Default PNG output to adaptiveFiltering=false, compressionLevel=9

PNG is mostly used for logos and line art, so these defaults will
produce the smallest files for most output most of the time.

docs/changelog.md

@@ -10,6 +10,10 @@ Requires libvips v8.6.0.
   [#868](https://github.com/lovell/sharp/issues/868)
   [@mirohristov-com](https://github.com/mirohristov-com)
 
+* PNG output now defaults to adaptiveFiltering=false, compressionLevel=9
+  [#872](https://github.com/lovell/sharp/issues/872)
+  [@wmertens](https://github.com/wmertens)
+
 ### v0.18 - "*ridge*"
 
 Requires libvips v8.5.5.

lib/constructor.js

@@ -186,8 +186,8 @@ const Sharp = function (input, options) {
     jpegOvershootDeringing: false,
     jpegOptimiseScans: false,
     pngProgressive: false,
-    pngCompressionLevel: 6,
-    pngAdaptiveFiltering: true,
+    pngCompressionLevel: 9,
+    pngAdaptiveFiltering: false,
     webpQuality: 80,
     webpAlphaQuality: 100,
     webpLossless: false,

lib/output.js

@@ -148,8 +148,8 @@ function jpeg (options) {
  * Use these PNG options for output image.
  * @param {Object} [options]
  * @param {Boolean} [options.progressive=false] - use progressive (interlace) scan
- * @param {Number} [options.compressionLevel=6] - zlib compression level
- * @param {Boolean} [options.adaptiveFiltering=true] - use adaptive row filtering
+ * @param {Number} [options.compressionLevel=9] - zlib compression level, 0-9
+ * @param {Boolean} [options.adaptiveFiltering=false] - use adaptive row filtering
  * @param {Boolean} [options.force=true] - force PNG output, otherwise attempt to use input format
  * @returns {Sharp}
  * @throws {Error} Invalid options

src/pipeline.h

@@ -179,8 +179,8 @@ struct PipelineBaton {
     jpegOvershootDeringing(false),
     jpegOptimiseScans(false),
     pngProgressive(false),
-    pngCompressionLevel(6),
-    pngAdaptiveFiltering(true),
+    pngCompressionLevel(9),
+    pngAdaptiveFiltering(false),
     webpQuality(80),
     tiffQuality(80),
     tiffCompression(VIPS_FOREIGN_TIFF_COMPRESSION_JPEG),

test/unit/io.js

@@ -628,6 +628,29 @@ describe('Input/output', function () {
       });
     });
 
+    it('default compressionLevel generates smaller file than compressionLevel=6', function (done) {
+      // First generate with default compressionLevel
+      sharp(fixtures.inputPng)
+        .resize(320, 240)
+        .png()
+        .toBuffer(function (err, defaultData, defaultInfo) {
+          if (err) throw err;
+          assert.strictEqual(true, defaultData.length > 0);
+          assert.strictEqual('png', defaultInfo.format);
+          // Then generate with compressionLevel=6
+          sharp(fixtures.inputPng)
+            .resize(320, 240)
+            .png({ compressionLevel: 6 })
+            .toBuffer(function (err, largerData, largerInfo) {
+              if (err) throw err;
+              assert.strictEqual(true, largerData.length > 0);
+              assert.strictEqual('png', largerInfo.format);
+              assert.strictEqual(true, defaultData.length < largerData.length);
+              done();
+            });
+        });
+    });
+
     it('without adaptiveFiltering generates smaller file', function (done) {
       // First generate with adaptive filtering
       sharp(fixtures.inputPng)