The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly.


PNG grayscale images support the widest range of pixel depths of any image type. Depths of 1, 2, 4, 8, and 16 bits are supported, covering everything from simple black-and-white scans to full-depth medical and raw astronomical images.[63]

[63] Calibrated astronomical image data is usually stored as 32-bit or 64-bit floating-point values, and some raw data is represented as 32-bit integers. Neither format is directly supported by PNG, although one could, in principle, design an ancillary chunk to hold the proper conversion information. Conversion of data with more than 16 bits of dynamic range would be a lossy transformation, however--at least, barring the abuse of PNG's alpha channel or RGB capabilities.

There is no direct comparison with GIF images, although it is certainly possible to store grayscale data in a palette image for both GIF and PNG. The only place a gray palette is commonly distinguished from a regular color one, however, is in VRML97 texture maps. Baseline TIFF images, on the other hand, support 1-bit ``bilevel'' and 4- and 8-bit grayscale depths. Nonbaseline TIFF allows arbitrary bit depths, but libtiff accepts only 1-, 2-, 4-, 8-, and 16-bit images. TIFF also supports an inverted grayscale, wherein 0 represents white and the maximum pixel value represents black.

The most common form of JPEG (the one that uses ``lossy'' compression, in which some information in the image is thrown away) likewise supports grayscale images in depths of 8 and 12 bits. In addition, there are two variants that use truly lossless compression and support any depth from 2 to 16 bits: the traditional version, known simply as ``lossless JPEG,'' and an upcoming second-generation flavor called ``JPEG-LS.''[64] But the first is extremely rare, and is supported by almost no one, despite having been standardized years ago, and the second is also currently unsupported (although that is to be expected for a new format). Lossy JPEG is very well supported, thanks largely to the Independent JPEG Group's free libjpeg (which, like libtiff, has become the de facto standard for JPEG encoding and decoding)--but, of course, it's lossy. Note that libjpeg can be compiled to support either 8-bit or 12-bit JPEG, but not both at the same time. Thus, from a practical standpoint, only 8-bit, lossy grayscale is supported.

[64] Be aware that even at the highest quality settings, the common form of JPEG is never lossless, regardless of whether the setting claims 100% or something similar.

Last Update: 2010-Nov-26