How does x264 decide on pic type?

[chen]

x264.exe -B 1500 --vbv-maxrate 1500 --vbv-bufsize 1500 --fps 25 --bframes 2 --pass 1 -v -o f:/test/bytestream.264 f:/test/ori_100f.yuv 720x576

Above is the parameter set for my encoding, but what i got was a sequence with only 1 B-frame between P-frames instead of 2 B-frames which i specified with --bframes 2. Will anybody please tell me how x264 makes decisions on pic type? Additionally, if i would like to have 2 B-frames anyway, how can i change the above paramter set?

[Audionut]

"--b-bias" i think would be your best bet.

Influences how often b-frames are used.

[squid_80]

Additionally, if i would like to have 2 B-frames anyway, how can i change the above paramter set?--b-bias will only increase the chance of a b-frame being used. --no-b-adapt will use a b-frame regardless of what the codec thinks.