Add to the ffmpeg docs an usage example showing how to extract images

from a video.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@14711 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

doc/ffmpeg-doc.texi

@@ -140,6 +140,25 @@ The mapping is particularly useful for DVD transcoding
 to get the desired audio language.
 
 NOTE: To see the supported input formats, use @code{ffmpeg -formats}.
+
+* You can extract images from a video:
+
+@example
+ffmpeg -i foo.avi -r 1 -s WxH -f image2 foo-%03d.jpeg
+@end example
+
+This will extract one video frame per second from the video and will
+output them in files named @file{foo-001.jpeg}, @file{foo-002.jpeg},
+etc. Images will be rescaled to fit the new WxH values.
+
+The syntax @code{foo-%03d.jpeg} specifies to use a decimal number
+composed of three digits padded with zeroes to express the sequence
+number. It is the same syntax supported by the C printf function, but
+only formats accepting a normal integer are suitable.
+
+If you want to extract just a limited number of frames, you can use the
+above command in combination with the -vframes or -t option, or in
+combination with -ss to start extracting from a certain point in time.
 @c man end
 
 @chapter Invocation