Ensure trim op supports image-in-alpha #1597

docs/api-resize.md

@@ -196,6 +196,8 @@ 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.
+
 The `info` response Object will contain `trimOffsetLeft` and `trimOffsetTop` properties.
 
 ### Parameters

docs/changelog.md

@@ -6,6 +6,9 @@ Requires libvips v8.8.1.
 
 #### v0.23.3 - TBD
 
+* Ensure `trim` operation supports images contained in the alpha channel.
+  [#1597](https://github.com/lovell/sharp/issues/1597)
+
 * Ensure tile `overlap` option works as expected.
   [#1921](https://github.com/lovell/sharp/pull/1921)
   [@rustyguts](https://github.com/rustyguts)

lib/resize.js

@@ -367,7 +367,10 @@ 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.
+ *
  * The `info` response Object will contain `trimOffsetLeft` and `trimOffsetTop` properties.
+ *
  * @param {Number} [threshold=10] the allowed difference from the top-left pixel, a number greater than zero.
  * @returns {Sharp}
  * @throws {Error} Invalid parameters

src/operations.cc

@@ -266,12 +266,22 @@ namespace sharp {
     if (HasAlpha(background)) {
       background = background.flatten();
     }
-    int top, width, height;
-    int const left = image.find_trim(&top, &width, &height, VImage::option()
+    int left, top, width, height;
+    left = image.find_trim(&top, &width, &height, VImage::option()
       ->set("background", background(0, 0))
       ->set("threshold", threshold));
     if (width == 0 || height == 0) {
-      throw VError("Unexpected error while trimming. Try to lower the tolerance");
+      if (HasAlpha(image)) {
+        // Search alpha channel
+        VImage alpha = image[image.bands() - 1];
+        VImage backgroundAlpha = alpha.extract_area(0, 0, 1, 1);
+        left = alpha.find_trim(&top, &width, &height, VImage::option()
+          ->set("background", backgroundAlpha(0, 0))
+          ->set("threshold", threshold));
+      }
+      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

@@ -87,6 +87,7 @@ module.exports = {
   inputPngTruncated: getPath('truncated.png'), // gm convert 2569067123_aca715a2ee_o.jpg -resize 320x240 saw.png ; head -c 10000 saw.png > truncated.png
   inputPngEmbed: getPath('embedgravitybird.png'), // Released to sharp under a CC BY 4.0
   inputPngRGBWithAlpha: getPath('2569067123_aca715a2ee_o.png'), // http://www.flickr.com/photos/grizdave/2569067123/ (same as inputJpg)
+  inputPngImageInAlpha: getPath('image-in-alpha.png'), // https://github.com/lovell/sharp/issues/1597
 
   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

@@ -41,6 +41,21 @@ describe('Trim borders', function () {
       });
   });
 
+  it('single colour PNG where alpha channel provides the image', () =>
+    sharp(fixtures.inputPngImageInAlpha)
+      .trim()
+      .toBuffer({ resolveWithObject: true })
+      .then(({ data, info }) => {
+        assert.strictEqual(true, data.length > 0);
+        assert.strictEqual('png', info.format);
+        assert.strictEqual(916, info.width);
+        assert.strictEqual(137, info.height);
+        assert.strictEqual(4, info.channels);
+        assert.strictEqual(-6, info.trimOffsetLeft);
+        assert.strictEqual(-20, info.trimOffsetTop);
+      })
+  );
+
   it('16-bit PNG with alpha channel', function (done) {
     sharp(fixtures.inputPngWithTransparency16bit)
       .resize(32, 32)