[FFmpeg-user] Apply format with overlay and -ss in front of inputfile is strange?

[nicolab]

Sorry my poor english.

> The "format" of the output of the overlay filter
> is yuva420p. This is not supported by x264, so
> the output "format" file-wise is yuv420p.
> The output is visually gray because you
> overlayed a gray frame (x1) over another
> frame (of the same size). 

Yes. This is right.

I want to know is why -ss <seconds> in front of input file applies overlayed
is visually gray?

Another command samples, and same ffmpeg.

ffmpeg -f lavfi -i rgbtestsrc=d=10,format=yuv420p -vcodec libx264
rgbtestsrc.mp4

This is visually rgb color[z] overlayed video.
ffmpeg -i rgbtestsrc.mp4 -vf split[x][z];[x]format=gray[x1];[x1][z]overlay
-vcodec libx264 10.mp4

This is visually rgb color[z] overlayed video.
ffmpeg -i rgbtestsrc.mp4 -ss 1 -vf
split[x][z];[x]format=gray[x1];[x1][z]overlay -vcodec libx264 11.mp4

This is visually rgb color[z] overlayed video, but output file is visually
gray.
ffmpeg -ss 1 -i rgbtestsrc.mp4 -vf
split[x][z];[x]format=gray[x1];[x1][z]overlay -vcodec libx264 12.mp4




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