From d705cffdd60a2941d4c27a9b8e2e292ae8acf31e Mon Sep 17 00:00:00 2001
From: Lovell Fuller <github@lovell.info>
Date: Sun, 12 Aug 2018 20:22:39 +0100
Subject: [PATCH] Ensure extractChannel works with 16-bit images #1330

---
 docs/changelog.md                              |   3 +++
 src/pipeline.cc                                |   5 ++++-
 test/fixtures/expected/extract-alpha-16bit.jpg | Bin 0 -> 685 bytes
 test/unit/extractChannel.js                    |  11 +++++++++++
 4 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 test/fixtures/expected/extract-alpha-16bit.jpg

diff --git a/docs/changelog.md b/docs/changelog.md
index 0a9988a2..e731b8db 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -26,6 +26,9 @@ Requires libvips v8.6.1.
 * Improve install time error messages for FreeBSD users.
   [#1310](https://github.com/lovell/sharp/issues/1310)
 
+* Ensure extractChannel works with 16-bit images.
+  [#1330](https://github.com/lovell/sharp/issues/1330)
+
 * Add experimental entropy field to stats response.
 
 #### v0.20.5 - 27<sup>th</sup> June 2018
diff --git a/src/pipeline.cc b/src/pipeline.cc
index 1328dd89..5cad5f01 100644
--- a/src/pipeline.cc
+++ b/src/pipeline.cc
@@ -694,9 +694,12 @@ class PipelineWorker : public Nan::AsyncWorker {
           (baton->err).append("Cannot extract channel from image. Too few channels in image.");
           return Error();
         }
+        VipsInterpretation const interpretation = sharp::Is16Bit(image.interpretation())
+          ? VIPS_INTERPRETATION_GREY16
+          : VIPS_INTERPRETATION_B_W;
         image = image
           .extract_band(baton->extractChannel)
-          .copy(VImage::option()->set("interpretation", VIPS_INTERPRETATION_B_W));
+          .copy(VImage::option()->set("interpretation", interpretation));
       }
 
       // Remove alpha channel, if any
diff --git a/test/fixtures/expected/extract-alpha-16bit.jpg b/test/fixtures/expected/extract-alpha-16bit.jpg
new file mode 100644
index 0000000000000000000000000000000000000000..58c02669bc6fe3a9b76b4cf202665094884e0fc4
GIT binary patch
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literal 0
HcmV?d00001

diff --git a/test/unit/extractChannel.js b/test/unit/extractChannel.js
index 2b599b7e..060bc968 100644
--- a/test/unit/extractChannel.js
+++ b/test/unit/extractChannel.js
@@ -69,6 +69,17 @@ describe('Image channel extraction', function () {
       });
   });
 
+  it('Alpha from 16-bit PNG', function (done) {
+    const output = fixtures.path('output.extract-alpha-16bit.jpg');
+    sharp(fixtures.inputPngWithTransparency16bit)
+      .extractChannel(3)
+      .toFile(output, function (err, info) {
+        if (err) throw err;
+        fixtures.assertMaxColourDistance(output, fixtures.expected('extract-alpha-16bit.jpg'));
+        done();
+      });
+  });
+
   it('Invalid channel number', function () {
     assert.throws(function () {
       sharp(fixtures.inputJpg)