I need to reduce the file size of an mp4 video. I don't need color information at all but pixel brightness resolution must be preserved. Videos consist of white background with a few (important) black dots of about 5px diameter. I am confused about what the best method/output would be.
Without knowing the video properties, my generic answer is
ffmpeg -i in.mp4 -vf geq=lum='p(X,Y)':cb=128:cr=128 out.mp4
This will reset the color information, but pass the luma plane intact. Add
-crf 0 for truly lossless re-encoding.
This is the faster way,
ffmpeg -i in.mp4 -vf extractplanes=y,format=yuv420p out.mp4
The luma plane is extracted, and then neutral chroma planes are added in the pixel format conversion. This 2nd step is added because otherwise ffmpeg will auto-convert pixel format to yuv444 which isn't widely compatible.