Add chroma subsampling options for JPEG output

test/bench/package.json

@@ -12,7 +12,7 @@
     "imagemagick-native": "^1.7.0",
     "gm": "^1.17.0",
     "async": "^0.9.0",
-    "semver": "^4.2.0",
+    "semver": "^4.3.0",
     "benchmark": "^1.0.0"
   },
   "license": "Apache 2.0",

test/bench/perf.js

@@ -347,6 +347,18 @@ async.series({
           }
         });
       }
+    }).add('sharp-without-chroma-subsampling', {
+      defer: true,
+      fn: function(deferred) {
+        sharp(inputJpgBuffer).resize(width, height).withoutChromaSubsampling().toBuffer(function(err, buffer) {
+          if (err) {
+            throw err;
+          } else {
+            assert.notStrictEqual(null, buffer);
+            deferred.resolve();
+          }
+        });
+      }
     }).add('sharp-rotate', {
       defer: true,
       fn: function(deferred) {

test/unit/io.js

@@ -459,6 +459,35 @@ describe('Input/output', function() {
 
   });
 
+  it('Without chroma subsampling generates larger file', function(done) {
+    // First generate with chroma subsampling (default)
+    sharp(fixtures.inputJpg)
+      .resize(320, 240)
+      .withoutChromaSubsampling(false)
+      .toBuffer(function(err, withChromaSubsamplingData, withChromaSubsamplingInfo) {
+        if (err) throw err;
+        assert.strictEqual(true, withChromaSubsamplingData.length > 0);
+        assert.strictEqual(withChromaSubsamplingData.length, withChromaSubsamplingInfo.size);
+        assert.strictEqual('jpeg', withChromaSubsamplingInfo.format);
+        assert.strictEqual(320, withChromaSubsamplingInfo.width);
+        assert.strictEqual(240, withChromaSubsamplingInfo.height);
+        // Then generate without
+        sharp(fixtures.inputJpg)
+          .resize(320, 240)
+          .withoutChromaSubsampling()
+          .toBuffer(function(err, withoutChromaSubsamplingData, withoutChromaSubsamplingInfo) {
+            if (err) throw err;
+            assert.strictEqual(true, withoutChromaSubsamplingData.length > 0);
+            assert.strictEqual(withoutChromaSubsamplingData.length, withoutChromaSubsamplingInfo.size);
+            assert.strictEqual('jpeg', withoutChromaSubsamplingInfo.format);
+            assert.strictEqual(320, withoutChromaSubsamplingInfo.width);
+            assert.strictEqual(240, withoutChromaSubsamplingInfo.height);
+            assert.strictEqual(true, withChromaSubsamplingData.length < withoutChromaSubsamplingData.length);
+            done();
+          });
+      });
+  });
+
   it('Convert SVG, if supported, to PNG', function(done) {
     sharp(fixtures.inputSvg)
       .resize(100, 100)