The SGI image file (.sgi, .rgb, .rgba, .bw)

SGI is the image format of Silicon Graphics, the maker of the IRIS workstations that were standard equipment for 3D graphics, film effects and scientific visualisation in the late 1980s and the 1990s. It was the file format of SGI's image library, libimage, which programs on IRIX used for screen dumps, greyscale and colour images, and image tools. Paul Haeberli of SGI wrote up the format as the SGI Image File Format specification, version 1.00.

The design is simple: a fixed 512-byte header, then each channel stored as a separate plane of rows, either raw or with run-length encoding (RLE). It handles greyscale, RGB and RGBA, at 8 or 16 bits per channel.

SGI workstations are long gone, but the files aren't:

• 3D and VFX tools still read them. Autodesk's FCheck image viewer, which comes with Maya, opens .rgb, .sgi and .bw files, and Blender reads and writes the format under the name Iris, at 8 or 16 bits with alpha.
• Textures and rendered image sequences in older 3D projects and archives.
• Video test material. The master files of the SVT High Definition Multi Format Test Set, a series of video test sequences, are SGI images.

For new work, PNG covers the same 8 and 16-bit lossless cases with better compression, and EXR is the VFX standard for high dynamic range.

Everything is big-endian. In a verbatim file, the pixels start at byte 512: all rows of the first channel, bottom row first, then all rows of the second channel, and so on. There is no row padding, so a verbatim file is exactly 512 + width × height × channels × bytes per channel. Our 3 × 2 example saved verbatim is 530 bytes.

In an RLE file, each row of each channel is compressed on its own. Two tables follow the header at byte 512: the offset of each compressed row, then the length of each, height × channels entries of 4 bytes each. Rows can be anywhere in the file and in any order, and identical rows can share data. Each compressed row is a series of packets:

The count is the low 7 bits, 1 to 127. For 16-bit images, the count and values are 16-bit instead of bytes. Note that the meaning of the top bit is the opposite of TGA's run-length encoding and of PackBits, where a set top bit means a run: porting a decoder from one to the other is a classic source of bugs.

RLE pays off on flat areas such as graphics, masks and renders with plain backgrounds, and does little for photos or noise, where it can make the file slightly bigger. Either way it's lossless. For our tiny example, RLE made the file larger (590 bytes against 530) because of the tables.

All of these are the same format. The extension only hints at the number of channels; the header's ZSIZE field is what counts.

SGI and TGA are siblings from the same era: simple headers, optional RLE, bottom-up rows. SGI's advantage was 16-bit channels; TIFF and PNG now do that and much more.

• GIMP opens and saves SGI files (.sgi, .rgb, .bw), on Windows, macOS and Linux.
• ImageMagick: magick texture.rgb texture.png, or magick input.png -compress RLE output.sgi to write an RLE file (convert in ImageMagick 6).
• Pillow: Image.open("texture.rgb").save("texture.png"). Pillow writes SGI files too, uncompressed.
• Netpbm: sgitopnm texture.rgb > texture.ppm, and pnmtosgi -rle or -verbatim the other way.
• FFmpeg: ffmpeg -i frame%04d.sgi frame%04d.png converts a numbered sequence in one go.
• Blender loads SGI files as Iris images and can render to the format.

Converting to PNG is lossless for 8 and 16-bit images, alpha included. Browsers don't display SGI, so convert before putting images on the web.

• The image is upside down. The reader ignored the bottom-up row order. Flip it, or use one of the converters above.
• The image is noise or a reader rejects it. The file is a raw .rgb dump without a header, not an SGI image. Check for 01 DA at the start.
• Stripes or wrong colours in an RLE file. A hand-written decoder used TGA's or PackBits' meaning of the top bit, or read the offset tables in the wrong order (all rows of one channel come before the next channel).
• A 16-bit image looks black or wrong. The program reads only 8-bit SGI files, or treats the 16-bit values as little-endian. Convert with ImageMagick or Pillow to 16-bit PNG.
• An alpha channel is ignored. The program reads only the first 3 channels. Check ZSIZE: 4 means there's alpha.

An SGI file contains pixels and a short name, nothing that runs. As with other formats, crafted files have hit bugs in decoders: Pillow fixed an out-of-bounds read in its SGI RLE decoder in version 8.1.1 (CVE-2021-25293). Keep image libraries updated, and check that RLE offsets and lengths stay inside the file before trusting them.