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I have a YUV 420NV12 image. I'm using ffmpeg to convert the YUV file to PNG and then back again to YUV.

ffmpeg -s 640x480 -pix_fmt nv12 -i testold.yuv test-640x480 test.png
ffmpeg -i test.png -s 640x480 -pix_fmt nv12 testnew.yuv

When I compare the two YUV files, there are few differences. I expected PNG to be a lossless compression. What is the reason for difference in two files?

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The PNG format is lossless for RGB24 image data. However the conversion from YUV to RGB24 is not lossless, as the two formats quantize the color space differently.

To see this, the following animated gif was made by applying your two ffmpeg operations 200 times back-and-forth and collating the resulting 200 images into the gif.

enter image description here

By contrast, the following gif was generated using yuv444p as the YUV format instead of nv12. This format does not do subsampling / as much quantization. enter image description here

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