Allow use of extend with greyscale input (#623)

src/pipeline.cc

@@ -521,11 +521,21 @@ class PipelineWorker : public Nan::AsyncWorker {
         // Scale up 8-bit values to match 16-bit input image
         double const multiplier = sharp::Is16Bit(image.interpretation()) ? 256.0 : 1.0;
         // Create background colour
-        std::vector<double> background {
-          baton->background[0] * multiplier,
-          baton->background[1] * multiplier,
-          baton->background[2] * multiplier
+        std::vector<double> background;
+        if (image.bands() > 2) {
+          background = {
+            multiplier * baton->background[0],
+            multiplier * baton->background[1],
+            multiplier * baton->background[2]
           };
+        } else {
+          // Convert sRGB to greyscale
+          background = { multiplier * (
+            0.2126 * baton->background[0] +
+            0.7152 * baton->background[1] +
+            0.0722 * baton->background[2]
+          )};
+        }
         // Add alpha channel to background colour
         if (baton->background[3] < 255.0 || HasAlpha(image)) {
           background.push_back(baton->background[3] * multiplier);

test/unit/extend.js

@@ -60,4 +60,19 @@ describe('Extend', function () {
         fixtures.assertSimilar(fixtures.expected('addAlphaChanelBeforeExtend.png'), data, done);
       });
   });
+
+  it('PNG with 2 channels', function (done) {
+    sharp(fixtures.inputPngWithGreyAlpha)
+      .background('transparent')
+      .extend({top: 0, bottom: 20, left: 0, right: 20})
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(true, data.length > 0);
+        assert.strictEqual('png', info.format);
+        assert.strictEqual(420, info.width);
+        assert.strictEqual(320, info.height);
+        assert.strictEqual(4, info.channels);
+        fixtures.assertSimilar(fixtures.expected('extend-2channel.png'), data, done);
+      });
+  });
 });