Re: mpeg2enc with MJPEG Tools 1.9.0rc2 buggy !

On Sun, 18 Mar 2007, Christophe Ducommun wrote:

> Hi I have compiled the latest version of MJPEG Tools 1.9.0rc2
> 
> But mpeg2enc doesn't encode correctly the frames.
> 
> I have an older version (just before rc1) which work correctly, but  
> rc2 doesn't work, the image
> pixellate (with br 9000 and q = 1).

	I  received this via private email - but it properly belongs on one
	of the mailing lists so I am replying here.

	First, if you are encoding for DVD do *NOT* use '-q 1 -b 9000'!  You
	are almost certain to generated a file with bitrate spikes that 
	exceed DVD specs.  This is because 9000 is very close to the maximum
	permitted and leaves almost no room for bit rate spikes.

	If the output is for software playback only then you're probably OK.

	On to the problem you mention.  I have known of this for some time and
	have communicated with the maintainer of mpeg2enc.  Errors were 
	introduced into the rate control logic in RC2 .

	If you see lines during the encoding like "q=40.0" you are seeing the
	effect of the bug - the encoder thinks it needs to apply EXTREME
	compression to maintain the specified bitrate.  I saw this behaviour
	with "-q 2".

	If you use "-q 3" or "-q 4" the problem appears to go away.  It is 
	unknown if the output is still being corrupted and it is not as
	noticeable OR if the problem goes away if a person does avoids 
	extreme (-q 1) or somewhat extreme (-q 2) values.

	Ideally and eventually '-q' should go away completely.  ALl that should
	be specified is a desired average bitrate and a tolerance (how much
	above/below the average is acceptable).

	Cheers,
	Steven Schultz


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