[x264-devel] x264-devel Digest, Vol 22, Issue 17

[Ajit Deshpande]

Hello Gabriel,
 yes its not H.264 per se.
But if i encode a 32x32 size MB with 8x8 transform size, then quantize it,
Huffman encode it, in one case, and with 32x32 transform size, quantize it
and Huffman encode it in second case;
Will the quantization values in both these cases be different as the DCT
coefficients obtained after these two differing block size transforms will
be different? I believe they should be different to be able to recover
same/similar pixels at decoder after Huffman decode, IQ, and IDCT? Am i
correct in saying ?

Thanks,
-Ajit



Rita Rudner  - "Before I met my husband, I'd never fallen in love. I'd
stepped in it a few times."

On Tue, Mar 17, 2009 at 4:30 PM, <x264-devel-request at videolan.org> wrote:

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> Today's Topics:
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>   1. Block DCT query (Ajit Deshpande)
>   2. Re: Block DCT query (Gabriel Bouvigne)
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> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 17 Mar 2009 10:59:06 +0530
> From: Ajit Deshpande <ajitsdeshpande at gmail.com>
> Subject: [x264-devel] Block DCT query
> To: x264-devel at videolan.org
> Message-ID:
>        <c2d46aba0903162229v659d7a03ob31798fbdee1ca75 at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Hello,
>   I am doing some basic Matlab simulations on DCT used in Video
> Compression. I have following questions arised out of that:
>
> 1.) In a 16x16 MB , if one does a 8x8 size block DCT (4-times) for that MB
> in first case and a one shot 16x16 size block DCT in second case, Will the
> reconstructed block at the decoder will have same/similar values in both
> cases?
>
> 2.) Which case would there be any benefit in terms of lesser residue,
> lesser
> bits needed for coding etc?
>
> 3.)  Will this analysis of block DCT also hold true for other transform
> like
> the integer transform used in H.264 codec?
>
> Regards
> -Ajit
>
> Douglas Adams  - "I love deadlines. I like the whooshing sound they make as
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> ------------------------------
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> Message: 2
> Date: Tue, 17 Mar 2009 10:41:01 +0100
> From: Gabriel Bouvigne <gabriel.bouvigne at joost.com>
> Subject: Re: [x264-devel] Block DCT query
> To: Mailing list for x264 developers <x264-devel at videolan.org>
> Message-ID: <49BF702D.1030102 at joost.com>
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>
> Ajit Deshpande a ?crit :
> > Hello,
> >    I am doing some basic Matlab simulations on DCT used in Video
> > Compression. I have following questions arised out of that:
>
> This is not really related to x264, is it?
>
> > 1.) In a 16x16 MB , if one does a 8x8 size block DCT (4-times) for that
> > MB in first case and a one shot 16x16 size block DCT in second case,
> > Will the reconstructed block at the decoder will have same/similar
> > values in both cases?
>
> An integer dct is lossless, while a floating point dct is usually lossy
> because of roundings.
> Now, if you are just using an iDct, there will be no loss, and thus
> reconstructed will be identical UNLESS you are quantizing the dct
> results. (in most perceptual compression schemes, it is not the
> transform which is lossy but the quantization of the results of the
> transform)
>
> Btw, there is no 16x16 transform within h.264.
>
>
> > 2.) Which case would there be any benefit in terms of lesser residue,
> > lesser bits needed for coding etc?
>
> Sometimes one would be better, sometimes the other one would be better.
> For "flat" residues, a bigger transform is usually better, while for
> "transcients" a smaller transform is usually better.
>
> > 3.)  Will this analysis of block DCT also hold true for other transform
> > like the integer transform used in H.264 codec?
>
> Likely
>
> --
> Gabriel
>
>
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> End of x264-devel Digest, Vol 22, Issue 17
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