Why is full processing worse than fast recompress?

[Mr. Conflict]

Hi,
It always gives me worse quality in Virtual Dub when compressing from mjpeg yuy2 to
divx, eg. while cropping the mjpeg source. Why is it like this?
I have already begun to crop when capture that is a bit troubling.

best regards,

Mr. Conflict

[DJ Bobo]

1) Full processing mode or not has nearly no impact on quality. Fast Recompress is faster and won't make a color room conversion, which gives IN THEORY better quality.

2) If you're willing to crop, you have to choose full processing mode anyway, unless you crop in avisynth and then choose fast recompress in VD. The AVS script will look like this:

avisource("...\filename.avi")
crop(x,y,a,b)

Note that AviSynth 1.05 and older doesn't interpret MJPEG colors correctly, so if you're using such a version, you may want to install the MJPEG patch for AviSynth.

3) if you're going to encode to DivX, be sure that the final values are divisible by 16.
For example, if your MJPEG has 384x288 and crop 2 pixel on the bottom, you have to resize again to 384x288 so that it is 100% DivX compatible.

[Mr. Conflict]

[B]1) Full processing mode or not has nearly no impact on quality. Fast Recompress is faster and won't make a color room conversion, which gives IN THEORY better quality.

In my case image is sharper after fast recompression that increases quality and file size is sometimes considerably bigger but it's ok for quality.


3) if you're going to encode to DivX, be sure that the final values are divisible by 16.
Isn't it 2 for hight and 4 for width now (DivX5.02)?

best

Mr. C.

[Mr. Conflict]

In my case image is sharper after fast recompression that increases quality and file size is sometimes considerably bigger but it's ok for quality.

p.s. After full processing colours are washed out and picture blurred.
But actually it is much more as in the mjpeg where there are no blocks, picture is smooth but it's blurred; then it gets more contrast in divx after fast recompression (at least I see it like this:)

best,

Mr. C.

[DJ Bobo]

F.R. or F.P.M. have as said no impact on quality, the sharpness is the same, all is same, but in FPM the colors are transformed to RGB first, which isn't really a quality loss, since conversion to RGB is a lossless conversion.
May be DivX is interpreting RGB & YUV differently, I don't know (and I don't think so)
And there is no difference in file size because of using FR or FPM, only the bitrate has influence on size: same bitrate, same size, same quality!

2 or 4 or what ever they say, stick to 16 and all will be fine. You can also read this somewhere by divx.com where they specifically recommend to stay by values divisible by 16 for 100% safety.

[Mr. Conflict]

So I did a test.


Took mjpeg captured film res.384x214
Converted to divx, target bps 1150, B-frames, the rest on default
1 Fast recomp gave 2.24MB file, avg bps 110Kb/s
2 Full recomp gave 2.02MB file, avg bps 99Kb/s (the same output with 24bit
and 32bit colour depth) with colour washed out comparing to FR

It was the same whatever codec I used, first noticed with divx3.11 a year ago

Mr.C.

[DJ Bobo]

different bitrate -> different quality

Use 2-pass encoding and you'll see: no difference in quality or in size

[Mr. Conflict]

:-)
2-pass, same bitrate 1150, same everything

Fast gave 145kb/s
Full gave 139kb/s

and differences in quality...


Mr. C.

[DJ Bobo]

weird! that's all I can say

[theReal]

are you sure you don't have any filters in the filter chain that kick in as soon as you're using full processing?

btw. I have once noticed a difference in filesize of a few kb between fast recompress and normal recompress, but it was almost nothing. Haven't noticed any differences in quality...

[Mr. Conflict]

[B]are you sure you don't have any filters in the filter chain that kick in as soon as you're using full processing?

I don't turn on any, only null transform when crop

btw. I have once noticed a difference in filesize of a few kb between fast recompress and normal recompress, but it was almost nothing. Haven't noticed any differences in quality...

As I said I always get washed out/blurred colours after full processing and consederably smaller files comparing to fast recompress

Mr. C.