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For Version 1.00 of this spec see:
http://reality.sgi.com/grafica/sgiimage.html
Draft version 0.97
The SGI Image File Format
Paul Haeberli
paul@sgi.com
Silicon Graphics Computer Systems
This is the definitive document describing the SGI image file format. This
is a low level spec that describes the actual byte level format of SGI image
files. On SGI machines the preferred way of reading and writing SGI image
files is to use the image library -limage. This library provides a set
of functions that make it easy to read and write SGI images. If you are
on an SGI workstation you can get info on -limage by doing:
% man 4 rgb
A note on byte order of values in the SGI image files
In the following description a notation like bits[7..0] is used to denote
a range of bits in a binary value. Bit 0 is the lowest order bit in
the value.
All short values are represented by 2 bytes. The first byte stores the
high order 8 bits of the value: bits[15..8]. The second byte stores
the low order 8 bits of the value: bits[7..0].
So, this function will read a short value from the file:
unsigned short getshort(inf)
FILE *inf;
{
unsigned char buf[2];
fread(buf,2,1,inf);
return (buf[0]<<8)+(buf[1]<<0);
}
All long values are represented by 4 bytes. The first byte stores the
high order 8 bits of the value: bits[31..24]. The second byte stores
bits[23..16]. The third byte stores bits[15..8]. The forth byte stores
the low order 8 bits of the value: bits[7..0].
So, this function will read a long value from the file:
static long getlong(inf)
FILE *inf;
{
unsigned char buf[4];
fread(buf,4,1,inf);
return (buf[0]<<24)+(buf[1]<<16)+(buf[2]<<8)+(buf[3]<<0);
}
The general structure of an SGI image file is as shown below:
The header indicates whether the image is run length encoded (RLE).
If the image is not run length encoded, this is the structure:
The Header
The Image Data
If the image is run length encoded, this is the structure:
The Header
The Offset Tables
The Image Data
The Header
The header consists of the following:
Size | Type | Name | Description
2 bytes | short | MAGIC | IRIS image file magic number
1 byte | char | STORAGE | Storage format
1 byte | char | BPC | Number of bytes per pixel channel
2 bytes | ushort | DIMENSION | Number of dimensions
2 bytes | ushort | XSIZE | X size in pixels
2 bytes | ushort | YSIZE | Y size in pixels
2 bytes | ushort | ZSIZE | Number of channels
4 bytes | long | PIXMIN | Minimum pixel value
4 bytes | long | PIXMAX | Maximum pixel value
4 bytes | char | DUMMY | Ignored
80 bytes | char | IMAGENAME | Image name
4 bytes | long | COLORMAP | Colormap ID
404 bytes | char | DUMMY | Ignored
Here is a description of each field in the image file header:
MAGIC - This is the decimal value 474 saved as a short. This
identifies the file as an SGI image file.
STORAGE - specifies whether the image is stored using run
length encoding (RLE) or not (VERBATIM). If RLE is used, the value
of this byte will be 1. Otherwise the value of this byte will be 0.
The only allowed values for this field are 0 or 1.
BPC - describes the precision that is used to store each
channel of an image. This is the number of bytes per pixel
component. The majority of SGI image files use 1 byte per
pixel component, giving 256 levels. Some SGI image files use
2 bytes per component. The only allowed values for this field
are 1 or 2.
DIMENSION - described the number of dimensions in the data stored
in the image file. The only allowed values are 1, 2, or 3. If
this value is 1, the image file consists of only 1 channel and
only 1 scanline (row). The length of this scanline is given by the
value of XSIZE below. If this value is 2, the file consists of a
single channel with a number of scanlines. The width and height
of the image are given by the values of XSIZE and YSIZE below.
If this value is 3, the file consists of a number of channels.
The width and height of the image are given by the values of
XSIZE and YSIZE below. The number of channels is given by the
value of ZSIZE below.
XSIZE - The width of the image in pixels
YSIZE - The height of the image in pixels
ZSIZE - The number of channels in the image. B/W (greyscale) images
are stored as 2 dimensional images with a ZSIZE or 1. RGB color
images are stored as 3 dimensional images with a ZSIZE of 3. An RGB
image with an ALPHA channel is stored as a 3 dimensional image with
a ZSIZE of 4. There are no inherent limitations in the SGI image
file format that would preclude the creation of image files with more
than 4 channels.
PINMIN - The minimum pixel value in the image. The value of
0 may be used if no pixel has a value that is smaller than 0.
PINMAX - The maximum pixel value in the image. The value of
255 may be used if no pixel has a value that is greater than 255.
This is the value that is considered to be full brightness in
the image.
DUMMY - This 4 bytes of data should be set to 0.
IMAGENAME - An null terminated ascii string of up to 79 characters
terminated by a null may be included here. This is not commonly
used.
COLORMAP - This controls how the pixel values in the file should be
interpreted. It can have one of these four values:
0: NORMAL - The data in the channels represent B/W values
for images with 1 channel, RGB values for images with 3
channels, and RGBA values for images with 4 channels.
Almost all the SGI image files are of this type.
1: DITHERED - The image will have only 1 channel of data.
For each pixel, RGB data is packed into one 8 bit value.
3 bits are used for red and green, while blue uses 2 bits.
Red data is found in bits[2..0], green data in bits[5..3],
and blue data in bits[7..6]. This format is obsolete.
2: SCREEN - The image will have only 1 channel of data.
This format was used to store color-indexed pixels.
To convert the pixel values into RGB values a colormap
must be used. The appropriate color map varies from
image to image. This format is obsolete.
3: COLORMAP - The image is used to store a color map from
an SGI machine. In this case the image is not displayable
in the conventional sense.
DUMMY - This 404 bytes of data should be set to 0. This makes the
header exactly 512 bytes.
The Image Data (if not RLE)
If the image is stored verbatim (without RLE), then image data directly
follows the 512 byte header. The data for each scanline of the first
channel is written first. If the image has more than 1 channel, all
the data for the first channel is written, followed by the remaining
channels. If the BPC value is 1, then each scanline is written as XSIZE
bytes. If the BPC value is 2, then each scanline is written as XSIZE
shorts. These shorts are stored in the byte order described above.
The Offset Tables (if RLE)
If the image is stored using run length encoding, offset tables
follow the header that describe what the file offsets are to the
RLE for each scanline. This information only applies if the value
for STORAGE above is 1.
Size | Type | Name | Description
tablen longs | long | STARTTAB | Start table
tablen longs | long | LENGTHTAB | Length table
One entry in each table is needed for each scanline of RLE data. The
total number of scanlines in the image (tablen) is determined by the
product of the YSIZE and ZSIZE. There are two tables of longs that
are written. Each consists of tablen longs of data. The first
table has the file offsets to the RLE data for each scanline in the
image. In a file with more than 1 channel (ZSIZE > 1) this table first
has all the offsets for the scanlines in the first channel, followed
be offsets for the scanlines in the second channel, etc. The second
table has the RLE data length for each scanline in the image. In a
file with more than 1 channel (ZSIZE > 1) this table first has all the
RLE data lengths for the scanlines in the first channel, followed
be RLE data lengths for the scanlines in the second channel, etc.
To find the the file offset, and the number of bytes in the RLE data
for a particular scanline, these two arrays may be read in and indexed as
follows:
To read in the tables:
unsigned long *starttab, *lengthtab;
tablen = YSIZE*ZSIZE*sizeof(long);
starttab = (unsigned long *)mymalloc(tablen);
lengthtab = (unsigned long *)mymalloc(tablen);
fseek(inf,512,SEEK_SET);
readlongtab(inf,starttab);
readlongtab(ing,lengthtab);
To find the file offset and RLE data length for a scanline:
rowno is an integer in the range 0 to YSIZE-1
channo is an integer in the range 0 to ZSIZE-1
rleoffset = starttab[rowno+channo*YSIZE]
rlelength = lengthtab[rowno+channo*YSIZE]
It is possible for two identical rows (scanlines) to share compressed
data. A completely white image could be written as a single compressed
row and having all table entries point to that row. Another little hack
that should work is if you are writing out a RGB RLE file, and a
particular scanline is achromatic (greyscale), you could just make the
r, g and b rows point to the same data!!
The Image Data (if RLE)
This information only applies if the value for STORAGE above is 1. If
the image is stored using run length encoding, the image data follows
the offset tables above. The RLE data is not in any particular order.
The offset tables above are used to locate the rle data for any scanline.
The RLE data must be read in from the file and expanded into pixel
data in the following manner:
If BPC is 1, then there is one byte per pixel. In this case the
RLE data should be read into an array of chars. To expand
data, the low order seven bits of the first byte: bits[6..0]
are used to form a count. If the high order bit of the first
byte is 1: bit[7], then the count is used to specify how many
bytes to copy from the RLE data buffer to the destination.
Otherwise, if the high order bit of the first byte is 0: bit[7],
then the count is used to specify how many times to repeat the
value of the following byte, in the destination. This process
continues until a count of 0 is found. This should decompress
exactly XSIZE pixels.
Here is example code to decompress a scanline:
expandrow(optr,iptr,z)
unsigned char *optr, *iptr;
int z;
{
unsigned char pixel, count;
optr += z;
while(1) {
pixel = *iptr++;
if ( !(count = (pixel & 0x7f)) )
return;
if(pixel & 0x80) {
while(count--) {
*optr = *iptr++;
optr+=4;
}
} else {
pixel = *iptr++;
while(count--) {
*optr = pixel;
optr+=4;
}
}
}
}
If BPC is 2, there is one short (2 bytes) per pixel. In this
case the RLE data should be read into an array of shorts. To
expand data, the low order seven bits of the first short: bits[6..0]
are used to form a count. If bit[7] of the first short is 1, then
the count is used to specify how many shorts to copy from the RLE
data buffer to the destination. Otherwise, if bit[7] of the first
short is 0, then the count is used to specify how many times to
repeat the value of the following short, in the destination. This
process proceeds until a count of 0 is found. This should decompress
exactly XSIZE pixels. Note that the byte order of short data in
the input file should be used, as described above.
Implementation notes
Implementation of both RLE and VERBATIM format for images with
BPC of 1 is required since the great majority of SGI images are in
this format. Support for images with a 2 BPC is encouraged.
If the ZSIZE of an image is 1, it is assumed to represent B/W
values. If the ZSIZE is 3, it is assumed to represent RGB data,
and if ZSIZE is 4, it is assumed to contain RGB data with alpha.
The origin for all SGI images is the lower left hand corner. The
first scanline (row 0) is always the bottom row of the image.
Naming Conventions
On SGI systems, SGI image files end with the extension .bw if
they are B/W images, they end in .rgb if they contain RGB image
data, and end in .rgba if they are RGB images with alpha channel.
Sometimes the .sgi extension is used as well.
An example
This program will write out a valid B/W SGI image file:
#include "stdio.h"
#define IXSIZE (23)
#define IYSIZE (15)
putbyte(outf,val)
FILE *outf;
unsigned char val;
{
unsigned char buf[1];
buf[0] = val;
fwrite(buf,1,1,outf);
}
putshort(outf,val)
FILE *outf;
unsigned short val;
{
unsigned char buf[2];
buf[0] = (val>>8);
buf[1] = (val>>0);
fwrite(buf,2,1,outf);
}
static int putlong(outf,val)
FILE *outf;
unsigned long val;
{
unsigned char buf[4];
buf[0] = (val>>24);
buf[1] = (val>>16);
buf[2] = (val>>8);
buf[3] = (val>>0);
return fwrite(buf,4,1,outf);
}
main()
{
FILE *of;
char iname[80];
unsigned char outbuf[IXSIZE];
int i, x, y;
of = fopen("example.rgb","w");
if(!of) {
fprintf(stderr,"sgiimage: can't open output file\n");
exit(1);
}
putshort(of,474); /* MAGIC */
putbyte(of,0); /* STORAGE is VERBATIM */
putbyte(of,1); /* BPC is 1 */
putshort(of,2); /* DIMENSION is 2 */
putshort(of,IXSIZE); /* XSIZE */
putshort(of,IYSIZE); /* YSIZE */
putshort(of,1); /* ZSIZE */
putlong(of,0); /* PIXMIN is 0 */
putlong(of,255); /* PIXMAX is 255 */
for(i=0; i<4; i++) /* DUMMY 4 bytes */
putbyte(of,0);
strcpy(iname,"No Name");
fwrite(iname,80,1,of); /* IMAGENAME */
putlong(of,0); /* COLORMAP is 0 */
for(i=0; i<404; i++) /* DUMMY 404 bytes */
putbyte(of,0);
for(y=0; y<IYSIZE; y++) {
for(x=0; x<IXSIZE; x++)
outbuf[x] = (255*x)/(IXSIZE-1);
fwrite(outbuf,IXSIZE,1,of);
}
fclose(of);
}
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