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1999-10-08
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About ViewILBM 1.9
==================
This is (yet another?) viewer program for picture files. But this one is
special, for a number of reasons:
* It shows only IFF-ILBM format picture files. And it can handle all
popular variants and formats.
* Unlike many other IFF-ILBM viewer programs, this one was specifically
written for graphics cards, using the CyberGraphX API (which is also
supported by the Picasso96 software); it does not run on a plain Amiga.
While this sounds like a disadvantage, it allows you to view HAM8
pictures on an ECS Amiga.
* This program supports colour cycling.
* Unlike other picture viewers, this program will try to display all
pictures in the correct aspect ratio. On a graphics card, every
pixel is square, yet there are IFF-ILBM pictures which require
pixels that are twice as wide as they are tall (or twice as tall
as they are wide).
And of course, there are the control options this program offers and the
source code is included, too.
How to use it?
==============
ViewILBM runs both from Shell and from Workbench. In both cases it responds
to the same set of options, only that with the Shell version you enter
them on the command line and with the Workbench you put the options into
icon tool types.
To view an IFF-ILBM file, just enter "viewilbm" and the name of the file to view,
then hit the [Return] key and watch how the picture comes to the screen.
The options and command line parameters are as follows:
FILES/M/A
The names of the files to show; instead of a files you can also specify
the names of a drawers to scan for IFF-ILBM files. And furthermore, you can
also use wild cards in place of the full file names.
CENTER/S
This will centre the picture(s) to load inside the screen's dimensions.
This is particularly useful if you want to view several pictures without
having the screen reopened every time. Note that it is sometimes
inevitable to reopen the screen in a new resolution.
WAIT/K/N
For a quick slide show feature, you can specify the number of seconds to
wait before the next picture is loaded.
QUIET/S
This option will suppress any program output except for error
messages.
ALL/S
This works like the "ALL" switch on the Shell "List" command. Just
point ViewILBM to a drawer and use the ALL option and it will descend
the entire filing system tree, displaying all IFF-ILBM files it finds
(and complain about every file it doesn't grok).
WAITAFTEROPEN/S
Apparently, all modern multiscan monitors take a second or two to
resync after the display is reopened for a new IFF-ILBM picture. This
is your chance to introduce a small delay (in seconds) to wait
before picture decoding starts.
CYCLE/S
Many pictures support a technique called "colour cycling" which is
often used for animation. ViewILBM supports both Deluxe Paint's
CRNG and GraphiCraft's CCRT colour cycling chunks. With this option
enabled, colour cycle will start immediately after the picture has
loaded (provided, of course, that the picture was stored with
colour cycling information attached). You can stop and restart
colour cycling at any time by hitting the [Tab] key.
HAM/S
EHB/S
These two switches work together and concern a feature (or rather,
the lack thereof) of "older" IFF-ILBM files. The original format
specification said nothing about the display mode to use for a
picture. That information was induced by the size and the number
of bit planes in a picture file. In this context, a picture with
six bit planes was expected to use the Hold-And-Modify format.
Still, some pictures exist which would use the Extra Halfbrite
format, giving no such indication. Once you saw the picture, you
would notice that something was wrong. By using the HAM and EHB
switches, you can tell ViewILBM to treat a picture with six
bit planes either as Hold-And-Modify format (HAM) or as
Extra Halfbrite format (EHB).
As said before, the program also works from Workbench: hold down any
[Shift] key, single-click on every IFF-ILBM file icon to display, then
double-click on the ViewILBM icon. You may put the tool types described
above into either the ViewILBM icon or the ILBM file icons. Note that
for the switches like CYCLE and CENTER you need to specify the option
as ON or OFF, as in CYCLE=ON or CENTER=ON.
Unsupported IFF-ILBM features
=============================
The IFF-ILBM container format saw several enhancements during its career.
New chunks were added, and not all of these are supported by this program.
Unsupported are the PCHG, DRNG, DYCP/CTBL chunks and many others. I'm
quoting these few since it could have made sense to support these, but due
to lack of documentation and/or sample files I didn't choose to implement
support for them. Not supported either are the GSPT and GSCC chunks as read
and written by Accolade's "The Graphics Studio" since no public
documentation exists which describes their format.
Requirements
============
Any Amiga with OS 3.x, an '020 CPU and CyberGraphX/Picasso96 installed will
do, but ViewILBM will refuse to display IFF-ILBM pictures if it cannot find a
screen display mode to match it.
Historic notes
==============
Listen up kids, here's something you probably didn't learn at school!
This program concerns itself with the perhaps oldest picture file format
used on the Amiga, the so-called IFF-ILBM format. The IFF format itself is
just a container for data, so there exist several variants which contain
different data, such as the IFF-AIFF format which contains sound data
instead of pictures. The IFF format specification was created back in 1985,
so when the Amiga finally hit the market, it had a solid backing of
standardized file formats used by the application software that existed back
then. You simply couldn't find a program to paint and process pictures with
that didn't support the IFF-ILBM format. Funny as it may sound today, the
"professional" IBM-PC computing market of these days had to deal with a
multitude of different file formats, each from a different vendor. Problems
which the Amiga did not have.
Today the GIF, JPEG and TIFF file format fill the niches the IFF-ILBM
format used to occupy. Up until the early 1990'ies the IFF-ILBM format was
very popular on the Amiga and plenty of images exist which "modern" software
cannot display properly. There's the problem of the pixel aspect ratio, for
example. On the Amiga we have display modes in which a pixel is not exactly
square; it can be twice as wide as tall (the so-called "low resolution
interlaced mode") or twice as tall as wide (the so-called "high resolution
mode"). The only display modes to offer square pixels are the so-called "low
resolution mode" and the "high resolution interlaced mode". And even in
these modes pixels are not exactly square, at least on NTSC machines, on
which each "square" pixel is about 10% taller than wide. Only on PAL
machines are pixels exactly square. If you show pictures with a non-square
pixel aspect ratio on displays with a square pixel aspect ratio, the result
may not look to pretty.
As a container format, IFF-ILBM has a few unique features with regard to
how the picture is stored. For example, no other picture file format
supports the Amiga typical Hold-And-Modify and Extra Halfbrite formats.
These two were attempts to squeeze more colour out of the chip set, with
Hold-And-Modify allowing 4,096 colours (262,144 on machines with the AGA
chip set) to be displayed simultaneously which would have only allowed a
maximum of 32 (256 on machines with the AGA chip set). Extra Halfbrite would
have allowed 64 colours to be shown on a display that would have allowed
only 32 (actually, only 32 colours could be used freely, the other 32
colours were half as bright as the ones that could be freely used). This
sure sounds strange today when every graphics card offers 256 or 16 millions
of colours.
Another unique feature of the IFF-ILBM format is the use of a simple
animation technique known as "colour cycling". With programs such as Deluxe
Paint, GraphiCraft, Aegis Images, etc. artists could define ranges of the
picture's colour palette which when cycled through would reveal otherwise
hidden parts of the picture.
That concludes our brief tour of the IFF-ILBM format.
Sample files
============
To make a point on how pictures are displayed by other programs and how
they are displayed with this one I have included a small selection of sample
images. These images were obtained from public sources, such as Fred Fish's
Amiga Library disks or the NewTek Demo Reel #1. The images are:
* ArtificialHeart
Created by artist Jack Haeger using GraphiCraft. This picture is
stored in uncompressed format and supports colour cycling.
* Knight1
Created by artist Jim Sachs using Deluxe Paint (apparently, using
Deluxe Paint II). This picture uses a non-square pixel aspect
ratio (high resolution mode).
* Clipper
Created by the late artist Rick Parks. This picture uses a non-square
pixel aspect ratio (low resolution interlaced mode).
* fashion
One of the very early Hold-And-Modify format pictures, created using
NewTek's DigiView product.
Author
======
This program was written by Olaf `Olsen' Barthel <olsen@sourcery.han.de>.
Comments and bug reports are welcome. Use at your own risk.