Add extractChannel operation to extract a channel from an image (#497)

test/unit/extractChannel.js

@@ -0,0 +1,72 @@
+'use strict';
+
+var assert = require('assert');
+
+var sharp = require('../../index');
+var fixtures = require('../fixtures');
+
+describe('Image channel extraction', function() {
+
+  it('Red channel', function(done) {
+    sharp(fixtures.inputJpg)
+      .extractChannel('red')
+      .resize(320,240)
+      .toBuffer(function(err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('extract-red.jpg'), data, done);
+      });
+  });
+
+  it('Green channel', function(done) {
+    sharp(fixtures.inputJpg)
+      .extractChannel('green')
+      .resize(320,240)
+      .toBuffer(function(err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('extract-green.jpg'), data, done);
+      });
+  });
+
+  it('Blue channel', function(done) {
+    sharp(fixtures.inputJpg)
+      .extractChannel('blue')
+      .resize(320,240)
+      .toBuffer(function(err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('extract-blue.jpg'), data, done);
+      });
+  });
+
+  it('Blue channel by number', function(done) {
+    sharp(fixtures.inputJpg)
+      .extractChannel(2)
+      .resize(320,240)
+      .toBuffer(function(err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('extract-blue.jpg'), data, done);
+      });
+  });
+
+  it('Invalid channel number', function() {
+    assert.throws(function() {
+      sharp(fixtures.inputJpg)
+        .extractChannel(-1);
+    });
+  });
+
+  it('No arguments', function() {
+    assert.throws(function() {
+      sharp(fixtures.inputJpg)
+        .extractChannel();
+    });
+  });
+
+});