Frame capture from video clip

[meRobs]

I am trying to determine the effect of bitrate, etc, (when using MediaCoder) on the quality of MPEG (MPEG2) files and MPEG4/AVI files.
To this end, I was hoping to compare individual frames from each clip as a means of assessing quality.
However, most software players render the video before displaying it or use hardware acceleration (Graphics card).
Thus, frames captures may not be representative of the 'real' video!

To get to the point:
If I use Tools > Extract Movie Frames in XnView for progressive video clips, are the captured frames a true reflection of what is sent to a Progressive Player?

Does anyone know what is involved in XnView during snapshot captures?
Does it faithfully convert the real frames to PNG, etc, which means P- or B-frames would look incomplete?
Or does it do what a Player does and correct P-frames and B-frames using adjacent I-frames -- to appear as they would when played?
Or,anything else?
Any comment on this topic would be very helpful.
Thanks