quantized more accurately than the chroma data by using separate 64-ele-
ment quantization tables. These tables were provided by the Joint
Photographic Experts Group and provide a scaling multiplier that the user
(you!) can adjust—the Quality slider bar in Photoshop.- Encodes the reduced coefficients using the Huffman method, which is a loss-
less mathematical model for compression. - Outputs the file with the appropriate header information, including the com-
pression information.
When the JPEG file is opened, it is decompressed based on the header information of
the file.The Replacement for JPEG: JPEG2000
New developments in the JPEG file format--specifically the JPEG2000 format--prom-
ise improved quality and greater compression. This format has been approved by
the JPEG group, but is not supported yet by many applications. Photoshop CS
includes a JPEG2000 plug-in and many high-end digital cameras support this for-
mat, but you need to install a plug-in for most browsers. The company Lura pro-
duces these plug-ins, which can be found atwww.luratech.com.
Key advantages of JPEG2000 over JPEG include the following:
■ Scalable by resolution, color channel, or quality. By organizing the
codestream in different ways, the compression can generate multiple levels of
compression.
■ 38-bit depth.Up to 38-bit depth is available in addition to the standard 24-
bit depth. This standard also allows for big images (up to 2^32 rows and
columns), 16,384 components, and different bit depths for each channel.
■ Wavelet-based compression.JPEG2000 uses the CWT (Cosine Wavelet
Transform) algorithm to compress images. Unlike the existing JPEG format,
the wavelet algorithm does not divide image into blocks. Instead, it analyzes
the entire image by treating it as three separate continuous waves (R, G, B),
centered along a zero axis. JPEG2000 images can be compressed more tightly,
preserve detail, and avoid the blockiness common in highly compressed JPEG
images.
An unusual benefit to wavelet compression is decomposition. Decomposition
creates a number of less detailed or smaller images. When you open this file,
the image is reconstructed and grows in pixel size and detail.
■ Lossless encoding.Lossless encoding is available in this format. The JPEG
format allowed for it as well, but few developers integrated it into their JPEG
format.90 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY