VIEWGIF (Version 0.8)
Copyright by Craig S. Buchanan
SHAREWARE
This program is copyrighted software. This program and its documentation may
be freely copied and shared with others, as long as no charge is made and the
program and documentation are unaltered.
If you use this program please support it by sending $15 and any suggestions
for improvements to the address listed below. Your support will ensure that
improvements are made.
4-319 Mackay St.
Ottawa, Ont.
K1M 2B7
Contents:
What's New?
1. Introduction
2. Function Overview
3. Known Problems
4. Enhancements Under Consideration
5. Program History
WHAT'S NEW? - Changes in VIEWGIF 0.8
1. All "translate..." conversions now work.
This includes CP8, AIM, and SPU. 256 colour GIF images can now be
converted to 256 gray scale AIM images for image processing using
the "Atari Image Manager". These gray scale images can also be
stored and viewed using the compressed CP8 format.
2. Load CP8 option added.
3. Improved monochrome support.
A dispersed dot dither mode has been added. While the dispersed
dot dither is much slower than the ordered dither the results are
much better. The dispersed dot dither has two modes, scaled and
histogram equalization.
4. Now supports OVERSCAN fully.
The OVERSCAN modification allows an ordinary Atari ST to display
an increased resolution screen in all three ST modes. When
activated OVERSCAN allows an entire Degas picture to be displayed
INSIDE A WINDOW. Viewgif looks great with OVERSCAN displaying full
320x200, 640x200 and 640x400 images inside GEM windows. Overscan
requires a very simple and inexpensive modification to your ST.
5. LOW and MED resolution option to display GIF colour images as GRAY
scale images.
Displaying a 256 colour GIF image on the ST's 16 or 4 colour
display involves a lot of compromise in the image quality.
Converting the image to a gray scale image and using an
intelligent dithering routine often results in much more image
detail being preserved. This can be most clearly seen in Medium
Resolution where the gray scale images are clearly superior to the
colour images.
1. INTRODUCTION
So what are GIF files and why should I be interested? GIF files are
computer graphic images stored in a compressed, hardware independent manner.
The hardware independent format makes it a standard for moving images between
different computers.
VIEWGIF allows your ST to show graphics created on a PC, an Amiga,
whatever. Even if the original image has more colours or pixels than the ST
can display. It also allows you to convert your Atari ST images into GIF
format for exchange with other systems.
VIEWGIF is GEM based and operates in all three resolutions. Images are
displayed in scrollable windows. Once an image is loaded it can be saved in
NEO or Degas Format. The image can also be cropped and shrunk to fit the ST
screen. VIEWGIF also allows a GIF image to be directly transformed into AIM
and Spectrum 512 pictures. This allows the 512 colour Spectrum mode to be used
to display 256 colour VGA images.
2. FUNCTION OVERVIEW
Most menu items have keyboard equivalents which are noted at the end of
each menu item. A caret (^) denotes a control key while a small triangle
denotes an alternate key. The equivalents are listed with their corresponding
functions in the list below.
2.1 FILE - menu
GIF INFO: (ALT I)
Given a GIF file it displays the image attributes including dimension,
number of colours, size of colour palette, etc. It also tries to
determine the image's original video format.
LOAD GIF/NEO/DEGAS/MAC/CP8: (ALT G, ALT N, ALT D, ALT M, ALT P)
Loads an image of the particular file type.
The GIF mode reads in the GIF file and then translates it to the
current ST format according to the options set in the MODE menu. The
two modes are COLOUR and GRAY. (See discussion of MODE.)
The DEGAS mode currently reads uncompressed DEGAS only.
The MAC mode will read and translate into the current resolution any
MacPaint image. Mac StartUpScreens will not read properly.
The CP8 mode always uses the dispersed dot dither to translate the
image into the current ST format regardless of the MODE options COLOUR
and GRAY. (See CP8 discussion in TRANSFORM.) The dispersed dot dither
MODE options HISTO and SCALED are used. (See MODE discussion.)
Each image is displayed in its own window and each window has its own
colour palette. The top window's colour palette is always used. The
original image dimensions are maintained. The CLIP and SHRINK
functions can be used to reduce the image size. The title bar of each
window contains the image filename and the information bar contains a
description of the image colours along with the dimensions of the
image. If the image is in colour then the information bar contains
[a:b] where 'a' is the number of colours in the original image and 'b'
is the number of Atari ST colours. If the image is displayed in gray
scale then the information bar contains [a:b] where 'a' is the number
of colours in the original image and 'b' is the number of grays in the
image.
SAVE NEO/DEGAS/GIF: (CTRL N, CTRL G, CTRL G)
Saves the current image (top window) in the particular file type. The
DEGAS mode writes only uncompressed DEGAS.
Save NEO/DEGAS shows the image to be saved in a full screen view. This
allows the clipping of the current window to be checked before saving.
When overscan or another non native ST video mode is used this full
screen view is not displayed. (It is presumed that in these cases a
full NEO or DEGAS picture will fit inside a window making the preview
unnecessary.)
CLOSE: (ALT C)
Closes the Top Window.
QUIT: (ALT Q or CTRL C)
Exits the program.
2.2 EDIT - Menu.
CLIP:
Crops the current image at one of the four window boundaries. Simply
size and scroll the window to the correct place, select CLIP, and then
select the clipping corner.
SHRINK: (ALT S)
Halves the image size using one of three methods. LINEAR works best on
256 colour images while the two STAGGERED modes work best on 4 and 16
colour images where dithering/patterning has been used to simulate
more colours. Many EGA (an IBM graphic mode) images use a checkerboard
pattern to simulate more than 16 colours. A linear shrink of the image
will produce only one colour in the checkerboard, the staggered modes
are designed to preserve the colour dither.
COLOURS:
Allows the Colour Palette of the Top Window to be edited. The modified
colours can be SAVED, kept with the window, USED, not stored with the
image but left the current colours, and CANCELLED.
DESKTOP: (Undo)
Returns the colour palette to your desktop colours. To return to an
image's palette use COLOURS or make another picture the active (top)
window.
TRANSFORM ...:
Allows a GIF image to be converted to either AIM, CP8 or SPU (Spectrum
uncompressed) without first being converted to screen format. This
allows the maximum of image data to be maintained.
When converting an image to SPU that is larger than 320x199 the top
left hand corner of the image will be used.
When converting to an AIM image both an .IM and a .HD file are
produced.
When converting an image to CP8 a false gray routine will be used if
the image contains less than 256 colours. The false gray routine tries
to ensure that two colours with the same gray value are encoded with
slightly different gray values. The false gray routine is used to
prevent 16 colour images from becoming 4 gray level images.
Why translate to CP8? The CP8 format is designed for 8 bit gray scale
images. It is a hardware independent, compressed format like GIF but
unlike GIF it can be compressed in very little time and decompressed
almost instantly. The CP8 format trades a decrease in compression
performance for a large increase in decompression speed. The CP8
format also allows the decompression routine to detect corrupted data
while the GIF format does not. (This is a small point as the picture
will show any file corruption dramatically.)
2.3 MODE - Menu.
COLOUR:
When COLOUR is selected LOAD GIF translates the GIF file into a colour
picture. This mode is obviously unavailable in monochrome mode.
GRAY:
When GRAY is selected LOAD GIF translates the GIF file into a gray
scale picture. This mode is preselected in monochrome mode.
ORDERED:
Only available if GRAY is selected, only available in monochrome mode.
When selected an ordered dither is used to convert the gray scale
image into monochrome. (This is the monochrome routine found in older
versions of VIEWGIF.)
DISPERSED:
This mode is preselected when GRAY is chosen. When selected a
dispersed dot dither is used to convert the gray scale image to 2, 4,
or 8 shades of gray depending on the current resolution.
HISTO:
When HISTO is selected all DISPERSED gray dithers use histogram
equalization to maximize the contrast of the resultant picture. This
mode is always available (even when COLOUR is selected) because it is
always used by LOAD CP8. HISTO alternative is SCALED. HISTO distorts
the gray scale linearity resulting in an artificial image.
SCALED:
When SCALED is selected all DISPERSED gray dithers use scaling to
insure full gray scale coverage in the resultant picture. This mode is
always available (even when COLOUR is selected) because it is always
used by LOAD CP8. SCALED alternative is HISTO. SCALE preserves the
gray scale linearity resulting in a truer image than HISTO.
3. KNOWN SHORTCOMINGS/BUGS
A. Full Size Window button does not work properly.
B. Edit colour did not seem to be compatible with QUICK-ST 1.6.
C. Window colours can still go to black on black on some images. This is
best fixed by remembering the DESKTOP colour menu key equivalent
(UNDO).
4. ENHANCEMENTS UNDER CONSIDERATION
A. Support 4096 Colour Mode of STE and Modified ST.
B. Add the following GIF and non-GIF "TRANSLATE..." options:
translate CP8 to AIM,
translate AIM to CP8,
translate GIF to Colour AIM,
translate CP8 to GIF.
C. Add support for 24-bit colour CP8 images.
This would be a pair of translate options:
translate a 24-bit colour image into a 256 colour GIF image,
translate 3 CP8 images into a 24-bit CP8 image.
D. Add Compressed Degas format load/save.
E. Add Memory Lean GIF read option.
The Lean option trades reduced memory requirements for a reduction
in image quality.
F. Add TT video mode support. (640x480x16, 1280x960x2, 320x400x256)
(When the TT makes its mass market appearance.)
G. Add an AVERAGE option to the SHRINK routines.
An average routine would allow gray dithered images to be shrunk.
It would return an error message when given a colour image.
H. Add a full screen option.
This would put the image on the full screen and use the cursor
keys to scroll the image. (I personally recommend the use of
OVERSCAN instead.)
5. PROGRAM HISTORY
0.6 First SHAREWARE release of VIEWGIF.
0.7 Fixed bugs due to differences between TOS 1.2 and TOS 1.1 [(c) 1985].
Low Res now works under TOS 1.1 [(c) 1985].
Neo/Degas save now works under TOS 1.1 [(c) 1985].
NOTE: Viewgif 0.6 worked under TOS 1.1 [(c) 1985] if key commands
were used instead of menu items.
WHAT'S NEW? - Changes in VIEWGIF 0.7.
1. Low Res now works under TOS 1.1. [(c) 1985].
2. Neo/Degas save now works under TOS 1.1 [(c) 1985].
3. Slight rearrangement of menu items.
4. Control, Alternate abbreviations changed in menu items.
0.8 Released February 25, 1990.
GIF is a trademark (tm) of Compuserve Inc, an H&R Block Company.