spelling/grammar fixes

Originally committed as revision 6757 to svn://svn.ffmpeg.org/ffmpeg/trunk

vhook/watermark.c

@@ -2,39 +2,39 @@
  * Watermark Hook
  * Copyright (c) 2005 Marcus Engene myfirstname(at)mylastname.se
  *
- * flags to watermark:
+ * parameters for watermark:
  *  -m nbr = nbr is 0..1. 0 is the default mode, see below.
  *  -t nbr = nbr is six digit hex. Threshold.
- *  -f file = File is the filename of watermark image. You must specify this!
+ *  -f file = file is the watermark image filename. You must specify this!
  *
  * MODE 0:
- * The watermarkpicture works like this. (Assuming colorintencities 0..0xff)
+ * The watermark picture works like this (assuming color intensities 0..0xff):
  * Per color do this:
- * If mask color is 0x80, no change to original frame.
- * If mask color is < 0x80 the abs difference is subtracted from frame. If
+ * If mask color is 0x80, no change to the original frame.
+ * If mask color is < 0x80 the abs difference is subtracted from the frame. If
  * result < 0, result = 0
- * If mask color is > 0x80 the abs difference is added to frame. If result
+ * If mask color is > 0x80 the abs difference is added to the frame. If result
  * > 0xff, result = 0xff
  *
- * You can override the 0x80 level with the -t flag. Eg if threshold is 000000
- * the color values of watermark is added to destination.
+ * You can override the 0x80 level with the -t flag. E.g. if threshold is
+ * 000000 the color value of watermark is added to the destination.
  *
  * This way a mask that is visible both in light pictures and in dark can be
- * made (fex by using a picture generated by gimp and the bump map tool).
+ * made (fex by using a picture generated by Gimp and the bump map tool).
  *
  * An example watermark file is at
  * http://engene.se/ffmpeg_watermark.gif
  *
  * MODE 1:
  * Per color do this:
- * If mask color > threshold color, watermark pixel is going to be used.
+ * If mask color > threshold color then the watermark pixel is used.
  *
  * Example usage:
  *  ffmpeg -i infile -vhook '/path/watermark.so -f wm.gif' -an out.mov
  *  ffmpeg -i infile -vhook '/path/watermark.so -f wm.gif -m 1 -t 222222' -an out.mov
  *
  * Note that the entire vhook argument is encapsulated in ''. This
- * way, arguments to the vhook won't be mixed up with those to ffmpeg.
+ * way, arguments to the vhook won't be mixed up with those for ffmpeg.
  *
  * This file is part of FFmpeg.
  *