Grayscale with Alpha Channel

The second kind of transparency supported by grayscale images is an alpha channel. This is a more expensive approach in terms of file size--for grayscale, it doubles the number of image bytes--but it allows the user much greater freedom in setting individual pixels to particular levels of partial transparency. Only 8-bit and 16-bit grayscale images may have an alpha channel, which must match the bit depth of the gray channel.

The full TIFF specification supports two kinds of interleaved ``extra samples'' for transparency: associated and unassociated alpha (though not at the same time). Unlike PNG, TIFF's alpha channel may be of a different bit depth from the main image data--in fact, every channel in a TIFF image may have an arbitrary depth. TIFF also offers the explicit possibility of treating a ``subfile,'' or secondary image within the file, as a transparency mask, though such masks are only 1 bit deep, and therefore support only completely opaque or completely transparent pixels.

Baseline TIFF does not require support for any of this, however. Current versions of libtiff can read an interleaved alpha channel as generic ``extra samples,'' but it is up to the application to interpret the samples correctly. The library does not support images with channels of different depths, and although it could be manipulated into reading a secondary grayscale subfile (which the application could interpret as a full alpha channel), that would be a user-defined extension--i.e., specific to the application and not supported by any other software.

As I just noted, standard JPEG (by which I mean the common JPEG File Interchange Format, or JFIF files) has no provision for transparency. The JPEG standard itself does allow extra channels, one of which could be treated as an alpha channel, but this would be fairly pointless. Not only would it require one to use a non-standard, unsupported file format for storage, there would also tend to be visual artifacts, since lossy JPEG is not well suited to the types of alpha masks one typically finds (unless the mask's quality setting were boosted considerably, at a cost in file size). But see Chapter 12, "Multiple-Image Network Graphics" for details on a MNG subformat called JNG that combines a lossy JPEG image in JFIF format with a PNG-style, lossless alpha channel.