SGI to PNG Converter

Encoding: the SGI uses uncompressed (verbatim) or per-row run-length encoding; PNG uses lossless DEFLATE with a prediction filter chosen per row, and the download is a lossless 8-bit PNG with full alpha.

Bit depth: 8 or 16 bits per channel in the SGI; PNG allows 1, 2, 4, 8 or 16 bits per channel, and the PNG written here has 8 bits per channel. 16-bit SGI files are reduced to 8 bits per channel.

Colour: greyscale (.bw), RGB (.rgb) or RGBA (.rgba) in the SGI; PNG supports RGB, greyscale, or an indexed palette of up to 256 colours, written here as 8-bit sRGB.

Transparency: the SGI stores it as a fourth channel plane; the PNG keeps it as a full alpha channel, so edges and shadows can fade smoothly.

Metadata: the SGI can carry only an 80-character image name in the header; PNG can carry text chunks, an ICC profile, physical resolution and EXIF in an eXIf chunk, but nothing is copied across.

SGI (also .rgb, .rgba, .bw) is the native image format of Silicon Graphics IRIX workstations, where much of 1990s 3D graphics and film work was done. It stores each colour channel as its own plane, uncompressed or run-length encoded.

An SGI file starts with the big-endian magic number 474 (01 DA), a storage byte (0 verbatim, 1 RLE), bytes per channel, dimensions and channel count in a 512-byte header. Channels are stored as separate planes and rows run from the bottom of the image up; RLE files add tables of row offsets and lengths.

SGI files usually come from SGI IRIX workstations, early 3D and VFX pipelines and old texture libraries.

PNG stores pixels losslessly with full alpha transparency. It is ideal for screenshots, logos, UI and anything with sharp edges, but photos saved as PNG are several times larger than JPEG.

A PNG opens with an 8-byte signature and is a chain of chunks, each with its own CRC: IHDR (size, depth, colour type), optional PLTE and tRNS for palettes, one or more IDAT chunks with the compressed rows, and IEND.

PNG is the better choice for screenshots, logos, graphics and anything that needs transparency, while SGI suits legacy SGI and 3D pipelines.

PNG strengths: Lossless: no quality loss, ever; Full transparency with smooth edges; Perfect for text, screenshots and graphics. Drawbacks to weigh: Large files for photographs; No standard animation (that is APNG).