home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Space & Astronomy
/
Space and Astronomy (October 1993).iso
/
mac
/
VIEWERS
/
AMIGA
/
GIF2IFF.ZOO
/
convert.doc
< prev
next >
Wrap
Text File
|
1989-04-26
|
5KB
|
132 lines
I was exploring BBS's one day and came across one for IBM's that
had several digitized images in GIF format. Out of curiousity, I
downloaded a couple and tried to display them using 'giffy'.
Nothing coherent. Still curious I put together a quick program to
decode GIF(which uses a form LZW compression-interesting in
itself) and saved the pictures in a more simpler format. When I
make reference to TMP files in this doc, I'm referring to this
format, which is as follows:
struct TMPFILE
{
SHORT ImageWidth,ImageHeight,NoOfColorRegisters;
Struct COLORREGISTER ColorRegisters[NoOfColorRegisters];
UBYTE Image[ImageHeight][ImageWidth];
}
where struct COLORREGISTER is
struct COLORREGISTER
{
UBYTE red;
UBYTE green;
UBYTE blue;
}
** Note that the picture is Image[y][x] not Image[x][y] **
The Image array indexes into the ColorRegisters array, which hold
the actual color values. For example-The red component of a pixel
(x,y) is given by the value of ColorRegisters[Image[y][x]].red.
A value of 255 is all the way on. A value of 0 is all the way
off. Since the Amiga can only display 4 bits per color component
the following equation should be used:
temp=ColorRegisters[Image[y][x]].red
RedComponent = (tmp>>4)&0x0f;
And'ing the value with 0x0f probably isn't necessary but helps
conceptually.
If the Image is Interlaced, GIFtoTMP will use exit(5) upon
completion in addition to a written message. This allows you to
check for this condition in a CLI script file. See the file
'convert' for an example. The file Unlace can then be used to
un-interlace the image.
The last program, TMPtoIFF, is the heart of this series. When
converting the TMP file into an Amiga IFF file, the number of
color registers must be reduced (usually from 256). TMPtoIFF
currently offers three algorithms to do this:
Popularity-Merely chooses the top most used colors
Median Cut-Thinks of the colors as points in three dimensional
space (Red,Green+Blue) and draws a box that
encloses all points. The program then divides that
box into two boxes with an equal number of colors in
each side. (A pure white screen would only have one
color-even though it is used in thousands of pixels)
The boxes are continuously divided along their
longest axis until there are as many boxes as the
number of colors that you want. Then the colors in
each box are averaged and those values are used in
the final color map.
Weighted Median Cut-I modified the above algorithm so that as
it's dividing each box along its longest axis, it
tries to put an equal number of pixels in each side
(which of course is impossible to do exactly)
This produces averaged colors closer to the majority
colors, but also provides the variety that the
Median Cut does
After one of the above algorithms is run, each pixel in the
original image is replaced with the closest value in the final
color map. If a HAM image is requested, the program uses one of
the above algorithms to produce the color register values. This
works very well in practice.
Summary of Files:
GIFtoTMP Converts GIF files into TMP files
Unlace Will unlace a TMP file. Use only if
GIFtoTMP indicates it as such
TMPtoIFF Converts TMP files into IFF files
convert the CLI file I use to convert the images
usage as such:
convert giffilepath giffilename [options for TMPtoIFF]
The usage of each file (except convert) can be found be typing
the name of each file and pressing return.
I've only tried this progam on 320x200 and 320x240 images. If you
try larger images, like 640x400 make sure you have a lot of
memory. Although I wrote this program specifically for converting
IBM GIF images to Amiga IFF images, it will hopefully work on
most GIF files. Let me know if you have any problems. One that I
already know of is if you specify a final image of 5 bit
planes(32 colors) and the original GIF image has less than 32
colors.
Mark Podlipec
111-4 BroadMeadow Rd
Marlboro MA 01752