From b9c38515157d0830fd2656fe29eae858dd4916be Mon Sep 17 00:00:00 2001
From: Lovell Fuller <github@lovell.info>
Date: Sat, 1 Apr 2023 12:08:14 +0100
Subject: [PATCH] Ensure linear op works with 16-bit input #3605

---
 docs/changelog.md                       |   3 +++
 src/operations.cc                       |   5 +++--
 test/fixtures/expected/linear-16bit.png | Bin 0 -> 1168 bytes
 test/unit/linear.js                     |  10 ++++++++++
 4 files changed, 16 insertions(+), 2 deletions(-)
 create mode 100644 test/fixtures/expected/linear-16bit.png

diff --git a/docs/changelog.md b/docs/changelog.md
index ec4ac5a7..59d52d6d 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -9,6 +9,9 @@ Requires libvips v8.14.2
 * Ensure use of `flip` operation forces random access read (regression in 0.32.0).
   [#3600](https://github.com/lovell/sharp/issues/3600)
 
+* Ensure `linear` operation works with 16-bit input (regression in 0.31.3).
+  [#3605](https://github.com/lovell/sharp/issues/3605)
+
 ### v0.32.0 - 24th March 2023
 
 * Default to using sequential rather than random access read where possible.
diff --git a/src/operations.cc b/src/operations.cc
index ea8652ec..026a0ffd 100644
--- a/src/operations.cc
+++ b/src/operations.cc
@@ -332,12 +332,13 @@ namespace sharp {
     if (a.size() > bands) {
       throw VError("Band expansion using linear is unsupported");
     }
+    bool const uchar = !Is16Bit(image.interpretation());
     if (HasAlpha(image) && a.size() != bands && (a.size() == 1 || a.size() == bands - 1 || bands - 1 == 1)) {
       // Separate alpha channel
       VImage alpha = image[bands - 1];
-      return RemoveAlpha(image).linear(a, b, VImage::option()->set("uchar", TRUE)).bandjoin(alpha);
+      return RemoveAlpha(image).linear(a, b, VImage::option()->set("uchar", uchar)).bandjoin(alpha);
     } else {
-      return image.linear(a, b, VImage::option()->set("uchar", TRUE));
+      return image.linear(a, b, VImage::option()->set("uchar", uchar));
     }
   }
 
diff --git a/test/fixtures/expected/linear-16bit.png b/test/fixtures/expected/linear-16bit.png
new file mode 100644
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diff --git a/test/unit/linear.js b/test/unit/linear.js
index 05bdad88..d668aabd 100644
--- a/test/unit/linear.js
+++ b/test/unit/linear.js
@@ -51,6 +51,16 @@ describe('Linear adjustment', function () {
       });
   });
 
+  it('applies linear levels adjustment to 16-bit w alpha ch', function (done) {
+    sharp(fixtures.inputPngWithTransparency16bit)
+      .linear(a, b)
+      .png({ compressionLevel: 0 })
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.expected('linear-16bit.png'), data, done);
+      });
+  });
+
   it('applies slope level adjustment w alpha ch', function (done) {
     sharp(fixtures.inputPngOverlayLayer1)
       .resize(240)