Ensure 16-bit PNG output uses correct bitdepth #2958

gforge · lovell · commit 2deced0fb93f · 2021-11-23T12:19:41.000Z
diff --git a/src/pipeline.cc b/src/pipeline.cc
@@ -824,7 +824,7 @@ class PipelineWorker : public Napi::AsyncWorker {
             ->set("filter", baton->pngAdaptiveFiltering ? VIPS_FOREIGN_PNG_FILTER_ALL : VIPS_FOREIGN_PNG_FILTER_NONE)
             ->set("palette", baton->pngPalette)
             ->set("Q", baton->pngQuality)
-            ->set("bitdepth", baton->pngBitdepth)
+            ->set("bitdepth", sharp::Is16Bit(image.interpretation()) ? 16 : baton->pngBitdepth)
             ->set("dither", baton->pngDither)));
           baton->bufferOut = static_cast<char*>(area->data);
           baton->bufferOutLength = area->length;
@@ -993,7 +993,7 @@ class PipelineWorker : public Napi::AsyncWorker {
             ->set("filter", baton->pngAdaptiveFiltering ? VIPS_FOREIGN_PNG_FILTER_ALL : VIPS_FOREIGN_PNG_FILTER_NONE)
             ->set("palette", baton->pngPalette)
             ->set("Q", baton->pngQuality)
-            ->set("bitdepth", baton->pngBitdepth)
+            ->set("bitdepth", sharp::Is16Bit(image.interpretation()) ? 16 : baton->pngBitdepth)
             ->set("dither", baton->pngDither));
           baton->formatOut = "png";
         } else if (baton->formatOut == "webp" || (mightMatchInput && isWebp) ||
diff --git a/test/unit/png.js b/test/unit/png.js
@@ -112,6 +112,18 @@ describe('PNG', function () {
       });
   });
 
+  it('16-bit grey+alpha PNG roundtrip', async () => {
+    const after = await sharp(fixtures.inputPng16BitGreyAlpha)
+      .toColourspace('grey16')
+      .toBuffer();
+
+    const [statsBefore, statsAfter] = await Promise.all([
+      sharp(fixtures.inputPng16BitGreyAlpha).stats(),
+      sharp(after).stats()
+    ]);
+    assert.deepStrictEqual(statsAfter.channels[1], statsBefore.channels[1]);
+  });
+
   it('Valid PNG libimagequant palette value does not throw error', function () {
     assert.doesNotThrow(function () {
       sharp().png({ palette: false });
