Amiga Computing 59/ac059.adf/Utilities/AGAOnly/Gif.doc/Gif.doc

Text File | 1993-02-05 | 4KB | 71 lines

How to use your GIF datatype. This version is 39.2.


This program can ONLY be used under WB3 (or above, I presume).

The Gif.datatype file must be copied into the directory:
SYS:Classes/Datatypes
The GIF and GIF.info files can be copied into one of two places:
SYS:Storage/Datatypes
If you just want to have the datatype available. You will have to double-
click on the GIF icon, or use AddDataType to actually use it. OR
SYS:Devs/Datatypes
If you want the datatype permanently installed. Doing this means that the
datatype is automatically added to the system every time you boot. This
means that the very first time you install it, you will have to double-click
on it or reboot to use it.

Once the GIF datatype has been installed, any datatype aware program can be
used to read GIF files as if they were IFF. For example, MultiView will
display them, and you can use them as screen/window backdrops. MultiView
will display the images on the workbench, and will remap the colours to
whatever workbench you have selected. If you run a 4 colour WB, you will get
a best effort in four colours. Actually, MultiView does quite a good job of
this. Normally, you will want to use a 256 colour workbench, in which case,
you will get all the colours. Due to the pen sharing in WB3, you can have
several pictures open simultaneously, and still get a good colour match.

By selecting the appropriate tooltypes, MultiView can also display pictures
on their own screen. Consult the MultiView documentation for more details.

The datatype was written to handle 87a and 89a type GIFs, although I haven't
been able to test the 89a ones. It will only read the first image in a file
containing multiple images, although I haven't been able to test this either.
A datatype is a library, and so naturally is fully re-entrant. This version
has also been tested on a 68000.

This version of the the datatype is a performance-enhanced version of the
first release, 39.1. A number of enhancements were made:

  1) General cleaning up of the code, and removal of redundant bits.
  2) Use of registerised parameters in selected places.
  3) A lot of SetAPen commands were eliminated by using multiple RastPorts.
  4) Some optimisation of the GIF decoder.
  5) Asynchronous file I/O. This means that while the decoder is working on
     one piece of data, the file system is prefetching the next piece. This
     should make a significant difference on DMA devices.
  6) Recompiled to pull the global data into the code space.
  7) Added call to BestModeID. This means that if you display a picture with
     the screen tooltype enabled, the datatype will choose the screen-mode
     it thinks is best-suited to displaying this picture from the selection
     available in the graphics database. The old version would always pick
     a Hires (640 by 256/200) screen.

As with everything, you never get something for nothing. The speed
enhancements in this version have been obtained at the expense of runtime
memory usage, which is approximately 30K greater than version 39.1. I do
not think this is onerous.

There are no restrictions on the distribution of this archive. The datatype
copes successfully with the modest collection of GIFs I have access to, but
testing has been somewhat less than rigorous. If you have any problems, I
can be contacted on [e-mail] or 100014,674 on CIS. There
are unlikely to be any further improvements on this version, unless any bugs
are reported, as the performance is as good as I can get it without major
rewriting.

The Graphics Interchange Format (c) is the Copyright property of CompuServe
Incorporated. GIF (sm) is a Service Mark property of Compuserve Incorporated.

Steve the G. 30/12/92.