SGI Format Guide

Available Conversions

Convert to SGI

About SGI Format

SGI (Silicon Graphics Image), also known as IRIS RGB, is a raster image file format developed by Silicon Graphics, Inc. (SGI) in 1984 for use on their IRIX workstations. The format was created as the native image format for SGI's graphics workstations, which were the dominant computing platform for visual effects, 3D animation, and scientific visualization throughout the 1980s and 1990s. SGI files store uncompressed or RLE-compressed (Run-Length Encoded) pixel data with support for 8-bit and 16-bit per channel color depth across one to four channels (grayscale, grayscale with alpha, RGB, and RGBA). The format uses a straightforward binary structure with a 512-byte header containing image dimensions, number of channels, pixel depth, minimum and maximum pixel values, and an optional image name field of up to 80 characters. SGI files are stored in big-endian byte order, reflecting the MIPS processor architecture of original Silicon Graphics workstations. The file extensions commonly associated with this format include .sgi, .rgb, .rgba, .bw (for grayscale), and .int (for 16-bit integer data), though .sgi is the most widely recognized extension today.

History of SGI

The SGI image format was introduced alongside Silicon Graphics' IRIX operating system and their pioneering line of graphics workstations in 1984. During the late 1980s and throughout the 1990s, SGI workstations such as the Indigo, Indy, O2, Octane, and Onyx were the industry standard for professional computer graphics, powering the visual effects in landmark films including Jurassic Park (1993), Terminator 2: Judgment Day (1991), Toy Story (1995), and The Matrix (1999). The SGI image format became deeply embedded in the visual effects and film production pipeline as the default format for texture maps, compositing plates, rendered frames, and scientific data visualization. Industrial Light & Magic (ILM), Pixar, Digital Domain, Weta Digital, and other major VFX studios relied on SGI workstations and the SGI image format as core components of their production infrastructure. When these studios developed custom compositing and rendering tools, SGI format support was a fundamental requirement. The format's influence extended into commercial software: early versions of Alias|Wavefront Maya, Side Effects Houdini, Softimage, The Foundry's Nuke (and its predecessor Shake by Nothing Real), and Pixar's RenderMan all provided native SGI format support. As the VFX industry gradually transitioned from SGI IRIX workstations to Linux-based render farms and macOS/Windows artist workstations in the early 2000s, the SGI format remained in use alongside newer formats like OpenEXR. Today, SGI files are still encountered in legacy VFX archives, scientific visualization datasets, and older 3D animation projects, and the format continues to be supported by modern image processing libraries including Pillow, ImageMagick, and FFmpeg.

Key Features and Uses

SGI files use a binary structure consisting of a 512-byte header followed by pixel data stored either in uncompressed (verbatim) or RLE-compressed form. The header's magic number is 474 (decimal), which identifies the file as an SGI image. The storage field in the header indicates whether the pixel data is stored verbatim (value 0) or with RLE compression (value 1). RLE compression in SGI files operates on individual scanlines, compressing runs of identical pixel values to reduce file size while maintaining fast random access to any scanline in the image. For RLE-compressed files, the header is followed by offset and length tables that provide the byte offset and compressed size of each scanline, enabling efficient random access without decompressing the entire file. Pixel data is stored in a channel-separated (planar) layout rather than interleaved: all red channel scanlines are stored first, followed by all green, then blue, and finally alpha channel scanlines. This planar organization was efficient for the SGI hardware architecture and for compositing operations that frequently operate on individual color channels. The format supports 8-bit per channel (bytes per pixel = 1) and 16-bit per channel (bytes per pixel = 2) storage, with 16-bit mode providing the extended dynamic range needed for scientific data and high-precision rendering. The optional colormap field supports normal RGB (value 0), dithered (value 1), screen (value 2), and colormap (value 3) modes, though only normal mode is widely used in practice.

Common Applications

SGI files have historically been used across the visual effects, film production, scientific visualization, and 3D animation industries. In VFX compositing, SGI images served as the standard format for film scan plates, rendered CG elements, matte paintings, and final composited frames in tools like Nuke, Shake, Inferno, and Flame. Film studios stored thousands of SGI frames per shot, with each frame representing a single film frame at resolutions of 2K (2048x1556) or 4K (4096x3112) for 35mm film scanning. In 3D animation, SGI textures were used as input maps for surface shading in Maya, Houdini, Softimage, and RenderMan, and as output rendered frames from these applications. Scientific visualization applications on SGI workstations used the format to store volumetric rendering outputs, medical imaging data, seismic analysis visualizations, weather simulation results, and astronomical imagery. The format was also used in flight simulator display systems, where SGI workstations generated real-time terrain and cockpit instrument imagery. Today, converting SGI files to modern formats like PNG, TIFF, or OpenEXR is common when digitizing legacy VFX archives, migrating old animation projects to current pipelines, or accessing historical scientific datasets. Conversely, converting modern images to SGI format may be necessary when working with legacy software or hardware that requires SGI input, or when integrating new assets into older pipeline tools that only accept SGI files.

Advantages and Disadvantages