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2023-02-26
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Gif Viewer 2.0 by David Jansen
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
oftens 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 "ddmiddle earth." 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 "ddmiddle earth."
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.
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