[FFmpeg-user] artefact when upload to youtube

[philippe.torgue at laposte.net]

Hi, 

I've used ffmpeg to make a movie from a sequence of png 
But, even with x264 lossless ffmpeg, i get a video on youtube with noticable artefact (the ugly square of cos discrete) 

(the .mp4 is clean before uploading : no artefact at all with quicktime or vlc) 

I guess it's not a ffmpeg problem but may be someone will lead me to the solution.