[x264-devel] PSNR vs the distance of reference frame

[john sisi]

Dear All,

I performed an experiment using x264 to find the effect of using
different reference frames on the PSNR performance. I got a somehow
weird result: it seems that the PSNR goes up (by about 0.2dB on
Foreman CIF @256kbps) as the distance to the reference frame is
increased (from 3 to 15).

I set the number of reference frame to 15 using "--ref 15" and I
changed the code such that I was able to select only one of these 15
reference frames for encoding the frames. For example, in
"x264_mb_analyse_inter_p16x16" I set "i_ref" to 5 just before
"REF_COST( 0, i_ref )" to use only the 6th reference frame. I applied
the same change on the other relevant functions like
"x264_mb_analyse_inter_p16x8".

My expectection is that the PSNR values must go down as we use farther
reference frames but is this result reasonable?  Thanks,
John