Premultiply alpha channel to avoid dark artifacts during tranformation

Add `Sharp.compare(file1, file2, callback)` function for comparing images
using mean squared error (MSE). This is useful for unit tests.

See:
- https://github.com/jcupitt/libvips/issues/291
- http://entropymine.com/imageworsener/resizealpha/

test/unit/alpha.js

@@ -1,9 +1,8 @@
 'use strict';
 
 var assert = require('assert');
-
-var sharp = require('../../index');
 var fixtures = require('../fixtures');
+var sharp = require('../../index');
 
 sharp.cache(0);
 
@@ -76,4 +75,30 @@ describe('Alpha transparency', function() {
       });
   });
 
+  it('Enlargement with non-nearest neighbor interpolation shouldn’t cause dark edges', function(done) {
+    var BASE_NAME = 'alpha-premultiply-enlargement-2048x1536-paper.png';
+    var actual = fixtures.path('output.' + BASE_NAME);
+    var expected = fixtures.expected(BASE_NAME);
+    sharp(fixtures.inputPngAlphaPremultiplicationSmall)
+      .resize(2048, 1536)
+      .interpolateWith('bicubic')
+      .toFile(actual, function(err) {
+        if (err) throw err;
+        fixtures.assertEqual(actual, expected, done);
+      });
+  });
+
+  it('Reduction with non-nearest neighbor interpolation shouldn’t cause dark edges', function(done) {
+    var BASE_NAME = 'alpha-premultiply-reduction-1024x768-paper.png';
+    var actual = fixtures.path('output.' + BASE_NAME);
+    var expected = fixtures.expected(BASE_NAME);
+    sharp(fixtures.inputPngAlphaPremultiplicationLarge)
+      .resize(1024, 768)
+      .interpolateWith('bicubic')
+      .toFile(actual, function(err) {
+        if (err) throw err;
+        fixtures.assertEqual(actual, expected, done);
+      });
+  });
+
 });

test/unit/compare.js

@@ -0,0 +1,77 @@
+'use strict';
+
+var assert = require('assert');
+var fixtures = require('../fixtures');
+var fs = require('fs');
+var sharp = require('../../index');
+
+sharp.cache(0);
+
+
+// Constants
+var MAX_ALLOWED_MEAN_SQUARED_ERROR = 0.0005;
+
+// Tests
+describe('sharp.compare', function() {
+  it('should report equality when comparing an image to itself', function(done) {
+    var image = fixtures.inputPngOverlayLayer0;
+
+    sharp.compare(image, image, function (error, info) {
+      if (error) return done(error);
+
+      assert.strictEqual(info.isEqual, true, 'image is equal to itself');
+      assert.strictEqual(info.status, 'success', 'status is correct');
+      assert(0 <= info.meanSquaredError &&
+        info.meanSquaredError <= MAX_ALLOWED_MEAN_SQUARED_ERROR,
+        'MSE is within tolerance');
+      done();
+    });
+  });
+
+  it('should report that two images have a mismatched number of bands (channels)', function(done) {
+    var actual = fixtures.inputPngOverlayLayer1;
+    var expected = fixtures.inputJpg;
+
+    sharp.compare(actual, expected, function (error, info) {
+      if (error) return done(error);
+
+      assert.strictEqual(info.isEqual, false);
+      assert.strictEqual(info.status, 'mismatchedBands');
+      assert(typeof info.meanSquaredError === 'undefined', 'MSE is undefined');
+      done();
+    });
+  });
+
+  it('should report that two images have a mismatched dimensions', function(done) {
+    var actual = fixtures.inputJpg;
+    var expected = fixtures.inputJpgWithExif;
+
+    sharp.compare(actual, expected, function (error, info) {
+      if (error) return done(error);
+
+      assert.strictEqual(info.isEqual, false);
+      assert.strictEqual(info.status, 'mismatchedDimensions');
+      assert(typeof info.meanSquaredError === 'undefined', 'MSE is undefined');
+      done();
+    });
+  });
+
+  it('should report the correct mean squared error for two different images', function(done) {
+    var actual = fixtures.inputPngOverlayLayer0;
+    var expected = fixtures.inputPngOverlayLayer1;
+
+    sharp.compare(actual, expected, function (error, info) {
+      if (error) return done(error);
+
+      var meanSquaredError = info.meanSquaredError;
+      assert.strictEqual(info.isEqual, false);
+      assert.strictEqual(info.status, 'success');
+      // ImageMagick reports: 42242.5
+      // `compare -metric mse 'actual' 'expected' comparison.png`
+      assert(41900 <= meanSquaredError && meanSquaredError <= 41950,
+        'Expected: 41900 <= meanSquaredError <= 41950. Actual: ' + meanSquaredError);
+      done();
+    });
+  });
+
+});

test/unit/overlay.js

@@ -6,19 +6,78 @@ var sharp = require('../../index');
 
 sharp.cache(0);
 
-// Main
+
+// Constants
+var MAX_ALLOWED_IMAGE_MAGICK_MEAN_SQUARED_ERROR = 0.3;
+
+// Helpers
+var getPaths = function(baseName, extension) {
+  if (typeof extension === 'undefined') {
+    extension = 'png';
+  }
+
+  var actual = fixtures.path('output.' + baseName + '.' + extension);
+  var expected = fixtures.expected(baseName + '.' + extension);
+  var expectedMagick = fixtures.expected(baseName + '-imagemagick.' + extension);
+
+  return {
+    actual: actual,
+    expected: expected,
+    expectedMagick: expectedMagick
+  };
+};
+
+var assertEqual = function (paths, callback) {
+  if (typeof callback !== 'function') {
+    throw new TypeError('`callback` must be a function');
+  }
+
+  fixtures.assertEqual(paths.actual, paths.expected, function (error) {
+    if (error) return callback(error);
+
+    sharp.compare(paths.actual, paths.expectedMagick, function (error, info) {
+      if (error) return callback(error);
+
+      if (info.meanSquaredError > MAX_ALLOWED_IMAGE_MAGICK_MEAN_SQUARED_ERROR) {
+        return callback(new Error('Expected MSE against ImageMagick to be <= ' +
+          MAX_ALLOWED_IMAGE_MAGICK_MEAN_SQUARED_ERROR + '. Actual: ' +
+          info.meanSquaredError));
+      }
+
+      callback();
+    });
+  });
+};
+
+// Test
 describe('Overlays', function() {
   it('Overlay transparent PNG on solid background', function(done) {
+    var paths = getPaths('alpha-layer-01');
+
     sharp(fixtures.inputPngOverlayLayer0)
       .overlayWith(fixtures.inputPngOverlayLayer1)
-      .toBuffer(function (error, data, info) {
+      .toFile(paths.actual, function (error) {
         if (error) return done(error);
 
-        fixtures.assertSimilar(fixtures.expected('alpha-layer-01.png'), data, {threshold: 0}, done);
+        assertEqual(paths, done);
+      });
+  });
+
+  it('Overlay low-alpha transparent PNG on solid background', function(done) {
+    var paths = getPaths('alpha-layer-01-low-alpha');
+
+    sharp(fixtures.inputPngOverlayLayer0)
+      .overlayWith(fixtures.inputPngOverlayLayer1LowAlpha)
+      .toFile(paths.actual, function (error) {
+        if (error) return done(error);
+
+        assertEqual(paths, done);
       });
   });
 
   it('Composite three transparent PNGs into one', function(done) {
+    var paths = getPaths('alpha-layer-012');
+
     sharp(fixtures.inputPngOverlayLayer0)
       .overlayWith(fixtures.inputPngOverlayLayer1)
       .toBuffer(function (error, data, info) {
@@ -26,26 +85,56 @@ describe('Overlays', function() {
 
         sharp(data)
           .overlayWith(fixtures.inputPngOverlayLayer2)
-          .toBuffer(function (error, data, info) {
+          .toFile(paths.actual, function (error) {
             if (error) return done(error);
 
-            fixtures.assertSimilar(fixtures.expected('alpha-layer-012.png'), data, {threshold: 0}, done);
+            assertEqual(paths, done);
           });
       });
   });
 
-  // This tests that alpha channel unpremultiplication is correct:
-  it('Composite three low-alpha transparent PNGs into one', function(done) {
-    sharp(fixtures.inputPngOverlayLayer1LowAlpha)
-      .overlayWith(fixtures.inputPngOverlayLayer2LowAlpha)
-      .toBuffer(function (error, data, info) {
+  it('Composite two transparent PNGs into one', function(done) {
+    var paths = getPaths('alpha-layer-12');
+
+    sharp(fixtures.inputPngOverlayLayer1)
+      .overlayWith(fixtures.inputPngOverlayLayer2)
+      .toFile(paths.actual, function (error, data, info) {
         if (error) return done(error);
 
-        fixtures.assertSimilar(fixtures.expected('alpha-layer-012-low-alpha.png'), data, {threshold: 0}, done);
+        assertEqual(paths, done);
+      });
+  });
+
+  it('Composite two low-alpha transparent PNGs into one', function(done) {
+    var paths = getPaths('alpha-layer-12-low-alpha');
+
+    sharp(fixtures.inputPngOverlayLayer1LowAlpha)
+      .overlayWith(fixtures.inputPngOverlayLayer2LowAlpha)
+      .toFile(paths.actual, function (error, data, info) {
+        if (error) return done(error);
+
+        assertEqual(paths, done);
+      });
+  });
+
+  it('Composite three low-alpha transparent PNGs into one', function(done) {
+    var paths = getPaths('alpha-layer-012-low-alpha');
+
+    sharp(fixtures.inputPngOverlayLayer0)
+      .overlayWith(fixtures.inputPngOverlayLayer1LowAlpha)
+      .toBuffer(function (error, data, info) {
+        if (error) return done(error);
+
+        sharp(data)
+          .overlayWith(fixtures.inputPngOverlayLayer2LowAlpha)
+          .toFile(paths.actual, function (error, data, info) {
+            if (error) return done(error);
+
+            assertEqual(paths, done);
+          });
       });
   });
 
-  // This tests that alpha channel unpremultiplication is correct:
   it('Composite transparent PNG onto JPEG', function(done) {
     sharp(fixtures.inputJpg)
       .overlayWith(fixtures.inputPngOverlayLayer1)