Jbig Basics

Bi-level images contain only two colors and are stored using a single bit per pixel. Black-and-white images, such as the pages of a book, are the most common type of bi-level images. However, any two colors may be represented by the 1 (foreground color) or 0 (background color) state of a single bit.

Typical bi-level compression algorithms encode only a single scan line at a time using a run-length encoding technique. Such algorithms are referred to as 1D encoding methods. 2D encoding methods encode runs of pixels by describing the differences between the pixel values in the current and the previous scan lines.

JBIG encodes redundant image data by comparing a pixel in a scan line with a set of pixels already scanned by the encoder. These additional pixels are called a template, and they form a simple map of the pattern of pixels that surround the pixel that is being encoded. The values of these pixels are used to identify redundant patterns in the image data. These patterns are then compressed using an adaptive arithmetic compression coder.

The adaptive nature of templates allows the color of the pixel values being encoded to be predicted with a high degree of success. For gray-scale images with halftoning, compression ratios are increased by as much as 80 percent over non-adaptive methods.

Although designed primarily as a method for compressing bi-level image data, JBIG is capable of compressing color or gray-scale images with a depth of up to 255 bits per pixel. Such multi-bit pixel images are compressed by bitplane rather than by pixel. For example, an 8-bit image compressed using JBIG would be encoded into eight separate bitplanes.

This type of encoding may be used as an alternative to lossless JPEG. JBIG has been found to produce better compression results than lossless JPEG (using the Q-coder) on images with two to five bits per pixel and to produce identical results on image data with pixels six to eight bits in depth.

It is recommended that each bitplane be preprocessed with a gray-coding algorithm to normalize the changes between adjacent byte values in the image data. This process increases the efficiency of the JBIG encoder.

JBIG images may be encoded sequentially or progressively. Sequentially encoded images are stored in a single layer at full resolution and without other lower resolution images being stored in the same data stream. This sequential JBIG image is equivalent in function and application to a G4 image. Such an image is decoded in a single pass and has at least as good a compression ratio as G4.

Progressively encoded images start with the highest resolution image and end with the lowest. The high-resolution image is stored in a separate layer and is then used to produce a lower resolution image, also stored in its own layer. Each layer after the first layer is called a resolution doubling. An image with three layers is said to have two doublings.

There is no imposed limit to the number of doublings that may be encoded. For example, a 1200-dpi image may be encoded as one layer (1200 dpi), three layers (1200, 600, and 300 dpi), or five layers (1200, 600, 300, 150, and 75 dpi). The lowest resolution is determined by whatever is considered useful. Even a 10-dpi image, though not legible, is still useful as an icon.

Progressive decoding is the opposite process, with the lowest resolution image being decoded first, followed by increased resolutions of the image until the full resolution is achieved. This technique has the advantage of allowing data to appear immediately on the output device. Only data up to the appropriate resolution of the output device need be decoded and sent.

Both sequential and progressive JBIG encoding are completely compatible. Images compressed using sequential encoding are readable by progressive JBIG decoders. Sequential JBIG decoders are only capable of reading the first, lowest-resolution layer within a progressively-encoded JBIG image.

Many applications that utilize JBIG may only have use for sequential encoding and decoding, especially those used for facsimile transmission. It is therefore possible to implement a simplified JBIG algorithm that encodes only the first layer in a JBIG data stream. Such encoders produce a valid JBIG-encoded data stream that is readable by all JBIG decoders.

Progressive encoding does not add much more data to a JBIG data stream than does sequential encoding, but it does have greater memory requirements. Because a lower resolution image is encoded from data of the next higher resolution image (and vice versa when decoding), a frame buffer must be used to store image data that is being used as a reference.