JPEG 2000 Image Compression
A lot of confusion exists as to what JPEG 2000 is and how it compares with other compression standards such as MPEG (Moving-Picture Experts Group) -2, MPEG-4, and the earlier JPEG. With brief comparisons to other compression standards, this article is primarily intended to highlight some of the often misunderstood and rarely mentioned potential-become-actual benefits of JPEG 2000.
Figure 1. JPEG 2000 applications.
An important feature of systems based on JPEG 2000 is the ability to extract a variety of resolutions, components, areas of interest, and compression ratios from a single JPEG 2000 code stream. This is not possible with any other compression standard because the image size, bit rate, and quality must be specified on the encode side and can not be determined or changed on the decode side.
For example, a closed-circuit TV (CCTV) security system can make use of this feature by sending a single JPEG 2000 code stream over a low bandwidth network. High-resolution images can be stored on a hard-disk drive (HDD), while several lower-resolution images are displayed on monitors. The operator on the receive side can decide what information to extract from the single code stream sent.
JPEG 2000 is frame accurate, in that every single frame of the input is contained in the compressed format. MPEG systems, on the other hand, reduce the amount of data through temporal compression (which does not encode each frame as a complete image), so MPEG compression is not frame-accurate. For this reason, legal issues restrict the use of MPEG compression in some security applications. To get around this problem, security system and equipment providers have had to develop their own compression schemesor use the highly inefficient motion JPEG
Internet Image Distribution
With the low-quality image instantly available, the user at the receiving end can decide whether to view the picture in its fully decoded version, or to pass it by and scan the next picture instead. Clients can view images at different resolutions or quality levels [compression rates] making them suitable for any transmission bandwidth, connection speed, or display device. In addition, JPEG 2000 coding provides the option to zoom in or out on a particular area of the imageor to display a particular region of the image at a different resolution or compression rate.
Many applications require exact bit-rate control, which only JPEG 2000 can provide. Exact bit-rate control is possible because an entire frame or field is transformed at once; it is then broken down into bit streams or code blocks that can be processed independently with the techniques described below. In systems using DCT, quantization is the only technique used, and this makes exact bit-rate control difficult. In order to control bit rate in DCT systems, the information must be repeatedly re-processed and re-quantized. The rate-control algorithm used in JPEG 2000 truncates each bit stream to meet a specific target bit rate, adjusting the truncation and re-quantization of each code blocks data as required. In addition to programming the target bit rate, the standard allows the user to specify a particular quality metric. In this case, the target bit rate will vary to meet the specified quality factor, as long as the performance does not fall below a specific peak signal-to-noise ratio. The PSNR is an indication of picture quality comparable to perceived picture quality.
JPEG 2000 Code Stream
Equally sized code blocks, which are essentially bit streams of data, are generated within each subband. This break-down is necessary for coefficient modeling and coding, and is done on a code-block-by-code-block basis. In essence, the actual compression is achieved by truncating and/or re-quantizing the bit streams contained in each code block. These bit streams are then optimally truncated using a technique knows as post-compression-rate-control (PCRC).
Code blocks can be accessed independently. Their bit streams are coded with three coding passes per bit plane. This process, called context modeling, is used to assign information about the importance of each individual coefficient bit. The code blocks can then be grouped according to their significance. On the decoding side it is then possible to extract information according to its significance, allowing the most significant information to be seen first.
JPEG 2000 can contain a user-defined number of layers, which are defined by PCRC and context modeling. Each layer stands for a particular compression rate, where the compression rate is achieved from the quantization-, rate-distortion-, and context modeling processes. Layer 0, for example, contains bit streams-from the lossy WT transform-that are heavily truncated, contain no coding passes, and thus provide the highest compression rate and the lowest quality. Layer 16 can then contain bit streams that are less truncated and use a higher number of coding passes, thus providing low compression and high quality.
Figure 2. ENCODEimage over wavelet transform into subbands and resolutions.
Tiles or images are further partitioned into precincts. Precincts contain a number of code blocks, and are used to facilitate access to a specific area within an image in order to process this area in a different way, or to decode only a specific area of an image. The JPEG 2000 bit stream is generated by arranging code blocks or precincts into an array of packets with the lower subbands coming first.
The JPEG 2000 stream starts with a main header containing information such as: uncompressed image size, tile size, number of components, bit depth of components, coding style, transform levels, progression order, number of layers, code block size, wavelet filter type, quantization level, etc. The entire image data, grouped in code blocks of LL, HL, LH, and HH subbands, follows the header. Data is not contained in the header information. Also, a table of contents can be stored on the encode side, and allows a decoder to call up a certain resolution on demand, without first having to decode or download the entire JPEG 2000 code stream.
Figure 3. DECODEone JPEG 2000 stream is received by several decoders.
DCT versus WT
To obtain the same amount of information as with one WT pass, the DCT must be used for every frequency; and each of these frequencies must be transformed at each time instant for each 8 × 8 pixel block. In addition, MPEG systems use inter-frame compression [motion estimation] in order to reduce the amount of data further for motion estimation. This requires storage of at least two entire fields in external memory. The computation-intensive motion estimation process requires a very powerful processor. Temporal compression can be used in JPEG 2000 systems, but it is not inherent in the JPEG 2000 standard.
JPEG 2000s Advantages Over Other Compression Standards
JPEG 2000 continues to gain popularity, even though MPEG-2 is the established standard for DVD and broadcast applications. JPEG 2000 is also very popular in HD applications that require high-quality storage or transmission of HD images over wireless or other links.
Figure 10. ADV202 block diagram.
Even though digital signal-processor (DSP) performance has improved significantly, a DSP would have to perform 20 billion instructions per second to match the performance of the ADV202 in a standard-definition encode application. Effectively serving as accelerators, the ADV202s three dedicated on-chip entropy codecs are responsible for the high throughput rate.
CONCLUSIONTHE OUTLOOK FOR JPEG 2000
Several major manufacturers of video or broadcast equipment have implemented JPEG 2000 into such future HD products as real-time encoding and decoding systems and video servers.
The Digital Cinema Initiative (DCI) has recently announced that it will use JPEG 2000 as the compression method in the delivery of digital motion pictures. The ADV202 has already found its way into many designs in the CCTV/security market in video-over-network applications.
Because of its flexibility and image-compression quality, the ADV202operating under JPEG 2000could find its way into virtually every design that uses image or video compression.
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