How does the enableLookahead & lookaheadDepth work?

Dear Experts,

I have been trying to config enableLookahead & lookaheadDepth in the H.264 encoder to support scene-cut related performance improvement. I found when I set enableLookahead=1 and lookaheadDepth=none-zero value,it came out the gop structure of IPPP…

Any ideas about it? Tnx in advance for your help~