11/20/90
 
 Gif Viewer 2.1  by David Jansen
 
 
 This version corrects a minor bug in the 320 multi-color mode viewer.  The
 only other change is in the file name prompt.  Other than these two changes
 it is identical to version 2.0.
 
 
 
 Option "A" displays the disk directory of the current device.  Press any key
 return to the main menu, press stop to abort.
 
 Option "B" sets the current device.  The device number can range from 8-11.
 
 Option "C" determines the graphic mode used when drawing a GIF, and also the
 file format to be used when saving an image (see option "H" below).  When this
 option is "Yes" the GIF is drawn in hi-res multi-color mode.  When "No"
 standard hi-res mode is used.
 
 Option "D" applies to multi-color mode only and determines the screen width.
 The physical screen size in multi-color hi-res mode is 160 x 200.  This option
 should normally be set to 160 so that it matches the screen display.  You can,
 however, set it to 320.  This will cause greater distortion of the image
 because you will be drawing a 320 pixel width on a 160 pixel wide screen.  The
 advantage of this is you see more of the image on screen.
 
 Option "E" allows you to scroll the viewing window left or right.  The value
 here determines the horizontal starting position.  The horizontal viewing range
 is:  value of option "E" to value of option "E" + screen width - 1
 
 Option "F" allows you to scroll the viewing window up or down.  The value here
 determines the vertical starting position.  The vertical viewing range is:
 value of option "F" to value of option "F" + 199
 
 Option "G" allows you to use gray scaling.  When this option is set to "Yes"
 all colors will be translated to a shade of gray.  While this mode strips the
 color from an image you will find that it often produces the best results.
 
 Option "H" allows you to save the previously viewed GIF.  Before using this
 option you must first view a GIF (it must be viewed completely for the save to
 work correctly).  Then select this option.  The save format is determined by
 the hi-res mode you used to view the GIF.  If option "C" is "yes" the save will
 be in Koala format.  If option "C" is "no" the save will be in Doodle format.
 It is your responsibility to provide proper names (use "DD" as the first 2
 characters in Doodle names.  For Koala use a temporary name and rename it later
 with the "pic" prefix).
 
 Option "I" allows you to view a GIF.
 
 Option "J" allows you to exit to BASIC.
 
 
 
 Notes:
 
     Windows are an important concept.  By changing the viewing window you can
 view virtually any part of a GIF.  If you set the window values to a range
 outside of the GIF image's range you will get a blank screen.  I suggest using
 either hi-res or multi-color hi-res with a width of 320, and set both window
 values to zero the first time you view a GIF.  When you get an idea of how the
 image appears you can move the window to the area you wish to view more
 accurately.  The best results are usually found in multi-color hi-res mode with
 a width of 160.  Gray scaling often increases the quality.  Experiment until
 you feel comfortable with these features.
 
 If you ever come across a GIF that was created from a C64 graphic you should
 always set your window values to a multiple of 8 so that you don't get
 "framing" errors.  To illustrate this point I have included a GIF
 ("midearth.gif") that I created from the Doodle file "ddmiddlearth."  To view
 this file set option "C" to "no" so that you are in hi-res mode, and set your
 window values to zero.  Then view the file.  If you did it correctly you will
 see a replica of "ddmiddleearth."  Now set the "Window X" value to 1.  View it
 again and you will see "framing" errors.  You will find that window values that
 are multiples of 8 will work correctly, but other values corrupt color.  The
 reason has to do with the way the C64 displays color.  In hi-res mode each 8 x
 8 matrix of pixels can display 2 colors.  Suppose this is the upper 16 x 8
 pixels in the image:
 
 bbbbbbbb yyyyyyyy
 gggggggg yyyyyyyy
 bgbgbgbg ybybybyb        Note that each 8 x 8 area has at most 2 colors
 bbbbbbbb yyyyyyyy        The first has blue & green, the second has
 bbbbbbbb yyyyyyyy        yellow and blue.
 bbbbbbbb yyyyyyyy
 bbbbbbbb yyyyyyyy
 bbbbbbbb yyyyyyyy
 
 b=blue g=green y=yellow
 
 
 With Window X equal to 1 you effective shift the picture 1 pixel left.  You the
 get:
 
 bbbbbbby yyyyyyy?
 gggggggy yyyyyyy?        Each pixel was shifted 1 to the left.  The "?"
 gbgbgbgy bybybyb?        represent the leftmost pixels from the third
 bbbbbbby yyyyyyy?        8 x 8 matrix (not shown in this example).
 bbbbbbby yyyyyyy?        The leftmost 8 x 8 matrix now has 3 colors in it,
 bbbbbbby yyyyyyy?        blue, green, and yellow.  The C64 cannot put 3
 bbbbbbby yyyyyyy?        colors in this area, and this causes a color
 bbbbbbby yyyyyyy?        conflict.
 
 
 This framing error occurs most notably in C64 format GIF's.