We are currently clearing the offscreen surface before rasterizing, but as we draw the image snapshot of the offscreen surface into the onscreen surface transparent pixels are blended with the current contents of the onscreen surface instead of replacing them. This is particularly noticeable when embedding platform views.
Flutter Engine
Flutter is a new way to build high-performance, cross-platform mobile apps. Flutter is optimized for today's, and tomorrow's, mobile devices. We are focused on low-latency input and high frame rates on Android and iOS.
The Flutter Engine is a portable runtime for hosting Flutter applications. It implements Flutter's core libraries, including animation and graphics, file and network I/O, accessibility support, plugin architecture, and a Dart runtime and compile toolchain. Most developers will interact with Flutter via the Flutter Framework, which provides a modern, reactive framework, and a rich set of platform, layout and foundation widgets.
Flutter is still under development and we continue to add features. However, it is ready for use by early adopters who are willing to deal with the odd wrinkle or two along the way. We hope you try it out and send us feedback.
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For information about using Flutter to build apps, please see the getting started guide.
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For information about contributing to the Flutter framework, please see the main Flutter repository.
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For information about contributing code to the engine itself, please see CONTRIBUTING.md.
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For information about the engine's architecture, please see the wiki.
Community
Join us in our Gitter chat room or join our mailing list, flutter-dev@googlegroups.com.