A key-frame-based bi-directional depth propagation algorithm.Multi-pass error correction to prevent depth artifacts from being further propagated.Better results against prior works in depth map quality and synthesized stereo views. In this paper, we propose a key-frame-based bi-directional depth propagation algorithm for semi-automatic 2D-to-3D stereoscopic video conversion. First, key-frames are identified from each video shot based on color motion-compensation errors to prevent high-motion content between any pair of consecutive key frames. Depths for key-frames are manually assigned or rendered by popular computer tools, and then bi-directionally propagated to non-key-frames there between. Our depth propagation algorithm is featured of a multi-pass error correcting procedure for each frame to prevent depth artifacts from being further propagated to adjacent frames. Our proposed algorithm is advantageous in solving the background occlusion/dis-occlusion problem that degrades the performances of traditional depth propagation algorithms. Experimental results show that our scheme is capable of achieving better results against three prior algorithms in view of the qualities of the estimated depth map (e.g., dis-occluded background and object boundaries) and the synthesized stereo views.