TopWare Tools/TOOLS.iso/tools/top1301/formats.txt

Text File | 1993-10-30 | 5.3 KB | 128 lines

FORMATS.TXT

File to accompany WinWall Plus
Copyright ⌐1993 by
Don Bradner
Arcata Pet
600 F Street
Arcata, CA 95521

WinWall Plus allows many standard graphics formats to be used
as Windows wallpaper, including .BMP, the only format directly
recognized by Windows.  A frequently asked question is "What is
the best format?"

The formats all involve a tradeoff of disk space versus time.
The most compressed formats take the longest to decompress.
For the user, the decision must be made based on available
space and the relative inconvenience of having the system pause
during wallpaper changes.

Users of disk compression software such as Stacker or
DoubleSpace will usually find that there is little advantage in
using a compressed format (other than JPEG), since the disk
utility will do as good a job as any of the compressed
formats.  Best choice in that case is almost always .BMP.

A discussion of each format follows:

.BMP

This is the fastest format available, but requires the most
disk space.  It does not require any conversion or
decompression by WinWall Plus, so the only time required to
display a new wallpaper is the time it takes to read the image
from the disk.  Excellent for small images that are "tiled"
(displayed as multiple images to cover the desktop space).
This format is usable for all color levels from 16 to
16-million.

.TIF and .TGA

These formats offer minimal if any disk space savings over
.BMP, while requiring time for WinWall Plus to convert them to
.BMP format for use by Windows.  The only reason to leave an
image in these formats is to maintain compatibility with some
other program.  Otherwise they would be best converted to .BMP or
one of the compressed formats.  .TIF is usable for all color
levels, 16 to 16-million, while .TGA is usable from 256 to
16-million.

.PCX

This format, which can be read and written by Windows
Paintbrush but not used directly as wallpaper, offers some
compression for images with color levels up to 256.
Decompression and conversion by WinWall Plus is fairly fast.
The .PCX format can use color levels up to 16 million, but file
size actually increases above the size of .BMP, offering no
advantage.  A .BMP test file in 256 colors that was 182,358 bytes
was compressed into .PCX format and had a size of 101,170, a
45% reduction.  A 24-bit (16 million color) .BMP file of
622,134 bytes resulted in a .PCX file of 743,165 bytes, a 19%
increase.

Images compressed in .PCX format do not lose any of their
quality.

.GIF

The Compuserve Graphics Interchange Format offers the greatest
compression of images up to 256 colors without loss of
quality.  It is not usable for images with more colors.  The
image noted under .PCX that was 182,358 bytes compressed into a
.GIF file of 46,074 bytes, a 75% reduction.  This format is
generally the best choice when disk space is limited and the
image has no more than 256 colors.  There is a noticable delay
while WinWall Plus decompresses the image and converts it to
.BMP for display, although the faster the computer the shorter
the delay.

.JPG
JPEG, which stands for Joint Photographic Experts Group - the
committee which formulated the standard, is designed to
compress 24-bit (16-million color) images.  Unlike .GIF and
.PCX compression, JPEG causes some loss of quality in the
image, so it is called "lossy."  There are no really good
loss-less methods for compressing 24-bit images.

JPEG does a very good job of compression, and with most pictures
the resulting image is comparable to the original.  There is a
variable amount of compression available, with greater loss of
quality as the image is further compressed.  WinWall Plus
allows a quality setting of 5 to 95, with 5 being the most
compressed and 95 being the least compressed.  The 622,134 byte
test file referenced above produced sizes of 5,381 bytes at a
setting of 5, and 75,809 bytes at a setting of 95.  The image
was very deteriorated at the lower setting, and was
indistinguishable from the original at 95.  Further testing for
this image showed very acceptable results at a setting of 75,
with a file size of 25,554 bytes.

JPEG does not work well with images of 256 colors or less, and
is particularly unsuited for images with large areas of solid
colors and sharp edges.

The delay in decompressing a JPEG file is considerable.  It
will vary depending on the speed of the computer, but is slow
even on a fast machine.  Users of the JPEG option may want to
turn off the timed wallpaper changes and use the manual change
options within WinWall Plus when the decompression time is not
going to hinder other work.  At some future time it may be
possible to completely perform decompression in the background;
at this time it is not possible with WinWall Plus.

Editing a JPEG image and saving it in .JPG format again will
result in additional losses of quality.  Editing should always
be performed on the original 24-bit image before compression is
done.

There are many non-standard forms of JPEG.  WinWall Plus should
read any JPEG image conforming to the JFIF standard, but may
not be able to read images produced by non-standard programs.
An example would be GIF2JPG, which can produce a proprietary
JPEG file called HSI.  Older versions of the program did this
by default; newer versions produce JFIF compatible files by
default.