I found FFmpeg too tedious to build and simply used the ffmpeg command line tool (as a prebuilt binary) from Perl - that's not hard either and saved me the hassle of collecting, configuring and compiling all the various libraries that ffmpeg needs to work. And luckily, ffmpeg can take the video stream from STDIN and can output the results to STDOUT.

$mplayer_path = "/path/to/mplayer";
$video_file = "test_file.avi";

#create a temporary directory.
my $temppath = "/tmp/screenshots." . int(rand(1e7));

if( ! mkdir($temppath)){ print("Error: $!\n"); exit(1); }

if( ! chdir($temppath)){ print("Error: $!\n"); exit(1); }


# Pick a mode:
# Every single frame
# system($mplayer_path, "-vo", "png:z=3", "-ao", "null", $video_file);
# Every 20th frame
system($mplayer_path, "-vo", "png:z=3", "-vf", "framestep=20", "-ao", 
+"null", $video_file);
# Every keyframe
# system($mplayer_path, "-vo", "png:z=3", "-vf", "framestep=I", "-ao",
+ "null", $video_file);        

#mplayer creates many files of the form "^\d{8}\.png$" in the current 
+directory.

my $dirh;
if( ! opendir($dirh, $temppath)) { print("Error: $!\n"); exit(1); }
my @frames = readdir($dirh);
closedir($dirh);

foreach(@frames){
  #do something useful, or delete the file.
}
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Awesome! Thanks guys!!

Also googling for something like "mplayer screenshot" should give good results.

#> I am looking for Linux tools for extracting the individual frames o
+f
#> an AVI movie file, and saving them to files in bitmap formats such 
+as
#> JPEG. Any suggestions?

mplayer -vo jpeg z-men.mpg
#(generates 1 JPEG per frame, plays all frames)

# mplayer -vo jpeg -ss 1:23:45 -frames 20 file.avi
#(-sseeks to 1 hour 23 minutes 45 seconds in file.avi, generates 20 JP
+EGs
#from the subsequent frames, quits)

#"man mplayer" for more stuff, like skipping to the start of a DVD
#chapter.
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