HSI Raw is a simple raster image format associated with the Image Alchemy software from Handmade Software. Images are usually uncompressed, and either 8-bit palette color, or 24-bit truecolor. Other image types, and PackBits compression, are possible, but are not documented.
The Image Alchemy documentation states that "HSI Raw format was originally intended as an internal format to Image Alchemy."
HSI presumably stands for "Handmade Software Inc."
HSI Raw is a full-fledged image format, not a raw bitmap format. Note that Image Alchemy also supports raw bitmap formats (which might also use the .raw file extension), in conjunction with its BIF description format.
The .hst file extension is used by Image Alchemy when it generates a (picture of a) histogram of another image.
MPlayer bitmap font format uses an extension of HSI Raw, to support images wider than 65535 pixels. The origin of this extension is unknown.
Files begin with ASCII "
Refer to the Image Alchemy documentation, Appendix F.