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

[sean darcy]

On 05/06/2011 01:27 PM, Baptiste Coudurier wrote:
> 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.
>

Lost. let me go back to basics. I've got an interlaced input with 38980 
"frames". But each of these frames is of 2 fields - each half the size 
of a progressive frame. And ~60 (59.94) fields are shown each second.

For yadif=0, 2 fields are combined into 1 frame. So with my input, I 
should get the same number of "frames". The framerate would be to ~30 
(29.97) frames per second.

For yadif=1, each field is reconstructed into a frame. "send 1 frame for 
each field" . So I have twice the number of "frames", and each frame is 
a full size progressive frame. And the framerate should now be ~60??

So if I use yadif=1 with a standard 29.97 frame rate, half the frames 
are discarded. Which means there's no benefit to yadif=1!

You'd need to set -r 59.94, and there'd be few if any players for your clip!

Am I getting closer?

Why would anyone ever use yadif=1 "bob" deinterlacing?

sean