blit alpha bug (#593)

jimp-dev · Sep 2, 2018 · c09d130 · c09d130
1 parent b5e5577
commit c09d130
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Showing 3 changed files with 36 additions and 5 deletions.
diff --git a/packages/jimp/test/blit.test.js b/packages/jimp/test/blit.test.js
@@ -1,4 +1,4 @@
-import { Jimp, mkJGD } from './test-helper';
+import { Jimp, mkJGD, getTestDir } from './test-helper';
 
 describe('Blit over image', () => {
   const targetJGD = mkJGD(
@@ -148,4 +148,16 @@ describe('Blit over image', () => {
         )
       );
   });
+
+  it('blit alpha', async () => {
+    const expectedImg = await Jimp.read(
+      getTestDir() + '/samples/blit-alpha.png'
+    );
+    const dice = await Jimp.read(getTestDir() + '/samples/dice.png');
+    const image = await Jimp.read(getTestDir() + '/samples/cops.jpg');
+
+    image
+      .blit(dice, 0, 0)
+      .bitmap.data.should.be.deepEqual(expectedImg.bitmap.data);
+  });
 });
diff --git a/packages/jimp/test/samples/blit-alpha.png b/packages/jimp/test/samples/blit-alpha.png
diff --git a/packages/plugin-blit/src/index.js b/packages/plugin-blit/src/index.js
@@ -67,10 +67,29 @@ export default () => ({
         maxh - y - sy > 0
       ) {
         const dstIdx = baseImage.getPixelIndex(x + sx - srcx, y + sy - srcy);
-        baseImage.bitmap.data[dstIdx] = this.bitmap.data[idx];
-        baseImage.bitmap.data[dstIdx + 1] = this.bitmap.data[idx + 1];
-        baseImage.bitmap.data[dstIdx + 2] = this.bitmap.data[idx + 2];
-        baseImage.bitmap.data[dstIdx + 3] = this.bitmap.data[idx + 3];
+        const src = {
+          r: this.bitmap.data[idx],
+          g: this.bitmap.data[idx + 1],
+          b: this.bitmap.data[idx + 2],
+          a: this.bitmap.data[idx + 3]
+        };
+
+        const dst = {
+          r: baseImage.bitmap.data[dstIdx],
+          g: baseImage.bitmap.data[dstIdx + 1],
+          b: baseImage.bitmap.data[dstIdx + 2],
+          a: baseImage.bitmap.data[dstIdx + 3]
+        };
+
+        baseImage.bitmap.data[dstIdx] =
+          ((src.a * (src.r - dst.r) - dst.r + 255) >> 8) + dst.r;
+        baseImage.bitmap.data[dstIdx + 1] =
+          ((src.a * (src.g - dst.g) - dst.g + 255) >> 8) + dst.g;
+        baseImage.bitmap.data[dstIdx + 2] =
+          ((src.a * (src.b - dst.b) - dst.b + 255) >> 8) + dst.b;
+        baseImage.bitmap.data[dstIdx + 3] = this.constructor.limit255(
+          dst.a + src.a
+        );
       }
     });