| Silicon Graphics Image | |
|---|---|
| Filename extension | .sgi,.rgb(among others) |
| Internet media type | image/sgi[1] |
| Type code | '.SGI' |
| Uniform Type Identifier (UTI) | com.sgi.sgi-image[2] |
| Magic number | 01 DA |
| Developed by | Paul Haeberli |
| Initial release | 0.95 / 1995; 31 years ago (1995)[3][4] |
| Latest release | 1.00 1996; 30 years ago (1996) |
| Type of format | image file |
| Standard | SGI Image File Format[5] |
Silicon Graphics Image (SGI) or theRGB file format is the nativerastergraphics file format forSilicon Graphics workstations.[4] The format was invented byPaul Haeberli.[4] It can berun-length encoded (RLE).FFmpeg andImageMagick, among others, support this format.
Common file extensions are:
.sgi or.rgb.rgba.bw or.int.intaThis format was originally developed forIRIX. The master files of theSVT High Definition Multi Format Test Set are SGIs.[6][7][8] Frame 200 of theParkJoy sequence in this set (15722.sgi, 3840×2160, 47.4 MB) was used inWebP comparisons.
SGI image format usesbig endian order. The image data scans from left to right from bottom to top.
| Field number | Hex offset | Length | Field name | Description |
|---|---|---|---|---|
| 1 | 0x00 | 2 byte | File signature/magic number | SGI Signature |
| 2 | 0x02 | 1 byte | Compression | 0 = Uncompressed,1 = RLE compressed |
| 3 | 0x03 | 1 byte | Bytes per channel | 1 = 8 bit,2 = 16 bit |
| 4 | 0x04 | 2 bytes | Dimension | Image dimension,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 channels in the image file,4 for RGBA image |
| 8 | 0x0C | 4 bytes | Minimum pixel value | Image's smallest pixel value |
| 9 | 0x10 | 4 bytes | Maximum pixel value | Image's largest pixel value |
| 10 | 0x14 | 4 bytes | Reserved | Unused, ignore |
| 11 | 0x18 | 80 bytes | Image name | NOTE: C strings end with last byte0x00 |
| 12 | 0x68 | 4 bytes | Color map ID | Only for color map image |
| 13 | 0x6C | 404 bytes | Dummy | Buffer to scale the header to 512 bytes. Ignore |
File Signature/Magic Number
Should equal0x010xda for SGI file.
Compression
0 if the file is uncompressed;1 for RLE compression.
Bytes Per Pixel Channel
Value is1 for 8 bits per channel ;2 for 16 bits per channel.
Dimension
Value is1 for 1 channel, 1 scanline image;2 for 1 channel, multiple scanlines image;3 for multiple channels, multiple scanlines image (most common).
X Size
Scanline width in pixels.
Y Size
Number of scanlines in the image.
Number of Channels
Number of channels in the image. Value is1 for greyscale images;3 for RGB;4 for RGBA. SGI supports more than 4 channels.
Minimum Pixel Value
Smallest pixel value in all channels. Since SGI uses unsigned char or short types for channels, the smallest possible value is typically0.
Maximum pixel value
Largest pixel value in all channels. Since SGI uses unsigned char or short types for channels, the largest possible value is typically0xff for 8-bit/channel and0xffff for 16-bit/channel.
Dummy
4 bytes unused.
Image name
Image name, 80-byte (maximum 79 characters ≠0x00) C string. If the name is shorter than 79 characters, all subsequent bytes are0x00 to fill the 80-byte space.
Color Map ID
SGI supports 4 types of color mapping:
0x00 normal: image has 1 channel (greyscale), 3 channels (RGB) or 4 channels (RGBA).0x01 dithered: image has packed RGB. First 3 bits are for R channel, next 3 bits are G channel, and last 2 bits are for B channel.0x02 screen: image uses 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. Typically unused. Scales the header to 512 bytes long.
No Compression
Image data starts at byte 512 in file. Data length can be calculated using the following:
(X size) × (Y size) × (Num. of channels) × (Bytes per channel)
The data is separated for each channel. For example, if an image uses RGBA, the first set of scanlines represents the R channel, the next set of scanlines are for the G channel, then B channel and, lastly, A channel. Therefore, each set of scanlines can be accessed directly via the following formula (whereC = channel index):
(X size) × (Y size) × (C ÷ Num. of channels) × (Bytes per channel)
RLE Compression
Offset tables start at byte 512. Their length can be calculated with the following:
(Y size) × (Num. of channels) × 4 bytes long
Each unsigned int in a table represents an offset (from file start) to the start of the compressed data of each scanline in each channel. The data is separated for each channel's scanline. If an image has RGBA channels, the order stays R, G, B, and A.
Next is the data length table which can also be calculated with the formula:
(Y size) × (Num. of channels) × 4 bytes long
This table holds the size of the compressed data (unsigned int) of each scanline. This size should equal the difference of numbers in the offset table.
The last data block holds RLE-compressed image data. If the number of bytes per channel is1, the compressed data is of unsigned char type. Otherwise, if the number of bytes per channel is2, the compressed data is of unsigned short type. The last byte of compressed scanline data has a value of0.
In either case of unsigned char or unsigned short, the lowest 7 bits will tell the count. If bit 8 is zero, the count is the number times copy value after count. If bit 8 is one, the count is number times copy series of values after count byte. For 2 bytes/channel image, count and value are unsigned short, and top byte of count always equals zero.
| Raster | |
|---|---|
| Raw | |
| Vector | |
| Compound | |
| Metadata |
|
