Isolating problems with MPEG2 VBR "best"

There have long been discussions here around problems with the quality of MPEG2 compression with Compressor v2.x, usually difficult to quantify or "solved" with "It doesn't work, use something else" such as a different product, Compressor v1 or a Quicktime MPEG2 export component. Maybe I'm an optimist, but I have to believe that someone who owns the code behind the Compressor product really does care to improve the quality of the compression, and is just waiting for a decent test case to figure-out what everyone is complaining about.
That in mind, I would like to propose that the most visible problems I've seen can be isolated to something to do with what is done when switching from "VBR" compression to "VBR best" compression, either in 1-pass or 2-pass mode. I would be curious if others who have had similar quality complaints could also check if they are using "VBR best" compression, such as the various default presets for "DVD: MPEG2 best quality..." which use "2-pass VBR best". In short, it seems as though one can get rid of the most serious artifacting problems by taking a "best quality" preset and changing the quality from "2-pass VBR best" to simply "2-pass VBR" (and similarly for 1-pass).
My own example: I've taken a short clip (less than 30 seconds) from the start of a recent home movie and published it along with some results on my iDisk's Public folder (Finder > Go > iDisk...): member name "dickermn" [sic], with no password. There you will find the folder CompressorTest, containing...
-> 2006winter - a self-contained QT movie with the DV source material.
-> dualG4 - a folder with the results of Compressor's MPEG2 compression with various settings, as run on a dual-1GHz G4 system.
-> coreDuo - a folder with the results of the same settings as above, but with Compressor run from a coreDuo mini.
-> *.png - two pairs of window-capture shots showing the same frames as compressed with "VBR" and "VBR best" settings.
Background: after having various odd and intermittent problems with MPEG2 compression results, I've used the recent upgrades to start with a clean slate: on the G4 system I ran Software Update, then removed all the components of DVD Studio Pro 4, Compressor and Qmaster, along with all preferences, application support files, frameworks, receipts, etc, as recommended in these web pages to remove these programs, then did a full install of my unversal upgrade to Final Cut Studio, and re-ran Software Update. This got me up to the current ProApps updates and Compressor v2.3.
Using only Compressor's default presets, selecting "DVD: Best Quality 150 minutes - 4:3" (2-pass VBR best, 3.7Mbps average rate), gives results with very visible artifacting. Using this same preset but changing the quality to "2-pass VBR" appears to clear up the most visible problems. Similarly, selecting "DVD: Fastest Encode 150 minutes - 4:3" (1-pass VBR) gives good results, but changing the preset's quality to "1-pass VBR best") produces visible artifacting.
For the purposes of testing, I also installed just Compressor on my coreDuo mini - a clean install - then ran Software Update to bring this to Compressor v2.3. Running the same tests as above on the Intel-based machine produced similar artifacting problems when selecting "best quality", but in different places in the media than on the G4. Using the same quality settings as above, but moving the average bitrate down to 3.0Mbps has similar behavior, but the artifacts are just more pronounced, and using a default preset with higher bitrate (5Mbps average, not included here) the artifacts are still quite visible, but less pronounced. From the 3Mbps results on the G4, the still frames captured show good examples of the problems at hand, though the artifacts are much more visible with the video in motion. These are the examples described below.
The clip shows some still titles against a still, horizontal-blurred background. At the end of the titles, the blurred background "un-blurs" and the motion starts: a horizontal pan, a wipe transition, and a downward pan. The subject is all falling snow, trees, our back deck, my son and me. The footage was taken with a consumer-grade Sony handheld camcorder.
The problems first noticable in the video appear as a blockiness to the images, particularly notable against the floor of snow where the image shifts visibly to different intensities of gray/white as the camera moves. More severe are blocks which don't move correctly with the camera, at first visible as a jerkiness to the opening background as it un-blurs, but more pronounced in the trees and decking during the downward pan at the end.
At about 14s is a good still of the blocking artifacts, seen here in the 1-pass VBR still frame. Note the large blocks in the snow in the "best" encoding, which are small enough to be lost in the noise for the "not-best" encoding. These shimmer between the different shades of gray as the camera moves.
At the last half of the clip, as the camera pans down, parts of the trees appear to jump independently of their branches. This is very clearly visible around the straight-lines of the railing in the lower part of the frame as the clip ends. The still is from about 23s into the clip, and you can see a block of the railing in the lower right which has clearly jumped-above the rest of the railing. Interestingly enough, this is the correct placement for those pixels in the next frame (as the camera continues to pan downward). In each of these "jumpy" cases, it would appear as though blocks of pixels show-up one frame early, and their surrounding blocks catch-up in the next frame (while these hold still). Again, when using the same settings without "best" quality, the jumping does not occur.
Note that with each pair of still (png) images you can group-select the pair and double-click to open them both in Preview, which allows you to toggle back-and-forth easily between the "best" and "not-best" versions. Comparing the two, you can easily spot several blocks in the frame which shift relative to their surroundings.
While I am aware that MPEG2 at low bitrates is a complicated thing (though I will leave open for debate how low 3-4Mbps is in today's world), and consumer-grade video has lots of noise which the encoder has to dance-around, I do believe it is fair to assume that the "best" settings should give visibly better results than their faster "non-best" counterparts, all else being equal - at the very least, the results should not be visibly worse. It is worth noting that the "best" VBR settings do appear (subjectively) to give better contrast or color-depth, so clearly something right is going-on - I just have to believe that there is something going awry in the whole process which creates artifacts negating the other benefits, but which, with any luck, could easily be fixed.
Please post with your comments, experiences or similar issues... or exactly what is the proper way to get this information to someone who could fix it?
dual 1GHz G4 + coreDuo mini   Mac OS X (10.4.8)   Compressor v2.3 / Final Cut Studio (univseral)

Well, I've found one more piece to this puzzle: hardware graphics accelerators.
In short, I tried this same test running Compressor on my G4 PowerBook (with Radion Mobility 9700) and got better results. The terrible blockiness and pixelation problems seem - dare I say it - gone from the MPEG2 video. In this regard, the video compressed as both the default "2-pass VBR best" and the same preset adjusted to use "1-pass VBR best" both looked cleaner than any of the VBR "not best" encodings produced on this machine, or either of the others.
Unfortunately, we're not completely out of the woods: the problems evidenced later in the clip - blocks which appear displaced in space/time for one frame - are still there, though they have moved to different locations in the frame.
This gets me to thinking: my PowerBook is just barely past current muster in terms of hardware-accelerated graphics (enough to run Motion, for example), whereas my other two machines are not (NVidia GForce4Ti & mini's Intel graphics). So this starts to get to the root of "If these presets don't work, why does Apple label them best - or distribute them at all?" I'm going to hazard a guess that there are various pieces to the codecs which can take either a software-only path or a hardware-accelerated path, and the bugs seen here exist only in the software (non-accelerated) path. Anyone testing on the latest top-notch hardware with a fancy-shmancy graphics card wouldn't go down that path and see any problems. And the problems are subtle enough that they'd even slip by the occasional testing of the slower path.
So now I'm out to find a more-current-than-my-PB graphics card to run the test, to see if I can get rid of the other problem. Updating to Compressor v2.3 got rid of most of the problems, and if buying new hardware is all it takes to fix MPEG2 compression, I might just spring for it - and hey, I'd get to run Motion, too.
So anyone out there with a whiz-bang graphics card care to run these tests and compare/contrast with what I've got here? I might know someone with a new MacBookPro, which I hear has a pretty current graphics chip in it...

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