Strangely high quality on compressibility test

[Lime]

Hi!

I often encode my TV captures (MPEG2) to XviD using StaxRip + VirtualDubMod. I run a compressibility test and I usually get quality results from 30% - 60%, but on a recent encode I got 117%, which seems pretty high! The source is 720x576 and I reencode to 640x352, and I often choose bitrate around 1000 kbps.

Well, I start my 117%-encode anyway but I realize when the encode is finished that the bitrate is only around ~750 kbps, so my questions are - what factors affect the comp test and what can I do to maintain choosen bitrate?

What I can say about the source is that it's often quite dark.

Also, I read somewhere that using ConvertToYV12() will speed up the encoding, is this true?

[thewebchat]

I don't know what these "compressibility ratios" are, but I know ConvertToYV12() is a NOP if your source is already in YV12 (which it is).

[LoRd_MuldeR]

While I don't know how exactly you came to those numbers and while expressing "quality" in numbers generally is highly problematic, a quality measure above 100% clearly seems nonsensical!

Re-encoding inherently sacrifices quality! Well, unless you re-encode to a lossless format, but then the re-encoded video is 100% identical to the source ;)

Never can the re-encoded version be better than the source. So did you actually look at the results with your own eyes? That's generally the one and only reliable method to judge quality...

[detmek]

Increase resolution or decrease bitrate or turn off B-frames and VAQ or set minimum quantizer to 1 in Xvid settings. It seams that this bitrate is too high for your source and resolution and current Xvid settings.

[Lime]

Setting minimum quantizer to 1 did the trick! Thank you!

If I turn the question around, if I get seriously low quality on the compressability test, and need to improve it, which filters (or settings) are preferable?

[detmek]

A bit late replay but... Don't go below 640xXXX for resolution. Use 2 B-frames and VAQ, increase bitrate(filesize), deinterlace your source using some non-bob filter like YADIF in non-bob mode, LeakKernel deinterlace or Decomb, denoise source with RemoveGrain, DegrainMedian, FFT3DFilter/FFT3DGPU or MCTemporalDenoise script (slow but does good job).

[foxyshadis]

You should never enable quant 1 in xvid unless you're using multi-megabit-per-second sizes. Instead, use a better custom quant matrix, the end result will look much sharper.