[FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[Kieran Kunhya]

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$subj as discussed at VDD

Kieran

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Re: [FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[Lynne]

Oct 29, 2023, 16:43 by kierank@obe.tv:

> $subj as discussed at VDD
>
> Kieran
>

This isn't Opus or JPEG-XL, there should be a single correct way of decoding something.
Looks good to me.
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Re: [FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[Michael Niedermayer]

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On Sun, Oct 29, 2023 at 03:43:27PM +0000, Kieran Kunhya wrote:
> $subj as discussed at VDD
> 
> Kieran

>  mpeg12dec.c |  265 ------------------------------------------------------------
>  1 file changed, 265 deletions(-)
> 94ecfc53b5e1b5c130eb68e1848078b7f1a43beb  0001-libavcodec-mpeg12-Remove-fast-mode.patch
> From aebab0825cb6c50d5f496fde2e9e9e5ad0cef87b Mon Sep 17 00:00:00 2001
> From: Kieran Kunhya <kierank@obe.tv>
> Date: Sun, 29 Oct 2023 15:32:49 +0000
> Subject: [PATCH] libavcodec/mpeg12: Remove "fast" mode

LGTM

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Republics decline into democracies and democracies degenerate into
despotisms. -- Aristotle

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Re: [FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[Leo Izen]

On 10/29/23 11:48, Lynne wrote:
> Oct 29, 2023, 16:43 by kierank@obe.tv:
> 
>> $subj as discussed at VDD
>>
>> Kieran
>>
> 
> This isn't Opus or JPEG-XL, there should be a single correct way of decoding something.
> Looks good to me.
> _______________________________________________

I'm not sure I understand what you mean - JPEG XL is fully specified. 
Are you thinking of mjpeg?

- Leo Izen

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Re: [FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[Lynne]

Oct 30, 2023, 01:02 by leo.izen@gmail.com:

> On 10/29/23 11:48, Lynne wrote:
>
>> Oct 29, 2023, 16:43 by kierank@obe.tv:
>>
>>> $subj as discussed at VDD
>>>
>>> Kieran
>>>
>>
>> This isn't Opus or JPEG-XL, there should be a single correct way of decoding something.
>> Looks good to me.
>> _______________________________________________
>>
>
> I'm not sure I understand what you mean - JPEG XL is fully specified. Are you thinking of mjpeg?
>

Anything that does not have a decoder fully defined with fixed-point math
or at least does not define a specific floating point format, with all constants
being given in that float format unambiguously (e.g. regular IEEE 32-bit floats, with all
constant values being given as 32-bit hex), along with a defined and bitexact
mathematical operations (i.e. IEEE). Giving only float constants up to a reasonable
decimal point and defining a maximum error is just not how video codecs
are defined.

Yeah, compared to Opus, JPEG-XL is really ordinary and shouldn't have been there,
and it can be argued that the error margins are small enough to permit bitexactness
up to a bit depth, but I nevertheless would've liked to see fixed-point specifications.
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Re: [FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[Leo Izen]

On 10/29/23 21:22, Lynne wrote:
> it can be argued that the error margins are small enough to permit bitexactness

For what it's worth, the error margins for compliance are specified in 
18181-3 and they're definitely within one pixel value for the various 
bit depths supported by the codestream format.




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Re: [FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[Matthias Dressel]

On 29.10.23 16:43, Kieran Kunhya wrote:
> $subj as discussed at VDD
> 
> Kieran

IMHO the commit message should have contained some reason as to *why* 
this was removed.
Now, someone looking at the log needs to find this patch on the ML, then 
follow the reference to find VDD meeting notes where they finally 
find... nothing. Because all the notes say is:

 > We should remove it

Unfortunately, I'm too late and it's pushed now, but maybe you can at 
least give a short explanation here on the ML?


Matthias
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Re: [FFmpeg-devel] [PATCH] libavcodec/mpeg12: Remove "fast" mode

[Kieran Kunhya]

On Mon, 30 Oct 2023 at 06:41, Matthias Dressel <lists.ffmpeg@deadcode.eu>
wrote:

> On 29.10.23 16:43, Kieran Kunhya wrote:
> > $subj as discussed at VDD
> >
> > Kieran
>
> IMHO the commit message should have contained some reason as to *why*
> this was removed.
> Now, someone looking at the log needs to find this patch on the ML, then
> follow the reference to find VDD meeting notes where they finally
> find... nothing. Because all the notes say is:
>
>  > We should remove it
>
> Unfortunately, I'm too late and it's pushed now, but maybe you can at
> least give a short explanation here on the ML?
>

"Fast" mode did some kind of out-of-spec VLC reading and dequant of MPEG-2
to allow faster decoding. This isn't really relevant in 2023 as MPEG-2
decoding is very fast in the official spec-compliant mode.

Kieran
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