SGI to JPG Converter

Encoding: the SGI uses uncompressed (verbatim) or per-row run-length encoding; JPG uses lossy DCT coding in 8×8 pixel blocks, with colour usually stored at half resolution, and the download is a standard JPEG at the quality you choose.

Bit depth: 8 or 16 bits per channel in the SGI; JPG allows 8 bits per channel, and the JPG 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; JPG supports YCbCr shown as RGB, greyscale, or CMYK from print software, written here as 8-bit sRGB.

Transparency: the SGI stores it as a fourth channel plane; the JPG has none, so transparent areas are filled with the colour you choose.

Metadata: the SGI can carry only an 80-character image name in the header; JPG can carry EXIF (camera, date, GPS), XMP, IPTC and an ICC profile in APP segments, 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.

JPEG is the universal photo format. It compresses by discarding fine detail the eye is least sensitive to, which makes photos small but blurs sharp edges and text if pushed too far.

A JPEG starts with the bytes FF D8 and is a run of marker segments: APP1 carries EXIF and often a thumbnail, APP2 an ICC profile, DQT the quantization tables that decide the quality, and SOS the compressed scan, which ends at FF D9.

JPG is the better choice for photographs that need to open anywhere, while SGI suits legacy SGI and 3D pipelines.

JPG strengths: Opens everywhere: every browser, phone, camera, printer and app; Small files for photographs; Adjustable quality from tiny to near-lossless. Drawbacks to weigh: No transparency; Lossy: every re-save loses a little more; Blocky artifacts around text, logos and sharp lines.