Using ffmpeg to capture frame by number

[hellgauss]

I need to capture several video frames as .png from .mkv files in an automated way, starting from the frame numbers.

I use the following .bat file (windows) which extracts e.g. 4 frames from a .mkv with yuv video.

set "f1=12345"
set "f2=55555"
set "f3=111111"
set "f4=141414"

ffmpeg -y -bitexact -i "myvideo.mkv" -map 0:v -vf "select='eq(n,%f1%)+eq(n,%f2%)+eq(n,%f3%)+eq(n,%f4%)'" -fps_mode passthrough -sws_flags accurate_rnd+full_chroma_int -bitexact "im_%%d.png"

The output seems to be ok for me. The problem is that ffmpeg parse (re-encode?) the whole video and it takes *a lot* of time, especially if I use the -sws_flags which I need for precision.

Is there a way to do that in a fast way without calculating the exact timestamp of the frames?

If needed I can also switch to another command-line windows utility, as long as it is well tested and the yuv-->rgb conversion is accurate.

Thanks
HG