minor(examples): update pixel-basics

thi-ng · Jul 28, 2019 · 6d2414e · 6d2414e
1 parent 969d6b8
commit 6d2414e
Showing 1 changed file with 11 additions and 6 deletions.
diff --git a/examples/pixel-basics/src/index.ts b/examples/pixel-basics/src/index.ts
@@ -1,6 +1,6 @@
 import {
-    ARGB4444,
     canvas2d,
+    GRAY_ALPHA88,
     GRAY8,
     imagePromise,
     PackedBuffer,
@@ -13,11 +13,16 @@ import LOGO from "../assets/logo-64.png";
 Promise.all([IMG, LOGO].map(imagePromise)).then(([img, logo]) => {
     // init 16bit packed RGB pixel buffer from image (resized to 256x256)
     const buf = PackedBuffer.fromImage(img, RGB565, 256, 256);
-    // use Porter-Duff operator to blend logo into main image
-    PackedBuffer.fromImage(logo, ARGB4444).blend(SRC_OVER_I, buf, {
-        dx: 10,
-        dy: 10
-    });
+
+    // create grayscale buffer for logo and use Porter-Duff operator to
+    // composite with main image. Since the logo has transparency, we
+    // need to premultiply alpha first...
+    PackedBuffer.fromImage(logo, GRAY_ALPHA88)
+        .premultiply()
+        .blend(SRC_OVER_I, buf, {
+            dx: 10,
+            dy: 10
+        });
 
     // extract sub-image
     const region = buf.getRegion(32, 96, 128, 64);