Background
Common file extensions are: ;.sgi
or .rgb
: 3 colour channels
; .rgba
: 3 colour channels and alpha
;.bw
or .int
: black and white
;.inta
: black and white and alpha
This format was originally developed for Technical Details
SGI image format useHeader
File Signature/Magic Number Should equal0x01
0xda
for SGI file.
Compression
If no compression equal 0; 1 for RLE compression.
Bytes Per Pixel Channel
Value is 1 for 8 bit per channel (most image); 2 for 16 bits per channel.
Dimension
Value equal 1 for 1 channel, 1 scanline image ; 2 for 1 channel, many scanline image (); 3 for many channel, many scanline image (most image).
X Size
Scanline width of image.
Y Size
Number of scanlines in image.
Number Of Channels
Number channels in image. Grey scale image is 1; RGB is 3; RGBA is 4. SGI can have more than 4 channels.
Minimum Pixel Value
Smallest pixel value in all channels. Because SGI use unsigned char or short for channels, smallest value can have is zero.
Maximum pixel value
Largest pixel value in all channels. Because SGI use unsigned char or short for channels, largest value can have for 8 bit/channel is 0xff
; 16 bit/channel image is 0xffff
.
Dummy
4 bytes not use.
Image name
Image name is 80 bytes (maximum 79 characters ≠ 0x00
) C string. If name is shorter than 79 characters, all bytes after equal 0x00
for fill all 80 bytes.
Color Map ID
Have 4 types:
# 0x00
normal: image have 1 channel is grey scale; 3 channels is RGB, 4 channels is RGBA
# 0x01
dithered: image is packed RGB. First 3 bits for R, next 3 bits G channel, and last 2 bits for B channel
# 0x02
screen: image use colormap. Pixel data is index in color map and image only have 1 channel.
# 0x03
colormap: file only have data for one color map (can use for different image file). This file no have image data.
Dummy
404 bytes for header 512 bytes long.
Image Data
No Compression Image data start at byte 512 in file. Data length equal (x size) × (y size) × (number of channels) × (bytes per channel). Data for each channel is separate. If image have RGBA channels, then all scanlines for channel R first, scanlines for channel G next, scanlines for channel B next, scanlines for channel A last. RLE Compression Offset table start at byte 512 and is (y size) × (number of channels) × 4 bytes long. Each unsigned int in table is offset (from file start) for start of compressed data of each scanline in each channel. Data for each channel's scanline is separate. If image have RGBA channels, order for channel data is R, G, B and A. Next have data length table also (y size) × (number of channels) × 4 bytes long. This table tell size of compressed data (unsigned int) of each scanline. This size should equal difference of numbers in offset table. Last is RLE compressed image data. If bytes per channel equal 1, compressed data is unsigned char. Else if equal 2, compressed data is unsigned short. End of compressed scanline data have value zero. For both case have unsigned char or unsigned short data, lowest 7 bits tell count. If bit 8 is zero, count is number times copy value after count. If bit 8 is one, count is number times copy series of values after count byte. For 2 bytes/channel image, count and value is unsigned short and top byte of count always equal zero.See also
*References