Ensure extractChannel sets bw colourspace interp #1257

lovell · Jun 19, 2018 · 94607b5 · 94607b5
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diff --git a/docs/changelog.md b/docs/changelog.md
@@ -10,6 +10,10 @@ Requires libvips v8.6.1.
   [#1241](https://github.com/lovell/sharp/issues/1241)
   [@anahit42](https://github.com/anahit42)
 
+* Ensure extractChannel sets correct single-channel colour space interpretation.
+  [#1257](https://github.com/lovell/sharp/issues/1257)
+  [@jeremychone](https://github.com/jeremychone)
+
 #### v0.20.3 - 29<sup>th</sup> May 2018
 
 * Fix tint operation by ensuring LAB interpretation and allowing negative values.

diff --git a/src/pipeline.cc b/src/pipeline.cc
@@ -694,7 +694,9 @@ class PipelineWorker : public Nan::AsyncWorker {
           (baton->err).append("Cannot extract channel from image. Too few channels in image.");
           return Error();
         }
-        image = image.extract_band(baton->extractChannel);
+        image = image
+          .extract_band(baton->extractChannel)
+          .copy(VImage::option()->set("interpretation", VIPS_INTERPRETATION_B_W));
       }
       // Convert image to sRGB, if not already
       if (sharp::Is16Bit(image.interpretation())) {

diff --git a/test/fixtures/expected/extract-lch.jpg b/test/fixtures/expected/extract-lch.jpg
diff --git a/test/unit/extractChannel.js b/test/unit/extractChannel.js
@@ -54,6 +54,21 @@ describe('Image channel extraction', function () {
       });
   });
 
+  it('With colorspace conversion', function (done) {
+    const output = fixtures.path('output.extract-lch.jpg');
+    sharp(fixtures.inputJpg)
+      .toColourspace('lch')
+      .extractChannel(1)
+      .resize(320, 240)
+      .toFile(output, function (err, info) {
+        if (err) throw err;
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('extract-lch.jpg'));
+        done();
+      });
+  });
+
   it('Invalid channel number', function () {
     assert.throws(function () {
       sharp(fixtures.inputJpg)