[FFmpeg-devel] [PATCH] Change default behaviour of scale filter from 'progressive' to 'auto'

Mark Himsley mark at mdsh.com
Wed Apr 4 23:13:39 CEST 2012

On 02/04/2012 22:11, Mark Himsley wrote:
> I'll demonstrate in another email (when I get my act together) the
> issues you can get if you scale progressive material with an interlaced
> scaler (it's not criticism of the scaler!).

Got my act together. I throw this out as observation and as a discussion 
point, and not as criticism.

I've knocked up a couple of images that demonstrate the "problem" with 
doing interlaced scaling with progressive media. If I recall I can't 
attach image files to this list so I've put them onto a web page.


The first image, http://mdsh.com/ffmpeg/lines.png, is made up of some 
over-sharp diagonal lines.

I converted that to a video file, which looks ok.

ffmpeg -loop 1 -r 25 -i lines.png -vframes 100 -vcodec libx264 -crf 21 
-top 1 -an lines-1920.mov

I scaled that to an SD video file in interlaced mode

ffmpeg -i lines-1920.mov -vf 
scale=704:576:interl=1,pad=720:576:8:0:black -aspect 16:9 -vcodec 
libx264 -crf 21 -top 1 -an lines-720.mov

And exported that to a picture again.

ffmpeg -i lines-720.mov -vframes 1 lines-720.png

You should see from the exported image 
http://mdsh.com/ffmpeg/lines-720.png that the diagonal lines are not 
diagonal. That demonstrates how PSNR figures of progressive material can 
be knackered by interlaced scaling.


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