November 26, 2020, 03:36:47 pm

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Xvid Macroblock Decision choices?

Started by xtro, October 11, 2020, 05:45:02 pm

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xtro

I am perplexed as to which of these choices Avidemux offers in Xvid encoding: 'none, dct, qpel, square etc' for macroblock decision. I thought qpel broke comptibility and I have no idea what the others are or if they will allow smooth playback on my Philips DVD player?

 Also the GOP is set to 200. I thought Xvid default was 300! Anyway, could anyone enlighten me on these option settings. Many thanks.

eumagga0x2a

"DCT" (discrete cosine transform) is the most basic method, "Qpel16" adds XVID_ME_HALFPELREFINE16_RD and XVID_ME_QUARTERPELREFINE16_RD flags (RD means Rate-Distortion optimisation), "Qpel8" adds further XVID_ME_HALFPELREFINE8_RD, XVID_ME_QUARTERPELREFINE8_RD and XVID_ME_CHECKPREDICTION_RD flags on top of Qpel16, "Square" adds XVID_ME_EXTSEARCH_RD on top of Qpel8.

(see http://websvn.xvid.org/cvs/viewvc.cgi/trunk/xvidcore/src/xvid.h?view=markup for some reference regarding the meaning of the flags)

In case that your target device doesn't support quarter-pixel motion vectors, the only option you have is DCT (or none).

The interval between keyframes (the GOP length) is a trade-off between compression rate and seek granularity, i.e. 200 at 25 fps equals max. 8 seconds between seek points. At 30000/1001 (29.97 fps) as in NTSC video broadcasts, a GOP length of 200 amounts to ~ 6.7 seconds. Choose whatever value suits your needs best.

xtro

Appreciate explanation, many thanks.
I was unsure if I used the'square' option that might break compatibility too, so I'll try DCT or None .A GOP of 250 should be fine. Thanks again.