Ensure greyscale images can be trimmed #3386

docs/changelog.md

@@ -19,6 +19,9 @@ Requires libvips v8.13.1
 * Ensure AVIF output is always 8-bit.
   [#3358](https://github.com/lovell/sharp/issues/3358)
 
+* Ensure greyscale images can be trimmed (regression in 0.31.0).
+  [#3386](https://github.com/lovell/sharp/issues/3386)
+
 ### v0.31.0 - 5th September 2022
 
 * Drop support for Node.js 12, now requires Node.js >= 14.15.0.

src/operations.cc

@@ -289,17 +289,20 @@ namespace sharp {
       background = image.extract_area(0, 0, 1, 1)(0, 0);
       multiplier = 1.0;
     }
-    if (background.size() == 4) {
+    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;
     }
-    background = {
-      background[0] * multiplier,
-      background[1] * multiplier,
-      background[2] * multiplier
-    };
-
+    if (image.bands() > 2) {
+      background = {
+        background[0] * multiplier,
+        background[1] * multiplier,
+        background[2] * multiplier
+      };
+    } else {
+      background[0] = background[0] * multiplier;
+    }
     int left, top, width, height;
     left = image.find_trim(&top, &width, &height, VImage::option()
       ->set("background", background)

test/unit/trim.js

@@ -124,6 +124,32 @@ describe('Trim borders', function () {
     assert.strictEqual(trimOffsetLeft, -13);
   });
 
+  it('Ensure greyscale image can be trimmed', async () => {
+    const greyscale = await sharp({
+      create: {
+        width: 16,
+        height: 8,
+        channels: 3,
+        background: 'silver'
+      }
+    })
+      .extend({ left: 12, right: 24, background: 'gray' })
+      .toColourspace('b-w')
+      .png({ compressionLevel: 0 })
+      .toBuffer();
+
+    const { info } = await sharp(greyscale)
+      .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: {