Use bounding box of alpha+non-alpha for trim op #2166

docs/api-resize.md

@@ -236,7 +236,8 @@ Returns **Sharp**
 ## trim
 
 Trim "boring" pixels from all edges that contain values similar to the top-left pixel.
-Images consisting entirely of a single colour will calculate "boring" using the alpha channel, if any.
+
+Images with an alpha channel will use the combined bounding box of alpha and non-alpha channels.
 
 The `info` response Object, obtained from callback of `.toFile()` or `.toBuffer()`,
 will contain `trimOffsetLeft` and `trimOffsetTop` properties.

docs/changelog.md

@@ -8,6 +8,9 @@ Requires libvips v8.13.0
 
 * Drop support for Node.js 12, now requires Node.js >= 14.15.0.
 
+* Use combined bounding box of alpha and non-alpha channels for `trim` operation.
+  [#2166](https://github.com/lovell/sharp/issues/2166)
+
 * Re-introduce support for greyscale ICC profiles (temporarily removed in 0.30.2).
   [#3114](https://github.com/lovell/sharp/issues/3114)
 

lib/resize.js

@@ -430,7 +430,8 @@ function extract (options) {
 
 /**
  * Trim "boring" pixels from all edges that contain values similar to the top-left pixel.
- * Images consisting entirely of a single colour will calculate "boring" using the alpha channel, if any.
+ *
+ * Images with an alpha channel will use the combined bounding box of alpha and non-alpha channels.
  *
  * The `info` response Object, obtained from callback of `.toFile()` or `.toBuffer()`,
  * will contain `trimOffsetLeft` and `trimOffsetTop` properties.

src/operations.cc

@@ -275,19 +275,31 @@ namespace sharp {
     left = image.find_trim(&top, &width, &height, VImage::option()
       ->set("background", background(0, 0))
       ->set("threshold", threshold));
-    if (width == 0 || height == 0) {
     if (HasAlpha(image)) {
-        // Search alpha channel
+      // Search alpha channel (A)
+      int leftA, topA, widthA, heightA;
       VImage alpha = image[image.bands() - 1];
       VImage backgroundAlpha = alpha.extract_area(0, 0, 1, 1);
-        left = alpha.find_trim(&top, &width, &height, VImage::option()
+      leftA = alpha.find_trim(&topA, &widthA, &heightA, VImage::option()
         ->set("background", backgroundAlpha(0, 0))
         ->set("threshold", threshold));
+      if (widthA > 0 && heightA > 0) {
+        if (width > 0 && height > 0) {
+          // Combined bounding box (B)
+          int const leftB = std::min(left, leftA);
+          int const topB = std::min(top, topA);
+          int const widthB = std::max(left + width, leftA + widthA) - leftB;
+          int const heightB = std::max(top + height, topA + heightA) - topB;
+          return image.extract_area(leftB, topB, widthB, heightB);
+        } else {
+          // Use alpha only
+          return image.extract_area(leftA, topA, widthA, heightA);
+        }
+      }
     }
     if (width == 0 || height == 0) {
       throw VError("Unexpected error while trimming. Try to lower the tolerance");
     }
-    }
     return image.extract_area(left, top, width, height);
   }
 

test/fixtures/index.js

@@ -94,6 +94,7 @@ module.exports = {
   inputPngSolidAlpha: getPath('with-alpha.png'), // https://github.com/lovell/sharp/issues/1599
   inputPngP3: getPath('p3.png'), // https://github.com/lovell/sharp/issues/2862
   inputPngPalette: getPath('swiss.png'), // https://github.com/randy408/libspng/issues/188
+  inputPngTrimIncludeAlpha: getPath('trim-mc.png'), // https://github.com/lovell/sharp/issues/2166
 
   inputWebP: getPath('4.webp'), // http://www.gstatic.com/webp/gallery/4.webp
   inputWebPWithTransparency: getPath('5_webp_a.webp'), // http://www.gstatic.com/webp/gallery3/5_webp_a.webp

test/unit/trim.js

@@ -128,6 +128,18 @@ describe('Trim borders', function () {
     )
   );
 
+  it('Ensure trim uses bounding box of alpha and non-alpha channels', async () => {
+    const { info } = await sharp(fixtures.inputPngTrimIncludeAlpha)
+      .trim()
+      .toBuffer({ resolveWithObject: true });
+
+    const { width, height, trimOffsetTop, trimOffsetLeft } = info;
+    assert.strictEqual(width, 179);
+    assert.strictEqual(height, 123);
+    assert.strictEqual(trimOffsetTop, -44);
+    assert.strictEqual(trimOffsetLeft, -13);
+  });
+
   describe('Invalid thresholds', function () {
     [-1, 'fail', {}].forEach(function (threshold) {
       it(JSON.stringify(threshold), function () {