Capture 'real' frames

[meRobs]

I wish to do some experiments to compare the quality of different encoders and the effect of B-frames when converting to MPEG2/MPEG2-PS and MPEG4/AVI Progressive.
To do this, I thought to frame step each clip in SMplayer 0.6.9 and compare the frames. Preferably, by capturing critical frames.

I did captures in Pause mode in VirtualDubMod for a bitrate of 1000 kbps and there were differences between B = 0 and 2, but the overall difference in quality was not significant.
The variation within each clip was as great as the change due to changing from 0 B-frames to 2. This finding is counter intuitive since B-frames are supposed to make more efficient use of the available bit rate! The conversions with 2 B-frames took 50% longer than with none.

On the other hand, early results for SMplayer suggest a different conclusion, using the “Stop/Start taking screen-shots” mode.
All the B- and P-frames for the file with B set at 2 looked better than for B=0! This is what one would expect. It could be, then, that SMplayer corrects any B- and P-frames using data from adjacent I-frames, as a player should.

However, this explanation fails to explain why the I-frames at both values of B were smoother, with fewer artefacts, than the same frames viewed in VirtualDubMod.
This is contrary to the fact that I-frames should not change when played (no correction).
Hence, maybe the B- and P-frames are better in SMplayer because it uses a smoothing technique?

Please: any comments on the 'reality' of frames captured by SMplayer?
I want the captured frames to be a realistic means of indicating video quality!

[meRobs]

A further point.
I just noticed that when a video clip is paused and the GUI window is moved across the desktop, the frame is left behind.
This suggests that for playing in the GUI, the frame is supplied by the graphics card memory as an overlay (hardware acceleration for quicker display).

Does this also aply to captures during play (Shift+D) using "Start/Stop taking Screenshots" ??

[rvm]

You should know that playback and video display is done by mplayer (a command line application), smplayer is just a frontend.

[meRobs]

Yes, thanks, I realise that.
But I don't know how SMplayer uses MPlayer.