Provided by: libmdc2-dev_0.14.1-1_amd64 
NAME
m-gif - GIF87a and annimated GIF89a format (MedCon)
DESCRIPTION
The Graphics Interchange Format from CompuServe allows between 1 and 8 bits of color
information with an RGB color palette. The image arrays are compressed with an LZW coding. The
extension of the file is `.gif'.
The basic defines for the format:
---------------------------------------------------------------------------
typedef struct {
char sig[6]; /* GIF87a or GIF89a */
Uint16 screenwidth,screenheight; /* screen dimensions */
Uint8 flags,background,aspect; /* background color, ratio */
} MDC_GIFHEADER;
#define MDC_GIF_GH_SIZE 13
typedef struct {
Uint16 left,top,width,height; /* image dimensions */
Uint8 flags;
} MDC_GIFIMAGEBLOCK;
#define MDC_GIF_IBLK_SIZE 9
typedef struct { /* display information */
Uint8 blocksize;
Uint8 flags;
Uint16 delay;
Uint8 transparent_colour;
Uint8 terminator;
} MDC_GIFCONTROLBLOCK;
#define MDC_GIF_CBLK_SIZE 6
typedef struct { /* plain text block */
Uint8 blocksize;
Uint16 left,top;
Uint16 gridwidth,gridheight;
Uint8 cellwidth,cellheight;
Uint8 forecolour,backcolour;
} MDC_GIFPLAINTEXT;
#define MDC_GIF_TBLK_SIZE 13
typedef struct { /* application block */
Uint8 blocksize;
char applstring[8];
char authentication[3];
} MDC_GIFAPPLICATION;
#define MDC_GIF_ABLK_SIZE 12
---------------------------------------------------------------------------
What does the format support or not support:
===========================================================================
Item Supported Not Supported
===========================================================================
Color Map : max 256 RGB colors -
File Endian : little big
Pixeltypes : Uint8 -
===========================================================================
Scaling factors : quantify & calibrate factors/image are NOT supported
---------------------------------------------------------------------------
Dimensions/Image : different dimensions for each image are supported
---------------------------------------------------------------------------
Pixeltypes/Image : different pixeltypes for each image are NOT supported
===========================================================================
Because of the flexible nature of the GIF format it could be possible to include scaling
factors per image with the GIF extension blocks, but more about this later. The image is
stored from left to right and from top to bottom, unless the images are interlaced.
First some explanation on the GIF format and its different structures.
=======================
The GIFHEADER structure
=======================
This data structure is the very first information in a GIF file:
sig[6] Holds the signature of the file "GIF87a" or "GIF89a".
screenwidth, screenheight
The required screen dimensions in pixels to display the images.
background
This represents the background color. It is in fact an index entry in the color
palette.
aspect The aspect ratio of the pixels in the image. If this field is not 0 the aspect
ratio is: ((gh.aspect + 15) / 64). This entry is always 0 for the GIF87a format.
flags This fields contains a number of bits of information.
if (gh.flags & 0x0080) is true, a global color map will follow.
The number of color bits: ((gh.flags & 0x0007) + 1)
The number of colors : (1 << ((gh.flags & 0x0007) + 1)
if (gh.flags > 0x0008) is true, the color palette is sorted with the most important
colors first. This bit is low in GIF87a.
Finally (1 << ((gh.flags >> 4) + 1) represents the number of color bits in the
original image. This bit is low in GIF87a.
After reading the GIFHEADER and any global colormap, there should be a `block separator' which
introduce the following block of GIF information. There are three kind of `block separators' :
a comma, an exclamation mark and a semicolon.
',' => the next block will be an image
'!' => the next block will be an extension
';' => the end of the GIF file
The image block after a comma consists of the IMAGEBLOCK structure and the compressed image.
The IMAGEBLOCK structure defines the nature of the image and supersedes the global definitions.
========================
The IMAGEBLOCK extension
========================
left, top
The upper left coordinate of the image relative to the screen.
width, height
The image dimensions. Width is the number of pixels in a line. Depth represents the
number of rows.
flags This field is similar to the global flags in the GIFHEADER structure. Number of
colors in the image is ((iblk.flags & 0x0007) + 1).
If (iblk.flags & 0x0040) is true, the image is interlaced. In this case the image
is split into four passes instead of sequential lines:
1st pass: lines 0 8 16 24 ... (+8)
2nd pass: lines 4 12 20 28 ... (+8)
3rd pass: lines 2 6 10 14 ... (+4)
4th pass: lines 1 3 5 7 ... (+2)
If (iblk.flags & 0x0080) is true, there is a local color map.
If (iblk.flags & 0x0020) is true, the color map is sorted.
The next byte, after the IMAGEBLOCK should be the initial image code size The compressed image
consists of subblocks of code, of which the first byte gives the amount of code bytes that
follow. The last block is a zero-length block. This is how you could skip an image:
FILE *fp;
int i,n;
do {
n = fgetc(fp); /* get code size */
if (n != EOF) {
for (i=0; i<n; i++) fgetc(fp); /* skip the block */
}
}while( (n != 0) && (n != EOF)) /* read the next block, if any */
After reading this hole image block, the next byte should be again a `block separator'. If
this separator is an exclamation mark, the following block is an extension. The GIF extension
blocks allow additional features.
=====================
The COMMENT extension
=====================
This is a very simple extension. The byte 0xfe after a block separator, introduces a comment
block. It contains text that does not make part of the image. The comment block is stored as
subblocks, ending with a zero-length subblock (or endblock).
=======================
The PLAINTEXT extension
=======================
This is identified by the byte 0x01 after the block separator. The data structure follows this
byte.
left, top
The items give the starting position of the displayed text.
gridwidth, gridheight
Two elements that specify the distance in pixels from one character to the next.
cellwidth, cellheight
These fields represent the actual dimensions in pixels of the characters to be
displayed.
forecolor, backcolor
Color map indices for the foreground and background respectively.
The next data after this structure is the text itself, stored in data subblocks just like the
comment block is.
==========================
The CONTROLBLOCK extension
==========================
A GIF file with more then one picture also contains a CONTROLBLOCK extension. The byte 0xf9
after the block separator, represents this graphics control block. Following this byte is the
data structure.
blocksize
This field always contains the value 0x04.
flags if (cb.flags & 0x01) is true, cb.transparent_color will contain a valid transparent
color index.
if (cb.flags & 0x02) is true, the viewing program should wait for user input before
displaying the next image. if (cb.delay) is greater than zero, the viewer should at
least wait for the number of seconds specified in the delay data field.
The value ((cb.flags >> 2) & 0x0007) tells the method to remove the present image
from the screen:
0 = do nothing
1 = leave it
2 = restore with the background color
3 = restore with the previous graphic
delay The delay in 1/100ths of a second to dispose the present graphic.
transparant_color
This fields represents the color index of the transparant color.
terminator
Any clues on this?
=========================
The APPLICATION extension
=========================
The final extension is the APPLICATION block. The application data structure is identified by
the byte 0xff just after the block separator.
blocksize
This contains the value 0x0b.
applstring
An 8-byte string that specifies the creator software.
authentication
This field should contain 3 bytes based on the applstring field to check the
integrity of the applstring field.
The APPLICATION block extension can be followed by subblocks, ending with a zero-length
subblock.
A special kind of APPLICATION block extension is the LOOPBLOCK extension used for annimated GIF
files in concern to Netscape Navigator. This block comes between the GIFHEADER and IMAGEBLOCK
data structures. It contains the following items:
1. An application block
ap.blocksize = 0x0b;
ap.applstring = "NETSCAPE";
ap.authentication = "2.0";
2. subblock of 3 bytes: 0x03
0x01,0xe8,0x03
3. endblock of 0 bytes: 0x00
NOTES
For complete information on the GIF format, we liked reading this book:
``Supercharged Bitmapped Graphics''
written by Steve Rimmer
published by Windcrest/McGraw-Hill
ISBN: 0-8306-3788-5
FILES
/usr/local/xmedcon/source/m-gif.h The header file. /usr/local/xmedcon/source/m-gif.c The source file.
SEE ALSO
medcon(1), xmedcon(1), xmedcon-config(1) m-acr(4), m-anlz(4), m-inw(4), m-intf(4), m-ecat(4) medcon(3)
AUTHOR
(X)MedCon project was originally written by Erik Nolf (eNlf) for the former PET-Centre at Ghent University (Belgium). e-mail: enlf-at-users.sourceforge.net www: http://xmedcon.sourceforge.net M-GIF(4)