[FFmpeg-user] I found the bugs

[Mark Filipak]

Hi Jim!

On 13/06/2024 17.04, Jim DeLaHunt wrote:
> On 2024-06-13 13:16, Mark Filipak wrote:
>> On 13/06/2024 02.16, Carl Zwanzig wrote:
>>> On 6/12/2024 2:44 PM, Mark Filipak wrote:
>>>>
>>>> I need someone more knowledgeable about FFmpeg to triage. 
>>>
>>> They're often called 'developers'.
>>
>> Okay, Carl. https://trac.ffmpeg.org/ticket/11055
>>
>> Since there are so many things wrong, I predict the devs will do nothing.
> 
> Mark, I agree with Carl that it is a better choice to turn your findings into a bug report than to 
> wait for replies to an ffmpeg-user list post. Good for you for writing ticket #11055.
> 
> However, let me anticipate the feedback of developers reading this ticket: it is not yet a good bug 
> report.  The FFmpeg project has advice on what to include in a bug report at 
> <https://ffmpeg.org/bugreports.html>. The advice includes:
> 
>> // When writing your bug report, please include (uncompressed):
>>
>>   * What you were trying to accomplish (e.g., "I am trying to
>>     transcode from this format to that format...")
>>   * The problem you encountered (e.g., "ffmpeg crashed, see the gdb
>>     and valgrind output below" or "The output video was all green")
>>   * The exact command line you were using (e.g., "|ffmpeg -i input.mov
>>     -an -vcodec foo output.avi|")
>>   * The full, uncut console output provided by |ffmpeg -v 9 -loglevel
>>     99 -i | followed by the name of your input file (copy/pasted from
>>     the console, including the banner that indicates version and
>>     configuration options), paste ffplay or ffprobe output only if
>>     your problem is not reproducible with ffmpeg.
>>   * Sufficient information, including any required input files, to
>>     reproduce the bug and confirm a potential fix.
>>
> I don't see a clear statement of what you are trying to accomplish. I think it is: "I am trying to 
> generate a video file which has correct PTS and DTS values and which plays correctly on MPV and 
> FFplay, by splicing two video files together." But maybe it is: "I am trying to get FFmpeg's -vf 
> showinfo filter and its -show_frames option to output consistent numbers."

I have no control over what '-vf showinfo' & '-show_frames' report. The issue is that they output 
different numbers and that the numbers are different from reality.

> I think you made a good faith effort to state the problem you encountered, but I think you did not 
> follow through and explain your data. You don't explain what each of your columns depict (though 
> maybe someone more expert than me would understand from just your column headers). You say, "'-vf 
> showinfo' disagree: 53 frames, gap & duplicates.", but you don't say with what showinfo disagrees, 
> and you don't point out which frames demonstrate the disagreement, which demonstrate gaps, which 
> demonstrate duplicates. Maybe an additional column or so would be a good place to codes to denote 
> disagreement, gaps, and/or duplicates.

I show the gaps. I guess that's not enough, eh?

> You do supply the exact command lines you used. You also specified which version of FFmpeg you used. 
> Good for you! But I don't see that you supplied "The full, uncut console output provided by ffmpeg 
> -v 9 -loglevel 99"….

The console output for the 3 command lines is summarized in the table. Their actual outputs are files.

> I don't see where you supply the required input files to reproduce the 99-frame output video or the 
> diagnostics you list.

I uploaded the 4 second video twice and it was acknowledged. Isn't it there? The ...whateveritis is 
so cryptic. It's made for programmers, not human beings.

> Part of the process of bug fixing is to convert information from the bug reporter's context and 
> frame of reference to the bug fixer's context and frame of reference. Someone has to do that work.  
> One of the reasons why bugs don't get fixed is that the work doesn't get done.  If you really want 
> the bug to get fixed, maybe you will have to do more of that work than you prefer to.

I spent 2 months in the investigation. It was an 'unending' series of discoveries of more and more 
things wrong with FFmpeg.

Thanks, Jim. What I'm flagging is not the commands. It's the internals of FFmpeg. It appears there 
are at least two internal routines -- duplicate functions -- that 'get' DTSes and PTSes. I really 
don't understand the source of the problem because DTS & PTS is in the TS packets. I don't know 
whether FFmpeg is getting packets mixed up or is rewriting the DTSes & PTSes. FFmpeg should never 
rewrite, but I know it does that _a_lot_.

--Mark.