Add unflatten operation to create an alpha channel (#3461)

test/unit/unflatten.js

@@ -0,0 +1,37 @@
+'use strict';
+
+const sharp = require('../../');
+const fixtures = require('../fixtures');
+
+// const assert = require('assert');
+
+describe('Unflatten', function () {
+  it('unflatten white background', function (done) {
+    sharp(fixtures.inputPng).unflatten()
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.expected('unflatten-white-transparent.png'), data, { threshold: 0 }, done);
+      });
+  });
+  it('unflatten transparent image', function (done) {
+    sharp(fixtures.inputPngTrimSpecificColourIncludeAlpha).unflatten()
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.expected('unflatten-flag-white-transparent.png'), data, { threshold: 0 }, done);
+      });
+  });
+  it('unflatten using threshold', function (done) {
+    sharp(fixtures.inputPngPalette).unflatten(true).threshold(128, { grayscale: false })
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.expected('unflatten-swiss.png'), data, { threshold: 1 }, done);
+      });
+  });
+  it('no unflatten', function (done) {
+    sharp(fixtures.inputPng).unflatten(false)
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.inputPng, data, { threshold: 0 }, done);
+      });
+  });
+});