[FFmpeg-user] Yes or No? About the processing pipeline.

[Jim DeLaHunt]

So Mark, for all his self-obstruction by insisting on terms which others 
don't understand, and for all the digressions into rants about free 
software in general, is asking a valid question here.

On 2025-06-19 14:05, Mark Filipak wrote:
> ...When FFmpeg reports "Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 
> 'c:\FATAL ATTRACTION [1987].mp4'", for example, it can't be reporting 
> the decoder's output. It must be the file's contents. So the question 
> comes up: What is the decoder's output, and what, if anything, does 
> FFmpeg do to the decoder's output prior to loading the head end of the 
> processing pipeline?
>
> I know that seems silly to you, but it's a valid question. It's a 
> simple 'Yes' or 'No' question, or at least that's what I thought.
>
> If the [video data of the input file] is [Y'CbCr 4:2:0 progressive 
> with 8-bit pixel values in SMPTE 170M colour space], then the [video 
> data at the] head end of the processing pipeline is also [Y'CbCr 4:2:0 
> progressive with 8-bit pixel values in SMPTE 170M colour space]. Yes 
> or No? ...

(I reworded his question to bypass the trivial objection that people 
don't understand his terminology.)

Putting aside the track record of the questioner, I think the question 
is reasonable. It would be great if an FFmpeg expert on this list would 
give a straightforward answer.


In my humble opinion, it is a reasonable thing to expect that a tool's 
documentation explains something as fundamental as whether the tool 
delivers the decoded data content of an input file unchanged to the 
start of a processing pipeline, or whether the tool makes a change to 
the data content.

In my experience, FFmpeg frequently lacks documentation with this sort 
of detail. The reflexive response of the FFmpeg experts active on this 
list is typically, "go read the source code". But the source code is 
complex and difficult to follow, so it makes poor documentation for this 
purpose.  And for some reason, the FFmpeg project has an immune 
response[2][3] to documentation improvements[1].

Best regards,
      —Jim DeLaHunt

[1] e.g. one proposed improvement 
<https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2020-April/261467.html>
[2] immune response to [1] 
<https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2020-April/261468.html>
[3] more immune response to [1] 
<https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2020-April/261586.html>