Add named 'alpha' channel to extractChannel op #2138

docs/api-channel.md

@@ -42,7 +42,7 @@ Extract a single channel from a multi-channel image.
 
 ### Parameters
 
--   `channel` **([number][1] \| [string][2])** zero-indexed band number to extract, or `red`, `green` or `blue` as alternative to `0`, `1` or `2` respectively.
+-   `channel` **([number][1] \| [string][2])** zero-indexed channel/band number to extract, or `red`, `green`, `blue` or `alpha`.
 
 ### Examples
 

docs/changelog.md

@@ -18,6 +18,9 @@ Requires libvips v8.9.1
   Expose `levels` metadata for multi-level images.
   [#2222](https://github.com/lovell/sharp/issues/2222)
 
+* Add support for named `alpha` channel to `extractChannel` operation.
+  [#2138](https://github.com/lovell/sharp/issues/2138)
+
 ### v0.25.3 - 17th May 2020
 
 * Ensure libvips is initialised only once, improves worker thread safety.

lib/channel.js

@@ -60,22 +60,19 @@ function ensureAlpha () {
  *     // green.jpg is a greyscale image containing the green channel of the input
  *    });
  *
- * @param {number|string} channel - zero-indexed band number to extract, or `red`, `green` or `blue` as alternative to `0`, `1` or `2` respectively.
+ * @param {number|string} channel - zero-indexed channel/band number to extract, or `red`, `green`, `blue` or `alpha`.
  * @returns {Sharp}
  * @throws {Error} Invalid channel
  */
 function extractChannel (channel) {
-  if (channel === 'red') {
-    channel = 0;
-  } else if (channel === 'green') {
-    channel = 1;
-  } else if (channel === 'blue') {
-    channel = 2;
+  const channelMap = { red: 0, green: 1, blue: 2, alpha: 3 };
+  if (Object.keys(channelMap).includes(channel)) {
+    channel = channelMap[channel];
   }
   if (is.integer(channel) && is.inRange(channel, 0, 4)) {
     this.options.extractChannel = channel;
   } else {
-    throw is.invalidParameterError('channel', 'integer or one of: red, green, blue', channel);
+    throw is.invalidParameterError('channel', 'integer or one of: red, green, blue, alpha', channel);
   }
   return this;
 }

src/pipeline.cc

@@ -645,9 +645,13 @@ class PipelineWorker : public Napi::AsyncWorker {
       // Extract an image channel (aka vips band)
       if (baton->extractChannel > -1) {
         if (baton->extractChannel >= image.bands()) {
+          if (baton->extractChannel == 3 && sharp::HasAlpha(image)) {
+            baton->extractChannel = image.bands() - 1;
+          } else {
             (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;

test/unit/extractChannel.js

@@ -80,6 +80,19 @@ describe('Image channel extraction', function () {
       });
   });
 
+  it('Alpha from 2-channel input', function (done) {
+    const output = fixtures.path('output.extract-alpha-2-channel.png');
+    sharp(fixtures.inputPngWithGreyAlpha)
+      .extractChannel('alpha')
+      .toColourspace('b-w')
+      .toFile(output, function (err, info) {
+        if (err) throw err;
+        assert.strictEqual(1, info.channels);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('extract-alpha-2-channel.png'));
+        done();
+      });
+  });
+
   it('Invalid channel number', function () {
     assert.throws(function () {
       sharp(fixtures.inputJpg)