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Old 19th August 2019, 10:21   #1635  |  Link
dm9656
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Join Date: Apr 2018
Posts: 5
Reasons for Full-TAB over Full-SBS for 3D MKV encoding

Hi,

I did tests lately on my LG OLED E6 in order to check the differences or similarities between Full-SBS and Full-TAB.

I have always encoded in FTAB, as I had the same logic ("it seems logical"), so just never tried FSBS until now.

As I always encode in at least placebo/CRF15 quality with x264/8bit/BR compatible, I am quite confident the result is humanly indiscernable from the original BR (typically the resulting MKV file is nearly the same size as the original BR), and that my comparison tests are reliable. As a note, I do use "motion interpolation" option when watching in 3D in order to achieve the full immersive experience, but I don't think this messes up with the tests (high frame rate for 3D content really gives a much more dramatic improvement than for 2D content).

Results:
- image in FULL-TAB result is excellent, the 3D is crisp, with 100% certainty the 4K TV displays both full frames perfectly (e.g. alternate the 2x1080 lines over full 2160 height)
- image in FULL-SBS is ok but NOT as good: it feels blurry, less defined, much more like "H-TAB" than "F-SBS". I tried to see if there was anything related to filters etc. but after many trials and settings tweaking, I couldn't achieve the same quality as FTAB. Altough it would require capturing the TV output and analyze it line by line to understand exactly what is happening, I'm quite confident that what is displayed is in fact H-TAB and that the TV does not make use of all the original video data.

My 2 cents is that FTAB certainly allows for "simpler decoding" as you hinted, because the image being already encoded vertically, all the TV has to do is decode/split/interlace the lines "as is". In FSBS, there is much more work needed as lines have also to be moved around to arrange them vertically, and it is possible they just didn't care doing it for FSBS. Anyway, it might be a flaw with my particular TV model, but all in all, SBS seems a more complicated and risky option.

In conclusion: choosing FTAB as a standard for encoding 3D MKVs seems the best option for maximum quality output on all equipments (as long as there are no problems related to FTAB on other TVs, but I have not seen any reports about such problems).

Quote:
Originally Posted by r0lZ View Post
LG TVs (HD and UHD) use always passive glasses that filter the polarized light. That means that they emit half the pixels with a particular polarization, and the other half with the opposite polarization. As far as I know, all LG TVs are based on the same model. One line of pixel contains the pixels for the left view, the next one for the right view and so on. (I have read somewhere that LG is now able to offer full resolution for the two eyes, but theoretically, it's impossible.) Therefore, it seems logical to use Half-TAB instead of Half-SBS, as there is exactly the same number of line for the left and right eyes in the encoded video and in the screen. You encode 1920x540 and you see exactly that. If you use Half-SBS, half the pixels are not at the right position and are useless. You see a 960x540 image.

However, if that logic is correct for Half-TAB, IMO there is absolutely no reason to prefer Full-TAB over Full-SBS or FS. Since the video resolution is full anyway, the passive HD TV will throw away half the resolution regardless of the 3D frame packing. But the habit to prefer TAB over SBS is well anchored, and there are certainly many web sites that continue to pretend, probably wrongly, that Full-TAB gives a better quality with all kind of LG TVs.

I don't know how 3D UHD LG TVs react to Half-TAB or Half-SBS. IMO, the quality loss when using Half-SBS is probably minimal, if not completely absent, but I'm not sure.

On the other hand, Half-SBS is largely considered better for active 3D TVs. I don't know why, as with active glasses, all pixels are used to display the left and right views. But it is a fact that Half-TAB gives less good results on my Samsung HD TV. Again, I think that Full-SBS/TAB/FS are strictly equivalent.
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