|Internet media type|
|Magic number||01 DA|
|Developed by||Paul Haeberli|
|Initial release||0.95 / 1995|
|Type of format||image file|
|Standard||SGI Image File Format|
Silicon Graphics Image (SGI) or the RGB file format is the native raster graphics file format for Silicon Graphics workstations. The format was invented by Paul Haeberli. It can be run-length encoded (RLE). FFmpeg and ImageMagick, among others, support this format.
Common file extensions are:
This format was originally developed for IRIX. The master files of the SVT High Definition Multi Format Test Set are SGIs. Frame 200 of the ParkJoy sequence in this set (15722.sgi, 3840×2160, 47.4 MB) was used in WebP comparisons.
SGI image format use big endian order. Image data scan from left to right from bottom to top.
|Field number||Hex offset||Length||Field name||Description|
|1||0x00||2 byte||File signature/magic number||For show file is SGI file|
|2||0x02||1 byte||Compression||0 = uncompressed, 1 = RLE compressed|
|3||0x03||1 byte||Bytes per pixel||1 = 8 bit, 2 = 16 bit|
|4||0x04||2 bytes||Dimension||Image dimension, equal 3 for RGBA image|
|5||0x06||2 bytes||X size||Image width|
|6||0x08||2 bytes||Y size||Image height|
|7||0x0A||2 bytes||Number of channels||Number channel in image, equal 4 for RGBA image|
|8||0x0C||4 bytes||Minimum pixel value||Smallest pixel value in image|
|9||0x10||4 bytes||Maximum pixel value||Largest pixel value in image|
|10||0x14||4 bytes||Reserved||No use, ignore|
|11||0x18||80 bytes||Image name||C string name have last byte equal 0x00|
|12||0x68||4 bytes||Color map ID||Only for color map image|
|13||0x6C||404 bytes||Dummy||For make head 512 bytes long. Ignore|
File Signature/Magic Number
0xda for SGI file.
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.
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).
Scanline width of image.
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
4 bytes not use.
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:
0x00normal: image have 1 channel is grey scale; 3 channels is RGB, 4 channels is RGBA
0x01dithered: image is packed RGB. First 3 bits for R, next 3 bits G channel, and last 2 bits for B channel
0x02screen: image use colormap. Pixel data is index in color map and image only have 1 channel.
0x03colormap: 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 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.
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.
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