Convert AVI to grayscale PNG

Large pixel-count or frame-count AVIs can be converted frame-by-frame to PNG. One can then use Irfanview or other common image browser to flick back and forth between movie frames.

The conversion uses ImageMagick, which calls FFmpeg.

magick -verbose CMOSvideoshorter_halfspeed.avi -type Grayscale CMOSvideo_%03d.png