Ensure trim is no-op when it would reduce to nothing #3223

docs/api-resize.md

@@ -239,6 +239,8 @@ Trim "boring" pixels from all edges that contain values similar to the top-left
 
 Images with an alpha channel will use the combined bounding box of alpha and non-alpha channels.
 
+If the result of this operation would trim an image to nothing then no change is made.
+
 The `info` response Object, obtained from callback of `.toFile()` or `.toBuffer()`,
 will contain `trimOffsetLeft` and `trimOffsetTop` properties.
 

docs/changelog.md

@@ -17,6 +17,9 @@ Requires libvips v8.13.0
 * Add support for WebP and PackBits `compression` options with TIFF output.
   [#3198](https://github.com/lovell/sharp/issues/3198)
 
+* Ensure `trim` operation is a no-op when it would reduce an image to nothing.
+  [#3223](https://github.com/lovell/sharp/issues/3223)
+
 ## v0.30 - *dresser*
 
 Requires libvips v8.12.2

lib/resize.js

@@ -433,6 +433,8 @@ function extract (options) {
  *
  * Images with an alpha channel will use the combined bounding box of alpha and non-alpha channels.
  *
+ * If the result of this operation would trim an image to nothing then no change is made.
+ *
  * The `info` response Object, obtained from callback of `.toFile()` or `.toBuffer()`,
  * will contain `trimOffsetLeft` and `trimOffsetTop` properties.
  *

src/operations.cc

@@ -297,10 +297,10 @@ namespace sharp {
         }
       }
     }
-    if (width == 0 || height == 0) {
-      throw VError("Unexpected error while trimming. Try to lower the tolerance");
+    if (width > 0 && height > 0) {
+      return image.extract_area(left, top, width, height);
     }
-    return image.extract_area(left, top, width, height);
+    return image;
   }
 
   /*

test/unit/trim.js

@@ -140,6 +140,25 @@ describe('Trim borders', function () {
     assert.strictEqual(trimOffsetLeft, -13);
   });
 
+  it('Ensure trim of image with all pixels same is no-op', async () => {
+    const { info } = await sharp({
+      create: {
+        width: 5,
+        height: 5,
+        channels: 3,
+        background: 'red'
+      }
+    })
+      .trim()
+      .toBuffer({ resolveWithObject: true });
+
+    const { width, height, trimOffsetTop, trimOffsetLeft } = info;
+    assert.strictEqual(width, 5);
+    assert.strictEqual(height, 5);
+    assert.strictEqual(trimOffsetTop, 0);
+    assert.strictEqual(trimOffsetLeft, 0);
+  });
+
   describe('Invalid thresholds', function () {
     [-1, 'fail', {}].forEach(function (threshold) {
       it(JSON.stringify(threshold), function () {