CODEWITHSUNDEEP
codecavconv
nrob
nrob

Reputation: 889

image -> video -> image not lossless using avconv

I'm trying to test the amount of information lost with some different video codecs. I've got a python script which uses PyPNG to write a series of 8 bit RGB images. I then encode it using avconv, for instance

avconv -r 1 -i ../frames/data%03d.png -c:v ffv1 -qscale:v 0 -r 1

outffv1.avi

I then decode this back into pngs like so

avconv -r 1 -i outffv1.avi -r 1 ./outffv1/frame%03d.png

But when I compare the images before and after the video compression, they are different (mean absolute error of ~~15%). The thing that is confusing me is that this is true (give or take) independent of the codec.

For instance, I get similar answers for libtheora for a range of qscale values.

The png encoding i.e. write to png, and immediately load back in without and video compression step, is lossless.

UPDATE - more precise worked example:

Single input frame here: https://www.dropbox.com/s/2utk1xs2t8heai9/data001.png?dl=0 Encoded to video like this: avconv -r 1 -i ./frames/data%03d.png -c:v ffv1 -qscale:v 0 -r 1 outffv1.avi resultant video here: https://www.dropbox.com/s/g1babae2a41v914/outffv1.avi?dl=0 decoded to a png again here: https://www.dropbox.com/s/8i8zg1qn7dxsgat/out001.png?dl=0 using this command: avconv -r 1 -i outffv1.avi -qscale:v 31 -r 1 out%03d.png and image magick differenced like this compare out001.png ./frames/data001.png diff.png to give this (non-zero) diff https://www.dropbox.com/s/vpouk54p0dieqif/diff.png?dl=0

Upvotes: 2

Views: 150

Answers (1)

szatmary
szatmary

Reputation: 31110

Your video file most likely uses the YUV color format. PNG uses RGB. The conversion of the color is not a lossless process.

Upvotes: 2

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