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

[Mark Filipak]

Hello!
On 04/15/2020 07:13 PM, pdr0 wrote:
> On 04/15/2020 00:25 AM, I wrote:
>> This:
>> 
>> 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)',split[E][F],[E][F]blend[D],[C][D]interleave"
>> OUT
>> 
>> outputs 598 frames. 'blend' outputs as expected.

May I make it plainer?

Filtergraph using 'blend':

IN (240 fr) > telecine (600 fr) > (cont. below)

split#1 [A] > select (480 progressive frames) > [C]
         [B] > select (120 combed frames) > [D]

                 [C] > (480 progressive frames) > interleave (600 fr)
[D] > split#2 > (120 Cframes) > blend (120 fr) >
               > (120 Cframes) >

Note: interleave output is actually 598 frames.

>> This:
>> 
>> 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[D],[C][D]interleave"
>> OUT
>> 
>> outputs 716 frames. 'tblend' (documented in the same article) outputs extra
>> frames.

Filtergraph using 'tblend':

IN (240 fr) > telecine (600 fr) > (cont. below)

                  [C] > (480 progressive frames) > interleave (720 fr)
[D] > separatefields (240 fr) > tblend (240 fr) >

Note: interleave output is actually 716 frames.

My point is that 'blend' outputs 120 frames while 'tblend' outputs 240 (actually, 219) frames.
Due to slipshod documentation -- the documentation is combined -- the difference is not noted.
Of course, my complaint began as a bug complaint, not a documentation complaint.
I can live with behavior differences.

> You would expect 719 frame output if you started with a 240 frame, 23.976p
> clip

Sure.

> tblend changes the frame count by -1 . Default mode doesn't appear to do
> anything except drop the original frame zero. When you use all_mode=average,
> it blends adjacent frames.  The new frame zero becomes 50/50 mix of (old
> frame 0,1). New frame one becomes a 50/50 mix of (old frame 1,2).  You can
> test the filter by itself to verify this
> 
> When you separate fields, you have 2 times the number of original frames.
> If you resize them to full height and treat them as frames, you still have 2
> times the number of frames on that selection set

Of course.

> If you started with a 240 frame clip , you should end up with a 600 frame
> clip after that telecine filter only with those settings.

Of course.

> If you take every 5th frame from the telecine output ; 600/5 =120 .
> Separating fields give you 120*2 =240. Applying tblend=all_mode=average
> after gives you 240-1=239 frames. This is [D] .

Of course.

> [C] is the other frame selection set; 600-120=480
> 
> Interleaving selection [C] with [D] should give you (600-120) + (240-1) =
> 719
> 
> The 716 might be from interleave dropping frames at the end when it hangs.
> Expect it to hang because there is no end-of-stream signal when used with
> select.
> 
> https://ffmpeg.org/ffmpeg-filters.html#interleave_002c-ainterleave

You have confirmed everything that I assert. Thank you.