Category : Display Utilities
Archive   : GIFINFO.ZIP
Filename : TGIF.DOC

Welcome to tgif, the Two-toned GIF utility!


Tgif is a utility program to display color graphic images on a plasma,
LCD or monochrome graphics display. The input format is the Compuserve
GIF format for color images. Tgif unpacks these GIF files and performs
a halftone process on the image so that the various colors of the
palette are displayed using patterns of black and white pixels that
approximate the intensities of each color. This is the same process
that allows images to be printed in black and white media such as books
and newspapers. The algorithm used to perform the halftone
transformation is an error distribution process developed by R. W.
Floyd and L. Steinburg.

Tgif starts by computing the intensity of each palette color by summing
the values of the red, green and blue intensities supplied in the GIF
image. A threshold intensity level is computed to represent the average
intensity of all colors in the palette. This threshold value is used to
make the decision as to whether a displayed pixel is black (pixel <=
threshold) or white (pixel > threshold). Unless the pixel displayed is
exactly black or white, an intensity error is incurred. This error
represents the difference between the pixel's intensity and what was
actually displayed. When distributed among the neighboring pixels, this
error influences the threshold decision for those pixels and changes
their display values thus creating a distribution of black and white
pixels that approximates the color intensity.

The above algorithm not only produces excellent visual results, it also
provides a remarkable and unexpected feature when the image is enlarged.
Tgif provides option switches to enlarge the image beyond the size
supplied by the GIF file. What this actually does is increase the
number of pixels in the drawing. In the instance of a double size
image, when a single pixel is unpacked from the GIF file tgif creates
four pixels of the same color; a block of four pixels where there was
just one. The result is the same image but with pixels twice the
original size. When this "big bit" image is halftoned by tgif, the "big
bits" are resolved into an image that actually has more detail than the
smaller version of the same image. In other words, the big pictures
look better, have more detail and look just as smooth as the smaller
images. Images can be enlarged to triple size with the same results.
Only when enlarged to quadruple size did the resulting image start to
get blocky and degraded, although this might be cured by increasing the
fractional part of the error accumulators used in the algorithm.

Tgif was developed to run using Toshiba plasma type display screens with
640 by 400 resolution (T3100, T3200, T5100). If no command line option
is used, tgif will attempt to set the video mode to the special mode
0x74 used by Toshiba plasma screens. A command line option forces tgif
to use any CGA type adapter and another option will force inverse video
for CGA compatible LCD displays. The CGA mode used by tgif is the 640
by 200 two color mode (mode 6). Since the aspect ratio of the pixels in
this mode is not 1:1 (that is, CGA pixels are taller than they are wide)
it is recommended that the command line option to widen the image be
used.















The usage of tgif is as follows:


Usage: tgif {/w} {/l} {/d} {/t} {/r} {/c} giffile{.ext}

Options: /w == widen image
/l == lengthen image
/d == double size
/t == triple size
/r == reverse video (for LCD screens)
/c == force CGA mode

Example: tgif/d/r picture
Would open picture.GIF and display a double size
image in reverse video.

Notes:

Command line options are stacked onto the program name and
separated by slashes.

If a filename extension is not included, ".GIF" is assumed.


Once invoked, tgif starts unpacking and halftoning the GIF image. If the
image is supplied in interlace mode, tgif uses two passes; one to unpack
and de-interlace the raster lines and a second to halftone the pixels.
For non-interlaced images, one pass performs both operations. As the
color image is halftoned, the black and white raster data is stored into
memory until the entire image has been completed. Tgif provides status
displays about the GIF file as it is being processed.

Once completed, tgif shifts into an image display mode where the user
may view the image. If the completed image is larger than the selected
display size, the arrow keys may be used move the display window to any
part of the image. The active keys are:

key usage

up-arrow move window up
down-arrow move window down
left-arrow move window left
right-arrow move window right
page-up big move up
page-down big move down
home display top of image
end display bottom of image
escape exit tgif















The source code module tgif.c has been provided for a number of reasons.
Hopefully it will help others avoid some of the pitfalls encountered
while unpacking and displaying GIF file images. The included tgif.exe
file was compiled directly from the included tgif.c source module using
MIX software's Power C compiler. This and many other "c" projects have
convinced me that this compiler is one of the greatest values in the
world of computing.

You will also notice that the source code statements in tgif.c are
uncommented. Since this is a shareware product, you are requested to
support this effort with a $10 shareware fee. Fully commented listings
as well as future enhancements will be available to registered users of
tgif. One enhancement that is foreseeable is the conversion of certain
procedures into assembly language to increase the speed of tgif's
processing.

The reporting of any bugs, anomalies or GIF files that tgif cannot
display would also be appreciated.

To all novice or beginning users, tgif is offered for your entertainment
and possibly educational uses. To all of you experts I offer a friendly
challenge: now that you have the source code, make tgif better in some
way and let me know how you did it.

This code was written and will be supported by:

James Lark
6101 W. Courtyard Dr.
Austin, TX. 78730