Re: Shift Histograms

Hi,

I tried turning off all MMX/SSE and the encoder crashed/locked up for each 
file I try.  I'm using VirtualDub and the VFW interface to XVid.  So I 
haven't been able to test the C implementation of RGB/YUV conversion :(

Regards,
Andrew



--------------------------------------------------
From: "Michael Militzer" <[email protected]>
Sent: Tuesday, May 12, 2009 12:52 AM
To: <[email protected]>
Subject: Re: [XviD-devel] Shift Histograms

> Hi,
>
> there have been tests. I especially remember that we fixed rounding issues
> for the YV12->RGB conversion. So what you look at now is already an
> improved version ;)
>
> In general, RGB->YV12 conversion is always lossy. You should avoid an
> unnecessary YV12->RGB->YV12 chain when aiming for best quality. Obviously,
> the conversion should not be biased. I'd say there however can be a small
> bias either because of rounding issues or because of limited precision of
> the conversion coefficients.
>
> When looking at the code I see that the coefficients for the RGB->YV12
> conversion have just 8 bits precision. At least for the c-code it should
> be easily possible to increase that number. If you can please try to
> increase SCALEBITS_IN to e.g. 12 (line 155 in src/image/colorspace.c) and
> check if this reduces the bias you measured for the c-code. If it does we
> could modify the MMX code accordingly then.
>
> Regards,
> Michael
>
>
> Quoting A Uio <[email protected]>:
>
>> Thank for your reply!  That makes total sense.
>>
>> Do you know if this bias is well known?  Has anyone besides me ever 
>> tested
>> this?  Random variations of plus or minus 1% for individual pixels is no 
>> big
>> deal, but having the whole image get 1% darker is.  It seems a shame to 
>> have
>> such a cool codec as Xvid be so accurate when given Yv12 source, and then
>> have it become so inaccurate if handed RGB.  Obviously I can choose to 
>> feed
>> it only YV12, but I'm guessing less tech savvy users would get confused
>> about this.
>>
>> Regards,
>> Andrew
>>
>>
>>
>> --------------------------------------------------
>> From: "Michael Militzer" <[email protected]>
>> Sent: Monday, May 11, 2009 2:01 AM
>> To: <[email protected]>
>> Subject: Re: [XviD-devel] Shift Histograms
>>
>>> Hi,
>>>
>>> Xvid's RGB <-> YUV conversions have been designed to be fast. It is 
>>> known
>>> that they aren't precise (especially the MMX versions). So if you 
>>> measure
>>> a ~1% deviation this may not be a big surprise.
>>>
>>> The implementation in C should be more precise. So for a test you may 
>>> want
>>> to disable MMX/SSE optimizations in the xvid GUI and check whether the
>>> C-code provides better results.
>>>
>>> In general especially the YUV -> RGB conversions have been included into
>>> xvid at a time when graphics cards with YUV overlay support were not the
>>> norm and when low-end PC processors still had a hard time to decode PAL
>>> resolution video in realtime. So the primary goal was to make the
>>> conversions fast to enable fluent playback...
>>>
>>> Regards,
>>> Michael
>>>
>>>
>>> Quoting A Uio <[email protected]>:
>>>
>>>> Howdy,
>>>>
>>>> I think I may have found a bug in Xvid's RGB/YUV conversions. I've been
>>>> posting about it at
>>>>
>>>> http://forum.doom9.org/showthread.php?p=1281193
>>>>
>>>> Despite what responders have posted, I'm fairly sure its not a PC/Video
>>>> levels or 601/709 issue.  No one took me up on my request to replicate
>>>> the
>>>> tests on their systems, so it is always could just be something wrong
>>>> with
>>>> my system.
>>>>
>>>> Hope this is helpful!
>>>>
>>>> Regards,
>>>> A
>>>>
>>>> _______________________________________________
>>>> Xvid-devel mailing list
>>>> [email protected]
>>>> http://list.xvid.org/mailman/listinfo/xvid-devel
>>>>
>>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>
>
>
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