x264 --qp 21 results in q18 @ iframes ?

[CruNcher]

Shouldn't constant quant be hard set and doesn't use RateControll, only if i set --ipratio 1 it is fully constraint ?

used cmdline = x264 --no-ssim --qp 21 --min-keyint 1 --no-fast-pskip --no-dct-decimate --bframes 0 --ref 0 --no-deblock --partitions none --subme 1 --no-cabac --keyint 250 --deadzone-intra 6 -o E:\paris.mp4 pariscif.yuv 352x288 <- results in q18 @ iframes

[jethro]

Seems to me that P frames are the reference here. There is I-to-P ratio and P-to-B ratio. With ipratio > 1, I-frames will be larger than normal, that is get quantizer 18 instead of 21.