[FFmpeg-user] Compressibility Check

[Brendan Brewster]

On Wed, Mar 28, 2012 at 11:42 AM, Brendan Brewster <
brendan.brewster at gmail.com> wrote:

> On Tue, Mar 27, 2012 at 10:14 PM, dE . <de.techno at gmail.com> wrote:
>
>> On 03/27/12 21:34, Brendan Brewster wrote:
>>
>>> On Mon, Mar 26, 2012 at 11:53 PM, dE .<de.techno at gmail.com>  wrote:
>>>
>>>  On 03/27/12 04:52, Brendan Brewster wrote:
>>>>
>>>>  Hi,
>>>>>
>>>>> I am trying to implement a compressibility check via ffmpeg to
>>>>> determine
>>>>> an
>>>>> appropriate resolution for the full encode.
>>>>>
>>>>>  Can you please be more clear about what you're trying to do? What do
>>>> you
>>>> mean by 'compressibility'. Changing resolution is just one of changing
>>>> compression level.
>>>>
>>>>  The idea is to take some percentage of the input, say 5%, at various
>>> points
>>> throughout and encode this with the default (input) resolution and
>>> bitrate.
>>> I have a specific file size to meet, such as DVD5, and therefore know the
>>> bitrate cap in order to fit my output onto the destination media (given a
>>> certain audio bitrate). I can then determine the number of bits per pixel
>>> for the default encode and prepare ratios with regard to lesser
>>> resolutions
>>> with the enforced bitrate. We choose a lower resolution if the ratio is
>>> poor. This is what I mean by a compressibility check. Similar
>>> functionality
>>> was implemented by the legacy Gordian Knot as well as within Tuxrip.
>>>
>>> I would just like to speed up my analysis by avoiding the majority of the
>>> seek time incurred especially in passes that pinpoint points later in the
>>> input video stream. I was really hoping that there might be a way to
>>> achieve the same end via one pass.
>>>
>>> Thanks,
>>> Brendan
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>>>
>>
>> From what I understand, you're asking for CBR, but that can be done using
>> a combination of -maxrate and -minrate. That will reduce on the quality not
>> resolution. This way you can keep the resolution constant.
>>
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>>
>
> I appreciate your response but no, I'm not looking for guidance on
> achieving CBR. My intent is to pick an appropriate resolution for the given
> bitrate. I have no issues staying within my target output size during the
> full encode no matter the resolution. All I'm trying to do is optimize my
> current solution for the initial analysis prior to the actual full encode.
>
> Maybe someone else has some thoughts for me :)
>
> Thanks,
> Brendan
>

Just putting this out there in case it's helpful for anyone else since I
found a solution that works for me per my initial requirements.

Basically, the following command will encode approximately 5%, 15 seconds
every 5 min segment except the latter dependent on its size, of the input
source (knowing the source frame rate is as indicated). This is a much more
efficient solution and affords the same analysis I was looking for.

ffmpeg -i 'VTS_01_PGC_01_1.VOB -filter:v 'select=lt(mod(t\,300)\,15),
setpts=N/(29.97*TB), yadif' -target ntsc-dvd -f mpeg2video -q 1 -mbd rd
-trellis 1 -cmp 2 -subcmp 2 -g 15 -an - 2>/dev/null | wc -c

-Brendan