Fix exported image artifacts on transparent background

CHANGELOG.md

@@ -4,6 +4,7 @@ All notable changes to this project will be documented in this file, following t
 Note that since we don't clearly distinguish between a public and private interfaces there will be changes in non-major versions that are potentially breaking. If we make breaking changes to less used interfaces we will highlight it in here.
 
 ## [Unreleased]
+- Fix exported image artifacts on transparent background with emissive, bloom, or antialiasing
 - Fix cel-shaded ambient color being stripped to luminance (now uses full RGB, matching the classic lighting path)
 - Fix empty transforms default in `ShapeFromPly`
 - Use morton order for spheres in dot visual with lod-levels

src/mol-util/image.ts

@@ -2,6 +2,7 @@
  * Copyright (c) 2019-2021 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
+ * @author Gianluca Tomasello <giagitom@gmail.com>
  */
 
 export { PixelData };
@@ -37,12 +38,14 @@ namespace PixelData {
     /** to undo pre-multiplied alpha */
     export function divideByAlpha(pixelData: PixelData): PixelData {
         const { array } = pixelData;
-        const factor = (array instanceof Uint8Array) ? 255 : 1;
+        // clamp: emissive, bloom and antialiasing can lift premul RGB above alpha; without it Uint8Array silently wraps.
+        const max = (array instanceof Uint8Array) ? 255 : 1;
         for (let i = 0, il = array.length; i < il; i += 4) {
-            const a = array[i + 3] / factor;
-            array[i] /= a;
-            array[i + 1] /= a;
-            array[i + 2] /= a;
+            const a = array[i + 3] / max;
+            if (a === 0) continue;
+            array[i] = Math.min(max, array[i] / a);
+            array[i + 1] = Math.min(max, array[i + 1] / a);
+            array[i + 2] = Math.min(max, array[i + 2] / a);
         }
         return pixelData;
     }