[SOLVED] ffmpeg re-scaling results in a ripple effect

GPGAgent · 02-20-2021, 08:26 AM

I'm re-encoding and resizing a few videos from .mpg to .mp4 using ffmpeg, from 704x576 to 640x524

Code:

-vf "scale=640:-1"

this results in a rippling effect on any rapid movement in that part of the video.

Removing the -vf scale switch and the mp4 re-encoded file is fine.
Anyone have a few pointers as to what I should look for, ddg hasn't uncovered any clues.

ondoho · 02-21-2021, 01:19 AM

I have difficulty understanding "ripple effect", a sample video would be nice.
Transcoding is never without pitfalls and requires careful adjustment for each type of video (codec, compression/bitrate, resolution).
I can imagine all sorts of artifacts popping up, esp. with relatively small (704 => 640) changes.

Meanwhile:
https://trac.ffmpeg.org/wiki/Scaling

GPGAgent · 02-21-2021, 07:08 AM

Quote:

Originally Posted by ondoho

I have difficulty understanding "ripple effect", a sample video would be nice.
Transcoding is never without pitfalls and requires careful adjustment for each type of video (codec, compression/bitrate, resolution).
I can imagine all sorts of artifacts popping up, esp. with relatively small (704 => 640) changes.

Meanwhile:
https://trac.ffmpeg.org/wiki/Scaling

Cheers,
I'll put together a n example but imagine as the camera pans down say, and a diagonal line will appear as a wavy line as the camera pans, if I don't scale, but simply re-encode at a lower video bitrate (to reduce the size of the video) it's straight line as the camera pans

GPGAgent · 02-21-2021, 07:56 AM

I think I found the culprit - I was using a filter to deinterlace the video, I tried a few and they all exhibit this affect

Code:

-vf "scale=640:-1,bwdif"

I also tried

Code:

w3fdif, kerndeint and yadif

They all just produce a different rippling affect. Leaving out the deinterlace filter it's fine but you can see the interlacing.

Or don't scale the video, but use the filter

Code:

-vf bwdif

which works fine

GPGAgent · 02-21-2021, 08:18 AM

Final bit of this puzzle, the original video has a size of 704x576 - it's a dvd

Now somewhere I think 16 comes into this so I divided 704 by 16 which gives 44

So to halve the size of the video 44 times 8 equals 352, and

Code:

-vf "scale=352:-1,bwdif"

works fine, and so does various other factors of 44, but not all.

teckk · 02-21-2021, 08:42 AM

You have not shown what you are doing. What video codec are you using inside of that mp4 container? Both of those ratios are about 1.22:1. Why that ratio, and why 640x524?

Quote:

I'm re-encoding and resizing a few videos from .mpg to .mp4

What kind of video, what kind of audio, how many fps in/out, at what bitrate?

How about:

Code:

ffmpeg -i Input.mpg -c:a aac -b:a 192k -af volume=1.5 -c:v libx264 -crf 18 -preset slow -maxrate 3000k -bufsize 1M -r 24 -s 640x524 Output.mp4

That's aac audio at 192k, volume made a little louder, x264 video using a -crf preset, maxrate of 3000k, a 1MB buffer that allows changing 30 fps to 24, resized to 640x524.

GPGAgent · 02-21-2021, 09:20 AM

This is what I use

Code:

ffmpeg -ss 1:29:56 -to 1:34:30 -i VideoIn.mpg  -c:v libx264  -preset veryfast -vf "bwdif"  VideoOut.mp4

The DVD I'm re-encoding is 704x576. it's one of the many official sizes for DVDs or broadcast media and I try to standardise my rips to a width of 640 - no reason why, the height of 524 is calculated by ffmpeg when you use scale=640:-1 - simples