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1987-11-17
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V G A G I F
GIF Decoder for VGA and compatible cards
Rev 2.0
Copyright (c) 1987 by Michael Vigneau
*****************************************************************************
VGAGIF is a program that will display a GIF (Graphics Interchange Format)
image using VGA mode 13h, also known as "multi-color" or "MCGA" mode.
VGAGIF can handle any image up to 720 x 400 pixels, and up to 256 total
colors. A Zoom/Scroll mode is also supported.
VGAGIF requires 40K of free memory to run, and at least 330K free to
use the Zoom feature.
The format for VGAGIF is:
[d:path\]VGAGIF [-options] [d:path\]gif-file[.gif]
Options are:
-z Zoom mode
If Zoom mode is selected, the image is stored in memory, and then
displayed. Otherwise, the image is displayed as it is decoded.
VGAGIF uses BIOS calls to set the Video Mode and Palette. Because of this,
the program -should- work correctly with "VGA compatible" cards. Note that
the program writes directly to video memory when displaying an image; boards
that do not handle this correctly will not work with VGAGIF.
PIXEL SKIPPING
If the image is larger than 320 pixels across or 200 pixels down, it will
be displayed by skipping every other pixel. This will allow the full image
to be seen on the screen.
Unfortunately, pixel skipping will often adversely affect the quality of
the image, especially those that contain pseudo-colors made by
"checkerboarding" two different colors. The pixel skipping will drop out
one color, displaying a solid (and wrong) color instead. This problem occurs
mostly with "paint" program images, and least with "smooth" digitized images.
Note that all pixel skipping problems can be overcome by using Zoom Mode
(see below).
WHAT?? NO HI-RES?
By now you may have realized that VGAGIF does not use the Hi-Res
(640x480) mode of the VGA, thought by many to be the "real" VGA mode. There
are several reasons for this.
Firstly, I believe that the best advantage of VGA over EGA, at least for
GIF files, is the much larger color palette, not the increased resolution.
The 640x480 mode has more lines than EGA, but it still can only do 16
colors. This is simply not good enough for rendering high-quality digitized
photos, regardless of resolution. Most Amiga-made GIF files are 320x200;
they look great because all those extra colors make up for the "lack" of
resolution. More colors allow greater range of shading and gradation; in
other words, more "realism".
The "multi-color" mode of the VGA allows 256 different colors to be
displayed at once. These colors are chosen from a pallette of 262,144
total colors. Using this mode, even 256-color GIF files like VANNA
and EARTH are quite accurately reproduced.
However, if you are longing for 640x480, you should use Ray McVay's
EGAGIF program. If you use "-m18" as an option, EGAGIF will use the VGA
hi-res mode.
ZOOM MODE
Zoom mode allows you to see full detail on images larger than 320x200,
as well as "blowing up" these images even further. Zoom mode uses quite
a bit of memory, in order to store an entire 640x400 image.
When Zoom mode is selected, VGAGIF will load the image into memory before
displaying it. To zoom in, press the grey "+" key next to the numeric
keypad. To zoom out, use the grey "-" key. You can scroll around the image
quickly with the cursor arrow keys; holding Shift down while using the arrows
will scroll by a very small amount (assuming NumLock is off). Pressing [Home]
will bring you to the top left corner, while [End] will bring you to the
bottom right. Any other key will clear the screen and end the program.
HANDY MS-DOS TIP
To display all GIF pictures in a directory, type the following line at the
DOS prompt:
FOR %X IN (*.GIF) DO VGAGIF %X
Put any options desired between the "VGAGIF" and the final "%X".
FEEDBACK
If you find VGAGIF to be useful to you, I would like to hear from you.
I am open to suggestions for improvements. Please correspond via Easyplex
to [76703,4275] or write a letter to:
Michael Vigneau
25 Emerald St.
Newton, MA 02158
I hope you enjoy using VGAGIF.
----------------------
GIF and 'Graphics Interchange Format' are trademarks (tm) of
CompuServe Incorporated, an H&R Block Company.