<html> <head> <title>JPEG -- JPEG JFIF File Format</title> </head> <body bgcolor="#ffffff"> <h1>JPEG -- JPEG JFIF File Format</h1> The JPEG JFIF format is supported for reading, and batch writing, but not update in place. JPEG files are represented as one band (greyscale) or three band (RGB) datasets with Byte valued bands.<p> The driver will automatically convert images whose color space is YCbCr, CMYK or YCbCrK to RGB, unless GDAL_JPEG_TO_RGB is set to NO (YES is the default). When color space translation to RGB is done, the source color space is indicated in the SOURCE_COLOR_SPACE metedata of the IMAGE_STRUCTURE domain.<p> There is currently no support for georeferencing information or metadata for JPEG files. But if an ESRI world file exists with the .jgw, .jpgw/.jpegw or .wld suffixes, it will be read and used to establish the geotransform for the image. If available a MapInfo .tab file will also be used for georeferencing. Overviews can be built for JPEG files as an external .ovr file.<p> The driver also supports the "zlib compressed mask appended to the file" approach used by a few data providers to add a bitmask to identify pixels that are not valid data. See <a href="http://trac.osgeo.org/gdal/wiki/rfc15_nodatabitmask">RFC 15</a> for further details.<p> The GDAL JPEG Driver is built using the Independent JPEG Group's jpeg library. Also note that the GeoTIFF driver supports tiled TIFF with JPEG compressed tiles.<p> <h2>Creation Options</h2> JPEG files are created using the "JPEG" driver code. Only Byte band types are supported, and only 1 and 3 band (RGB) configurations. JPEG file creation is implemented by the batch (CreateCopy) method. YCbCr, CMYK or YCbCrK colorspaces are not supported in creation. If the source dataset has a nodata mask, it will be appended as a zlib compressed mask to the JPEG file.<p> <ul> <li> <b>WORLDFILE=YES</b>: Force the generation of an associated ESRI world file (with the extension .wld). <p> <li> <b>QUALITY=n</b>: By default the quality flag is set to 75, but this option can be used to select other values. Values must be in the range 10-100. Low values result in higher compression ratios, but poorer image quality. Values above 95 are not meaningfully better quality but can but substantially larger.<p> <li> <b>PROGRESSIVE=ON</b>: Enabled generation of progressive jpegs. In some cases these will display a reduced resolution image in viewers such as Netscape, and Internet Explorer, before the full file has been downloaded. However, some applications cannot read progressive jpegs at all. GDAL can read progressive jpegs, but takes no advantage of their progressive nature.<p> </ul> See Also: <p> <ul> <li> <a href="http://www.ijg.org/">Independent JPEG Group</a> </ul> </body> </html>
