, entries] of Object.entries(options.exif)) {
+          if (is.object(entries)) {
+            for (const [k, v] of Object.entries(entries)) {
+              if (is.string(v)) {
+                this.options.withMetadataStrs[`exif-${ifd.toLowerCase()}-${k}`] = v;
+              } else {
+                throw is.invalidParameterError(`exif.${ifd}.${k}`, 'string', v);
+              }
+            }
+          } else {
+            throw is.invalidParameterError(`exif.${ifd}`, 'object', entries);
+          }
+        }
+      } else {
+        throw is.invalidParameterError('exif', 'object', options.exif);
+      }
+    }
   }
   return this;
 }