[MPlayer-DOCS] CVS: main/DOCS/man/en mplayer.1,1.834,1.835
lorenm at u.washington.edu
Fri Jan 14 07:07:20 CET 2005
On Thu, 13 Jan 2005, Torinthiel wrote:
> On Sun, Jan 02, 2005 at 11:38:23PM +0100, Loren Merritt CVS wrote:
>> CVS change done by Loren Merritt CVS
>> +.B qp_constant=<1\-51>
>> +This selects the quantizer to use for P-frames.
>> +I- and B-frames are offset from this value by ip_factor and pb_factor, respectively.
>> +20\-40 is a useful range (default: 26).
>> +Lower values result in better fidelity, but higher bitrates.
>> +Note that quantization in H.264 works differently from MPEG.
>> +H.264's quantization parameter (QP) is on a logarithmic scale.
>> +As an example, the bitrate difference between QP=20 and QP=40
>> +is about a factor of 10.
>> +Useful quantizers in H.264 tend to be very large compared to MPEG.
> I don't get it. If H.264 uses logarithmic scale and MPEG does not, then the values in H.264 should be much smaller. (as log x < x).
+The mapping is approximately H264QP = 12 + 6*log2(MPEGQP).
+For example, MPEG at QP=2 is equivalent to H.264 at QP=18.
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