Performance Discrepancy: Direct getDisplayMedia vs. Canvas Capture for 2K Screen Sharing (OpenH264 )

[xuanshu wu]

Hi all,

I encountered a performance issue with 2K screen sharing using openH264 encoding and would appreciate your insights.

Test Setup:

• Source: 2K display capture via getDisplayMedia().

• Test URL: https://sh7mzr.csb.app/

• Encoding: OpenH264 (video/H264 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42e01f ).

Results:

1. getDisplayMedia → PeerConnection:

   • Encode time: ~500ms

   • Frame rate: 1–2 FPS

   • 
     

2. getDisplayMedia → canvas.captureStream() →  PeerConnection:

   • Encode time: ~20ms

   • Frame rate: ~30 FPS

   • 

Is this behavior expected? If so, why does routing through a canvas dramatically improve encoding performance for 2K content?

My hypotheses: Could GPU rasterization of the canvas bypass CPU bottlenecks?