Ensure integral output of linear op #3468

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)
 
+* Ensure integral output of `linear` operation.
+  [#3468](https://github.com/lovell/sharp/issues/3468)
+
 ### v0.31.2 - 4th November 2022
 
 * Upgrade to libvips v8.13.3 for upstream bug fixes.

src/operations.cc

@@ -345,9 +345,9 @@ namespace sharp {
     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).bandjoin(alpha);
+      return RemoveAlpha(image).linear(a, b, VImage::option()->set("uchar", TRUE)).bandjoin(alpha);
     } else {
-      return image.linear(a, b);
+      return image.linear(a, b, VImage::option()->set("uchar", TRUE));
     }
   }
 

test/unit/linear.js

@@ -73,6 +73,28 @@ describe('Linear adjustment', function () {
       });
   });
 
+  it('output is integer, not float, RGB', async () => {
+    const data = await sharp({ create: { width: 1, height: 1, channels: 3, background: 'red' } })
+      .linear(1, 0)
+      .tiff({ compression: 'none' })
+      .toBuffer();
+
+    const { channels, depth } = await sharp(data).metadata();
+    assert.strictEqual(channels, 3);
+    assert.strictEqual(depth, 'uchar');
+  });
+
+  it('output is integer, not float, RGBA', async () => {
+    const data = await sharp({ create: { width: 1, height: 1, channels: 4, background: '#ff000077' } })
+      .linear(1, 0)
+      .tiff({ compression: 'none' })
+      .toBuffer();
+
+    const { channels, depth } = await sharp(data).metadata();
+    assert.strictEqual(channels, 4);
+    assert.strictEqual(depth, 'uchar');
+  });
+
   it('Invalid linear arguments', function () {
     assert.throws(
       () => sharp().linear('foo'),