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1994-05-14
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Documentation for the GEM-View Savingmodule JPEG.GVS
""""""""""""""""""""""""""""""""""""""""""""""""""""
(C) by Guido Vollbeding, May 14 1994
""""""""""""""""""""""""""""""""""""
Changes
#######
- Got rid of problems in dialog handling after Load/Save for
braindamaged file selectors (like the original TOS one).
- Little code optimizations in color conversion routines.
May 12 1994:
- First release.
Introduction
############
Yeah, here it is, the hardly wanted JPEG-Savingmodule for the
exciting Shareware program GEM-View (Version 3.xx) from Dieter
Fiebelkorn. Since more and more people more and more asked me for
this in the last time (Hi Dieter, Georg, Harald, ...), I left all
other things aside some days to get this part ready by hardly
working.
Installation
############
Unlike the internal complexity of the subject, the usage is very
simple: Just copy the file JPEG.GVS into the GVWSAVE folder of
your GEM-View installation.
Within GEM-View you can now select this module like any other.
Please note, that this module only supports True-Color-Images and
therefore is only available if you select the TC-Button in the
Saving and Conversion dialogs. With the GEM-View Conversion
module it is possible to process True-Color pictures indepen-
dently from the actual screen resolution, even in monochrom.
During saving a progress monitor is shown by the module in the
GEM-View Log-window as a sequence of dots. 8 dots are totally
displayed. In case of successful completion of saving, the
achieved compression ratio (Original Image Size in relation to
the JPG-File-Size) is displayed.
Options
#######
If you hold the Alternate key down at entering the saving
function, a dialog appears where you can choose some JPEG
compression parameters.
Alternatively you can set the parameters offline by starting the
module as a program without GEM-View from the Desktop. In that
case exactly the same dialog appears as with the module in GEM-
View.
To achieve this, you can for instance rename the module to
JPEG.PRG or JPEG.APP. A more likely way, however, is to insert
the following line in your NEWDESK.INF desktop configuration file
at the marked point:
...
#W 00 00 06 01 34 09 00 @
#G 03 04 000 C:\1STGUIDE.ACC@ *.*@ @
#G 03 FF 000 *.AC*@ @ @
#G 03 FF 000 *.GV?@ @ @ <<--
#G 03 FF 000 *.APP@ @ @
#G 03 FF 000 *.PRG@ @ @
...
This avoids unlikely renaming and allows you to directly start
any GEM-View modules as programs from the Desktop.
The changes you made in case of start as a program have, of
course, only an effect by saving them.
The method of calling the dialog with the Alternate key is
consistent with the method used in GEM-View to load images with
the Alternate key.
If you, however, should not like this for any reasons, the dialog
provides the possibility to change the calling condition to any
combination of special keys Shift/Control/Alternate. Here a
selected button represents a pressed keystate, otherwise non-
pressed. So, for example, if you deselect all four buttons, then
the dialog would appear normally EVERY time the saving is called.
So, if you convert many pictures, you could set individual
parameters for each picture immediately before saving.
Now let's have a look at the parameters in special:
Quality
-------
The most important parameter certainly is the predefined quality.
Possible values are 0 to 100 %. The default setting is 75 %. This
guaranties in allmost situations a good compromise between visi-
ble image quality and compression ratio. The resulting images are
nearly visually indistinguishable from the original (very good
image quality), and the achieved compression ratios range from
10:1 to 20:1 depending on the image contents.
To control the resulting image quality, you should read in the
picture after saving to display it in the highest possible
screen-color-depth (!). To do this you can either use GEM-View
itself, or users of 1STGUIDE can also use this instead.
If you see defects at default quality 75, then go up 5 or 10
counts at a time until you are happy with the output image.
Quality values above about 95 are NOT recommended for normal use.
The compressed file size goes up dramatically for hardly any gain
in output image quality.
In the other direction, if you prefer small files to be produced,
set the quality value down until about 50. Lower values should be
avoided because of visible loss of image quality.
Entropy Optimization
--------------------
Activating this option usually makes the resulting file a little
smaller, but runs somewhat slower and needs much more memory.
Image quality and speed of decompression are, however, unaffected
by this option.
This option is not yet supported, but is reserved for future
versions.
Smoothing
---------
This value filters the input to eliminate fine-scale noise. This
is useful when converting dithered palette-type pictures: A
moderate smoothing factor of 10 to 50 gets rid of dithering
patterns in the input image, resulting in a smaller JPG file and
a better-looking image. Too large a smoothing factor will visibly
blur the image, however.
This option is not currently supported, since it is not necessary
for real True-Color input. If I feel free to support reduced
palette-type pictures in the future, this option will be used.
Greyscale Output
----------------
With this option activated, a pure greyscale image is produced by
trashing away the color information completely.
ATTENTION! Some programs can't read in such files! This is a
disadvantage of these programs. Insure, that a certain program
you want to use with such files can handle this correctly.
GEMVIEW and 1STGUIDE don't have any problems with this!
The resulting files are, of course, somewhat smaller and take
less time to decode. The corresponding compression ratio, how-
ever, (only the greyscale information taken into account) is
relatively small (at Q = 75 % about 5:1), because the color
components normally can be harder compressed (only considering
the color-subsampling).
Distribution
############
I decided to put this project as real FREEWARE to the public,
although much work has been spent in it and is expected to do in
the future.
The package, consisting of the following 3 files:
JPEG.GVS -> the real GEM-View-Module, also
executable as PRG or APP;
JPEG_GVS.GER -> the german description;
JPEG_GVS.ENG -> this text as english description
can be freely distributed, as long as all files remain together
and unchanged. Any commercial distribution (including PD-Series
or so) is strictly excluded, because nobody else should be paid
for my work if I myself also don't get anything.
I accept NO LIABILITY for damages of any kind. A certain kind of
data loss is unavoidable, however ;-). This is caused by the
subject. So BE CAREFUL in usage, and read the above instructions.
For questions, hints, critics (constructive!) or thanks here is
my contact address:
Guido Vollbeding
Turmstraβe 61
D-06110 Halle (Saale)
(Germany)
Email: Guido Vollbeding @ HAL (Mausnet)
Bank: Stadt- u. Saalkreissparkasse Halle
Blz.: 800 537 62
Kto.: 486 260 725
To questions I can answer preferably only if they come from
registered 1STGUIDE-users.
It should be clear, that anybody who uses this module MUST be a
registered GEMVIEW-user.
BTW: A DSP-Support for the Falcon is not currently in question. I
have spent much time in optimizing the code, however, but
somewhat can be done in the future to increase performance a bit.
As programming environment Pure-C is used (what else ?).
References
##########
If you want to learn more about the theory behind JPEG, the
following article should be read as introduction:
Wallace, Gregory K. "The JPEG Still Picture Compression Standard"
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44
At the end of this article you will find more detailed referen-
ces.
Last Words
##########
At this point a great thank goes to Dieter Fiebelkorn for the
fruitable cooperation (over years).
Only the flexibility and comfort of GEM-View made it possible,
that now an in my opinion ideal environment for JPEG compression
is available.
A further thank goes, of course, to the Independent JPEG Group.
It's excellent sources built the basic of my work at all, and
without them the subject JPEG certainly would be pure theory for
a long time yet.
Now I wish you much fun by using this module and hope to see much
beautiful JPEG-images in the future.