From 9d760f39587aa986fcd9f8c854c045de44618f3f Mon Sep 17 00:00:00 2001
From: Lovell Fuller <github@lovell.info>
Date: Mon, 5 Dec 2022 20:40:41 +0000
Subject: [PATCH] Improve perf of ops that introduce non-opaque background
 #3465

---
 docs/changelog.md   | 3 +++
 src/common.cc       | 2 +-
 test/unit/rotate.js | 2 +-
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/docs/changelog.md b/docs/changelog.md
index c6374becf..f04b7aaba 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -18,6 +18,9 @@ Requires libvips v8.13.3
 * Ignore `sequentialRead` option when calculating image statistics.
   [#3462](https://github.com/lovell/sharp/issues/3462)
 
+* Small performance improvement for operations that introduce a non-opaque background.
+  [#3465](https://github.com/lovell/sharp/issues/3465)
+
 * Ensure integral output of `linear` operation.
   [#3468](https://github.com/lovell/sharp/issues/3468)
 
diff --git a/src/common.cc b/src/common.cc
index 0c6302a35..3a208951f 100644
--- a/src/common.cc
+++ b/src/common.cc
@@ -913,7 +913,7 @@ namespace sharp {
     // Add non-transparent alpha channel, if required
     if (colour[3] < 255.0 && !HasAlpha(image)) {
       image = image.bandjoin(
-        VImage::new_matrix(image.width(), image.height()).new_from_image(255 * multiplier));
+        VImage::new_matrix(image.width(), image.height()).new_from_image(255 * multiplier).cast(image.format()));
     }
     return std::make_tuple(image, alphaColour);
   }
diff --git a/test/unit/rotate.js b/test/unit/rotate.js
index a08bc51a6..33dfb18fa 100644
--- a/test/unit/rotate.js
+++ b/test/unit/rotate.js
@@ -67,7 +67,7 @@ describe('Rotation', function () {
   it('Rotate by 30 degrees with solid background', function (done) {
     sharp(fixtures.inputJpg)
       .resize(320)
-      .rotate(30, { background: { r: 255, g: 0, b: 0, alpha: 0.5 } })
+      .rotate(30, { background: { r: 255, g: 0, b: 0 } })
       .toBuffer(function (err, data, info) {
         if (err) throw err;
         assert.strictEqual('jpeg', info.format);