[Didée]

After so much elaborating about which cells of the quantization table correspond to which frequencies, and to what kind of image detail, I feel the need to remind of this:

All of the above written is generally correct for intra-frames, resp. intra-blocks, only.
In I-frames the actual image gets coded, and therefore all the frequency stuff is directly related to image detail.

Now, in the usual way we all encode mpeg-4, most of the video stream (~ 98%) is P- and/or B-frames. And for these, its a quite different story.
What gets DCT'ed is the image after the motion compensation, and therefore it is not possible to directly draw the conclusion "fine detail" -> "high frequency", or the other way round. For example, it is perfectly possible that you have a block with pretty fine detail, but that block gets very nicely catched by ME, so the result that gets DCT'ed consists only out of low frequencies!

Because of that, the relation between quantization coefficients and image detail is not of that kind like our straight imagination would suggest. Keep that in mind!

Another point in this context:
For the above reasons, it makes a big difference if you "zero-out" some frequencies by either the encoder's quantization table (so: after ME), or by Tom's DCTfilter (before ME).
Zeroing by DCTfilter will directly remove image detail, whereas zeroing by encoder's matrix will remove the differences that are left over by ME. That are two different pairs of shoes!


Regards

Didée