Add ensureAlpha op, adds alpha channel if missing #1153

lib/channel.js

@@ -29,6 +29,23 @@ function removeAlpha () {
   return this;
 }
 
+/**
+ * Ensure alpha channel, if missing. The added alpha channel will be fully opaque. This is a no-op if the image already has an alpha channel.
+ *
+ * @example
+ * sharp('rgb.jpg')
+ *   .ensureAlpha()
+ *   .toFile('rgba.png', function(err, info) {
+ *     // rgba.png is a 4 channel image with a fully opaque alpha channel
+ *   });
+ *
+ * @returns {Sharp}
+ */
+function ensureAlpha () {
+  this.options.ensureAlpha = true;
+  return this;
+}
+
 /**
  * Extract a single channel from a multi-channel image.
  *
@@ -120,6 +137,7 @@ module.exports = function (Sharp) {
   Object.assign(Sharp.prototype, {
     // Public instance functions
     removeAlpha,
+    ensureAlpha,
     extractChannel,
     joinChannel,
     bandbool

lib/constructor.js

@@ -144,6 +144,7 @@ const Sharp = function (input, options) {
     joinChannelIn: [],
     extractChannel: -1,
     removeAlpha: false,
+    ensureAlpha: false,
     colourspace: 'srgb',
     // overlay
     overlayGravity: 0,