High performance many-to-many intranet screen sharing with DisplayCast

Abstract

DisplayCast is a many to many Intranet screen sharing system. Its screen capture mechanism creates a sequence of pixmap images of the screen updates. Prior systems that used a similar approach were designed to operate over constrained wide-area networks and did not exploit the Intranet network conditions to achieve high capture rates. First we empirically analyzed the screen contents for a variety of scenarios. We showed that screen updates were sporadic with long periods of inactivity. When active, screens were updated at far higher rates than was supported by earlier systems. The mismatch was pronounced for interactive scenarios. Even during active screen updates, the number of updated pixels were frequently small. We showed that crucial information can be lost if individual updates were merged. When the available system resources could not support high capture rates, we showed ways in which updates can be effectively collapsed. Next, we investigate compression mechanisms for streaming these updates. Even while using a hardware encoder, lossy compressors such as H.264 were unable to sustain high frame rates. Though Zlib lossless compression operated within the latency and compression rate requirements, the compression efficiency was poor. By analyzing the screen pixels, we developed a practical transformation that significantly improved compression rates. DisplayCast incorporates these observations. It shares the processor and network resources required for screen capture, compression and transmission with host applications whose output needs to be shared. DisplayCast is agile and uses faster processing capability to achieve even higher performance. Our system components operate natively in Windows 7, Mac OS X and iOS and is deployed in a production setting. DisplayCast is released under a New BSD License.