Fix transparency in capturePage (#12561)

* Fix transparent window capture. Transparency is preserved and not converted to black pixels anymore * Add test to make sure aplha channel exists in captured image
electron · Apr 10, 2018 · 6fc819d · 6fc819d
1 parent eca0436
commit 6fc819d
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diff --git a/atom/browser/api/atom_api_web_contents.cc b/atom/browser/api/atom_api_web_contents.cc
@@ -276,6 +276,9 @@ content::ServiceWorkerContext* GetServiceWorkerContext(
 void OnCapturePageDone(const base::Callback<void(const gfx::Image&)>& callback,
                        const SkBitmap& bitmap,
                        content::ReadbackResponse response) {
+  // Hack to enable transparency in captured image
+  // TODO(nitsakh) Remove hack once fixed in chromium
+  const_cast<SkBitmap&>(bitmap).setAlphaType(kPremul_SkAlphaType);
   callback.Run(gfx::Image::CreateFrom1xBitmap(bitmap));
 }
 

diff --git a/spec/api-browser-window-spec.js b/spec/api-browser-window-spec.js
@@ -442,6 +442,29 @@ describe('BrowserWindow module', () => {
         done()
       })
     })
+
+    it('preserves transparency', (done) => {
+      w.close()
+      const width = 400
+      const height = 400
+      w = new BrowserWindow({
+        show: false,
+        width: width,
+        height: height,
+        transparent: true
+      })
+      w.loadURL('data:text/html,<html><body background-color: rgba(255,255,255,0)></body></html>')
+      w.once('ready-to-show', () => {
+        w.show()
+        w.capturePage((image) => {
+          let imgBuffer = image.toPNG()
+          // Check 25th byte in the PNG
+          // Values can be 0,2,3,4, or 6. We want 6, which is RGB + Alpha
+          assert.equal(imgBuffer[25], 6)
+          done()
+        })
+      })
+    })
   })
 
   describe('BrowserWindow.setSize(width, height)', () => {