[FFmpeg-user] Decombing via screening - 'tblend' bug (?)

Mark Filipak markfilipak.windows+ffmpeg at gmail.com
Wed Apr 15 07:25:17 EEST 2020

Request for bug confirmation is included.

On 04/14/2020 10:24 AM, Paul B Mahol wrote:
> On 4/14/20, Mark Filipak <markfilipak.windows+ffmpeg at gmail.com> wrote:
>> In the command line below,
>>                  I will eventually insert a screen maker --+
>>                                                            ¦
>> separatefields,scale=height=2*in_h:sws_flags=neighbor,--HERE--,tblend=normal[D]
>> but first, I need to persuade 'tblend' to accept the 2 frames made by
>> 'separatefields' (and scaled
>> by 'scale').
>> But as you can see in the next section (extracted from the log), though the
>> 'height=2*in_h'
>> directive works (i.e., the 1920x540 frames from 'separatefields' are scaled
>> to 1920x1080), ffmpeg is
>> not happy. Is it because the SARs don't match? How can I overcome that?
>> Thanks!
> Insert setsar=1 after scale?

Thank you, Paul. It works. I apologize for not finding that solution on my own ...how embarrassing.

Next problem: 'tblend' appears to be malfunctioning.

"The tblend (time blend) filter takes two consecutive frames from one single stream, and outputs the 
result obtained by blending the new frame on top of the old frame."

#1 - This works as expected:
ffmpeg -i IN -filter_complex "telecine=pattern=5, split[A][B], 
[A]select='not(eq(mod(n+1\,5)\,3))'[C], [B]select='eq(mod(n+1\,5)\,3)', 
datascope=size=1920x1080:x=45:y=340:mode=color2[D], [C][D]interleave" OUT

#2 - This malfunctions:
ffmpeg -i IN -filter_complex "telecine=pattern=5, split[A][B], 
[A]select='not(eq(mod(n+1\,5)\,3))'[C], [B]select='eq(mod(n+1\,5)\,3)', separatefields, 
scale=height=2*in_h:sws_flags=neighbor, setsar=1, tblend, 
datascope=size=1920x1080:x=45:y=340:mode=color2, datascope=size=1920x1080:x=45:y=340:mode=color2[D], 
[C][D]interleave" OUT

#1 brings up datascope in frames (n, zero-based): 2 7 12 17 etc., as expected.
#2 brings up datascope in frames (n, zero-based): 4 7 10 13 etc.

Another clue is:
#1 OUT has 598 frames.
#2 OUT has 716 frames.

Would someone who has the latest nightly build kindly confirm this. I can supply the test video -- 
contact me off-list.


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