From 8408e99aa38a84be63986ca0210eae106d13cbc2 Mon Sep 17 00:00:00 2001
From: Lovell Fuller <github@lovell.info>
Date: Thu, 20 Apr 2023 10:49:39 +0100
Subject: [PATCH] Ensure trim op works with CMYK input #3636

---
 docs/changelog.md |  3 +++
 src/operations.cc | 28 +++++++++-------------------
 test/unit/trim.js | 26 ++++++++++++++++++++++++++
 3 files changed, 38 insertions(+), 19 deletions(-)

diff --git a/docs/changelog.md b/docs/changelog.md
index 662da39b0..1bf8733b3 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -26,6 +26,9 @@ Requires libvips v8.14.2
 * Add support for `modulate` operation when using non-sRGB pipeline colourspace.
   [#3620](https://github.com/lovell/sharp/issues/3620)
 
+* Ensure `trim` operation works with CMYK images (regression in 0.31.0).
+  [#3636](https://github.com/lovell/sharp/issues/3636)
+
 ### 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 a3b480a53..e59157ff9 100644
--- a/src/operations.cc
+++ b/src/operations.cc
@@ -269,30 +269,20 @@ namespace sharp {
     if (image.width() < 3 && image.height() < 3) {
       throw VError("Image to trim must be at least 3x3 pixels");
     }
-
-    // Scale up 8-bit values to match 16-bit input image
-    double multiplier = sharp::Is16Bit(image.interpretation()) ? 256.0 : 1.0;
-    threshold *= multiplier;
-
-    std::vector<double> backgroundAlpha(1);
     if (background.size() == 0) {
       // Top-left pixel provides the default background colour if none is given
       background = image.extract_area(0, 0, 1, 1)(0, 0);
-      multiplier = 1.0;
-    }
-    if (HasAlpha(image) && background.size() == 4) {
-      // Just discard the alpha because flattening the background colour with
-      // itself (effectively what find_trim() does) gives the same result
-      backgroundAlpha[0] = background[3] * multiplier;
+    } else if (sharp::Is16Bit(image.interpretation())) {
+      for (size_t i = 0; i < background.size(); i++) {
+        background[i] *= 256.0;
+      }
+      threshold *= 256.0;
     }
-    if (image.bands() > 2) {
-      background = {
-        background[0] * multiplier,
-        background[1] * multiplier,
-        background[2] * multiplier
-      };
+    std::vector<double> backgroundAlpha({ background.back() });
+    if (HasAlpha(image)) {
+      background.pop_back();
     } else {
-      background[0] = background[0] * multiplier;
+      background.resize(image.bands());
     }
     int left, top, width, height;
     left = image.find_trim(&top, &width, &height, VImage::option()
diff --git a/test/unit/trim.js b/test/unit/trim.js
index 1025e6971..8d2c76d44 100644
--- a/test/unit/trim.js
+++ b/test/unit/trim.js
@@ -153,6 +153,32 @@ describe('Trim borders', function () {
     assert.strictEqual(trimOffsetLeft, -12);
   });
 
+  it('Ensure CMYK image can be trimmed', async () => {
+    const cmyk = await sharp({
+      create: {
+        width: 16,
+        height: 8,
+        channels: 3,
+        background: 'red'
+      }
+    })
+      .extend({ left: 12, right: 24, background: 'blue' })
+      .toColourspace('cmyk')
+      .jpeg()
+      .toBuffer();
+
+    const { info } = await sharp(cmyk)
+      .trim()
+      .raw()
+      .toBuffer({ resolveWithObject: true });
+
+    const { width, height, trimOffsetTop, trimOffsetLeft } = info;
+    assert.strictEqual(width, 16);
+    assert.strictEqual(height, 8);
+    assert.strictEqual(trimOffsetTop, 0);
+    assert.strictEqual(trimOffsetLeft, -12);
+  });
+
   it('Ensure trim of image with all pixels same is no-op', async () => {
     const { info } = await sharp({
       create: {