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Am I misunderstanding something: how do you get different filesizes for the same bitrate? Bitrate is bits/time, for the same clip (time) you should get the same filesizes, modulo some header information for the codec.

What those graphs are describing are how the bitrate corresponds to the quality for a given profile. If you take the graphs as a whole they tell you how your quality will increase as you increase the filesize (bitrate) for a given clip, cheers. "

http://en.wikipedia.org/wiki/Variable_bitrate

"Variable bitrate (VBR) is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate (CBR), VBR files vary the amount of output data per time segment. VBR allows a higher bitrate (and therefore more storage space) to be allocated to the more complex segments of media files while less space is allocated to less complex segments. The average of these rates can be calculated to produce an average bitrate for the file.

MP3, WMA, Vorbis, and AAC audio files can optionally be encoded in VBR. Variable bit rate encoding is also commonly used on MPEG-2 video, MPEG-4 Part 2 video (Xvid, DivX, etc.), MPEG-4 Part 10/H.264 video, Theora, Dirac and other video compression formats."

The better the VBR compression, the higher the average bitrate can be stored within the same filesize.

WebM has better compression for relatively slow-moving video, such as this one:

http://www.youtube.com/watch?v=rLxQiI8c1Bs

If WebM dedicates the same number of bits to such a clip as H264, the still frames captured from the WebM rendered video playback will be sharper and better quality than H264.

In areas and segments of high motion, WebM throws a lot of data away, and when you look at a still frame, whatever part of the picture is moving quickly will be rendered as a blur. H264 will be sharper, but it will exhibit artefacts. This gives WebM a poorer score in objective measures such as PSNR, but since the human eye sees high motion as blur anyway, subjectively it looks fine.

Cheers.

Edited 2012-11-17 07:12 UTC

How can you have a "higher average bitrate" within the same filesize?

It's like saying you and I both ran a marathon (filesize) in the same amount of time (clip running time), but you went faster.

One average bitrate can't be higher than the other for a clip of the same length and filesize; they are the same.

As to your other point: so you have no benchmarks (even screenshots?) to support your claim that WebM has higher quality/bitrate than x264? Okie.

Edited 2012-11-17 07:34 UTC