From 2deced0fb93f51667d75250457f08d3d62f5207d Mon Sep 17 00:00:00 2001
From: Max Gordon <max@gforge.se>
Date: Sun, 31 Oct 2021 22:22:35 +0100
Subject: [PATCH] Ensure 16-bit PNG output uses correct bitdepth #2958

---
 src/pipeline.cc  |  4 ++--
 test/unit/png.js | 12 ++++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/pipeline.cc b/src/pipeline.cc
index 270c7424..0cb161f5 100644
--- 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
index 99694133..19d9c1f2 100644
--- 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 });