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1993-02-24
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As this software act as a RiscOs interface for the
Independant JPEG Group C routines , this documentation will be
largely inspired by the USAGE file for CJPEG and DJPEG of IJG .
What is JPEG ?
--------------
JPEG is a standardized compression method for full-color and gray-scale
images . JPEG is designed to handle "real-world" scenes, for example
scanned photographs. Cartoons, line drawings, and other non-realistic
images are not JPEG's strong suit; on this sort of material you may get
poor image quality and/or little compression.
JPEG is lossy, meaning that the output image is not necessarily identical to
the input image. Hence you should not use JPEG if you have to have identical
output bits. However, on typical real-world images, very good compression
levels can be obtained with no visible change, and amazingly high compression
is possible if you can tolerate a low-quality image. You can trade off image
quality against file size by adjusting the compressor's "quality" setting.
Using Jewel :
-------------
Double click on !Jewel icon , it then install on the icon bar .
You can now drag a GIF , PPM or TARGA file on the jewel icon
jewel will open a standard save as box , drag the icon on a filer
window it will create a JPEG file according to the compression settings .
Dragging a JPEG file create a GIF , PBM or TARGA file according
to the Decompression settings .
It should be pointed out that files dragged on jewel icon must be
properly filetyped .
This filetypes are :
- 0x695 for GIF files
- 0x69E for PPM files
- 0x69D for TARGA files
- 0xC85 for JPEG files
The JPEG supported file format is JFIF .
Compression menu :
------------------
- Quality settings : Scale quantization tables to adjust image quality.
Quality is 0 (worst) to 100 (best) , default is 75.
- Optimization : Perform optimization of entropy encoding parameters.
Without this, default encoding parameters are used.
Optimization usually makes the JPEG file a little smaller , but
jewel runs somewhat slower .
Image quality and speed of decompression are unaffected by optimization .
Decompression menu :
--------------------
- Greyscale : Force gray-scale output even if input is color.
- Quantization menu : Quantize to N colors. This reduces the number of
colors in the output image so that it can be displayed on a colormapped
display or stored in a colormapped file format. For example for a 8-bit
display, you'd need to quantize to 256 or fewer colors.
The dithering option perform Floyd-Steinberg dithering when quantizing
colors, but on some images dithering may result in objectionable
"graininess". If that happens, you can turn off dithering with .
The one pass option select one-pass instead of two-pass color quantization.
The one-pass method is faster but it produces a lower-quality image.
Also,the one-pass method is always used for gray-scale output
(the two-pass method is no improvement then).
- Smoothing : Perform cross-block smoothing. This only seems to improve
the image at very low quality settings (10 to 20 or so).
At normal Quality settings it may make the image worse.
Hints :
-------
Avoid running an image through a series of JPEG compression/decompression
cycles. Image quality loss will accumulate; after ten or so cycles the image
may be noticeably worse than it was after one cycle. It's best to use a
lossless format while manipulating an image, then convert to JPEG format when
you are ready to file the image away.
About the quality setting :
---------------------------
The quality settings lets you trade off compressed file size against quality
of the reconstructed image : the higher the quality setting, the larger the
JPEG file, and the closer the output image will be to the original input.
Normally you want to use the lowest setting (smallest file) that
decompresses into something visually indistinguishable from the original
image. For this purpose the setting should be between 50 and 95 .
The default of 75 is often about right.
If you see defects at 75, then go up 5 or 10 counts at a time until you are
happy with the output image . (The optimal setting will vary from one image
to another.)
A 100 quality setting will generate a quantization table of all 1's,
eliminating loss in the quantization step (but there is still information
loss in subsampling, as well as roundoff error).
This setting is mainly of interest for experimental purposes.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 , values below 50 will produce very small files of
low image quality. Settings around 5 to 10 might be useful in preparing an
index of a large image library, for example. Try 2 (or so) for some
amusing Cubist effects. (Note: values below about 25 generate 2-byte
quantization tables, which are considered optional in the JPEG standard.
Conditions of distribution :
----------------------------
This software is freeware and is ⌐ Frank Lyonnet 1992 .
It may be copied and distributed freely as long as no profit
is made and all the files listed bellow remained unaltered .
Contents :
----------
!Boot
!Help
!Run
!RunImage
!Sprites
!Sprites22
Docs.Arm'sTech
Docs.Credits
Docs.Futur
Docs.History
Docs.ReadMe this file
Templates
Jewel is another production of ARM's TECH the new french programmers
group .
You can contact me on internet : lyonnet@ufrima.imag.fr .
lyonnet@imag.fr .
on french RTEL : bal ARCHETYPE .
by mail : Frank Lyonnet
420 , Chemin de la Cassine
73000 ChambΘry
France