Joint Photographic Experts Group JFIF Files (JPG)

The GDB library supports single-scan JPEG files that contain 8-bit grayscale or 24-bit color images. GDB will read both YCC and the less common RGB color space JPEG files, but will always write the YCC variant.

The GDB JPEG support is based on the Independent JPEG Group (IJG) version 4 code. A suite of public domain Unix tools for working with JPEG files is available by anonymous ftp from ftp.uu.net in the directory graphics/jpeg.

Projection information is read from an ESRI World File with extension .jgw or from a MapInfo TAB file with a .tab file name extension when either of these files exists in your project. For more information on how georeferencing is assigned using a World File, see TIFF (TIF). If no such file is present, a projection type Pixel will be assigned.

Export of JPEG JFIF files is supported. These files are exported with a type of "JPG".

JPEG files do not usually contain any auxiliary information, such as georeferencing, lookup tables (LUT), or pseudocolor table (PCT); however, a .pox file is written on export, and this will be recognized when reading PCI- generated JPEG files. Georeferencing can be generated when the file is generated by specifying the WORLD option. This generates an ESRI World File in a .jgw file.

You can only export one (grayscale) and three (RGB color) bands to a JPEG file. You can specify an available option for the numerical percent 'quality' of the resulting JPEG file. The default quality is 75. More quality implies a bigger image. The useful range of this parameter is from approximately 15 through 90. You specify the parameter by entering the applicable number.

JPEG is a lossy compression, and under no circumstances will the integrity of the image be preserved exactly, even if the quality is 100.

When reading data, Exchangeable image file format (Exif) tags are imported as metadata, which contain specific image properties.