[FFmpeg-user] dv => mp4: deinterlace or not, and how?

[Baptiste Coudurier]

On 05/06/2011 10:02 AM, sean darcy wrote:
> On 05/06/2011 12:04 PM, sean darcy wrote:
>> On 05/05/2011 07:42 PM, Baptiste Coudurier wrote:
>>> On 05/05/2011 04:34 PM, sean darcy wrote:
>>>> On 05/05/2011 04:36 PM, Baptiste Coudurier wrote:
>>>>> Hi,
>>>>>
>>>>> On 05/05/2011 01:19 PM, sean darcy wrote:
>>>>>> I have an interlaced dv file. I'm transcoding it with x264 to mp4.
>>>>>>
>>>>>> 1. ffmpeg -i file.dv -an -vcodec libx264 -b<x> out.mp4
>>>>>>
>>>>>> If I just leave it like that, is out.mp4 interlaced or progressive?
>>>>>
>>>>> progressive. By default encoding is progressive.
>>>>>
>>>>>> 2. ffmpeg -i file.dv -an -vcodec libx264 -b<x> -deinterlace out.mp4
>>>>>>
>>>>>> Here I assume out.mp4 is progressive. The ffmpeg documentation says:
>>>>>>
>>>>>> "The alternative is to deinterlace the input stream with
>>>>>> `-deinterlace',
>>>>>> but deinterlacing introduces losses."
>>>>>
>>>>> Correct, it is progressive. Use -vf yadif instead of -deinterlace
>>>>> Deinterlacing may be a bit destructive, especially if the input is
>>>>> _not_
>>>>> interlaced.
>>>>>
>>>>>> [...]
>>>>>>
>>>>>> Given this note about losses, am I right we should never deinterlace?
>>>>>> Almost never? When is deinterlacing required/better?
>>>>>
>>>>> You have options:
>>>>> if the receiving end playback interlaced (CRT tv):
>>>>> encode interlaced
>>>>> else if the receiver is going to deinterlace if the file is marked as
>>>>> interlaced and you trust this deinterlacer, then you may encode
>>>>> interlaced (deinterlacing will take cpu time)
>>>>> otherwise you should deinterlace yourself using a good deinterlacer.
>>>>
>>>> Right. I knew that! Just passed right out of my mind, though.
>>>>>
>>>>> I suggest always deinterlace using -vf yadif if the source content is
>>>>> interlaced
>>>>>
>>>>
>>>> Well I found http://guru.multimedia.cx/deinterlacing-filters/
>>>>
>>>> so I thought I'd try:
>>>>
>>>> -vf "yadif=3:0,mp=mcdeint=2:0:10"
>>>>
>>>> (dv is bottom-field first, right?)
>>>>
>>>> That generates a lot of perplexing output:
>>>>
>>>> [snow @ 0x230df40] pass:4mv changed:1384
>>>> [snow @ 0x230df40] pass:0 changed:1083
>>>> [snow @ 0x230df40] pass:1 changed:407
>>>> [snow @ 0x230df40] pass:2 changed:147
>>>> [snow @ 0x230df40] pass:3 changed:50
>>>> [snow @ 0x230df40] pass:4 changed:17
>>>> [snow @ 0x230df40] pass:5 changed:11
>>>> [snow @ 0x230df40] pass:6 changed:2
>>>> [snow @ 0x230df40] pass:7 changed:1
>>>> [snow @ 0x230df40] pass:8 changed:1
>>>> [snow @ 0x230df40] pass:9 changed:1
>>>> [snow @ 0x230df40] pass:10 changed:1
>>>> [snow @ 0x230df40] pass:11 changed:2
>>>> [snow @ 0x230df40] pass:12 changed:2
>>>> [snow @ 0x230df40] pass:13 changed:2
>>>> [snow @ 0x230df40] pass:14 changed:0
>>>> [snow @ 0x230df40] pass:4mv changed:1864
>>>
>>> This is some debug messages, ignore them.
>>>
>>>> I realize the filter comparison is from five years ago, and yadif may
>>>> have changed significantly since then. Does mcdeint still add anything
>>>> to yadif?
>>>
>>> I think nothing has changed much since then :)
>>>
>>
>> Now I've tried yadif=1:0. As I understand it, this is "bob"
>> deinterlacing - field doubling (each field becomes a frame) - with
>> spatial and temporal weaving.
>>
>> But the output is strange:
>>
>> [yadif @ 0xfbd9c0] mode:1 parity:0
>> .........
>> frame=38981 fps= 15 q=-1.0 Lsize= 624133kB time=1300.60
>> bitrate=3931.2kbits/s dup=0 drop=38979
>>
>> There's a "drop" for each input frame. I'd understand this for yadif=0,
>> where (as I understand it) 2 fields are combined into 1 frame. But
>> yadif=0 shows _no_ drops.
>>
>> sean
> 
> Ran it with yadif=0:
> 
> [yadif @ 0x1d359c0] mode:0 parity:0
> ........
> frame=38980 fps= 16 q=-1.0 Lsize=  622885kB time=1300.57
> bitrate=3923.4kbits/s    s
> video:622275kB audio:0kB global headers:0kB muxing overhead 0.097974%
> frame I:166   Avg QP:16.67  size: 50398
> 
> No drops. And the resulting file size is approximately the same. But
> shouldn't the yadif=0 file be ~1/2 the size of the yadif=1 file? That
> is, 2 fields are becoming 1 frame, so 1/2 the number of frames. Or is
> x264 just compressing the related "bob" frames so effectively?  Or am I
> misunderstanding this entirely?
> 
> But I still don't get why yadif=1 drops a frame for each input frame.

No, when using mode 1, please read the documentation:
  * 1: send 1 frame for each field

You are outputting 2 frames for one field. If you want no drop you need
to double the frame rate.

-- 
Baptiste COUDURIER
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