MrSID was originally developed for Geographic Information Systems (GIS). With this format, large raster image files such as aerial photographs or satellite imagery are compressed and can be quickly viewed without having to decompress the entire file.
MrSID (pronounced “Mister Sid”) stands for “Multi-resolution Seamless Image Database.” We call it SID for short. (That also happens to be the extension used by MrSID-format files. Coincidence? We think not!)
The MrSID format is an industry standard image format that provides very high compression ratios (both lossless and lossy) while maintaining high visual accuracy.
MrSID has gone through several iterations (called “generations” in SIDville); these are referenced as MG2, MG3, and MG4 (MrSID Generation 2, etc.). All MrSID files use the same extension (
Throughout this documentation, we’ll generally refer to SID files or SID images, meaning a MrSID Generation 4 (MG4, that’s right!) image or file. SquishPic can read MG2 and MG3 files, but will always save them as MG4 files.
MrSID technology uses lossless wavelet compression to create an initial image. Then the encoder divides the image into zoom levels, subbands, subblocks and bitplanes. After the initial encoding, the image creator can apply zero or more optimizations. While 2:1 compression ratios may be achieved losslessly, higher compression rates are lossy much like JPEG-compressed data.
MrSID uses selective decoding meaning that the decoder does not have to decode the entire file to view a specific zoom level, image quality or scene for example.