Joint Picture Experts Group(JPEG) The Still Picture Compression Standard

The Need for Data Compression Ali Mustafa Zaidi: The key obstacle for many applications is the vast amount of data required to represent a digital image directly. To reduce storage costs and data transmission times of images. Large archival systems such as medical databases store huge amounts of high resolution images. Storage would be prohibitively expensive without effective compression. Ali Mustafa Zaidi: explain that for medical images resolution is critical to reveal detail.

An Example A 1024X1024 pixel image with 24 bits/pixel requires 3.15 Mbytes.

Video at this resolution and at 30 fps, would require a data rate of 94 Mbytes/s. State-of-the-art compression techniques can compress the same image to 1/50 (62 Kbytes) of its uncompressed size. For a 30 fps video, this translates to data rate of only 1.88 Mbytes/s

Data Compression Basics Ali Mustafa Zaidi: Every kind of data has some form of repetition like repeating characters in text files, patches of similar pixels in an image. This property is utilized in compression techniques such as Huffman compression. Data compression exploits repetitive patterns in data and transforms data into a smaller representation. Compression techniques are classified as Lossless Lossy

Lossless Compression Lossless compression allows exact original data to be recovered from the compressed form. Compression ratios are 1/2 to 1/3. Huffman coding and Run-Length Encoding are examples of lossless techniques. Ali Mustafa Zaidi: Give example of lossless compression. E.g Text files, executables, no loss can be tolerated. Zip files are lossless.

Lossy Compression Lossy techniques discard some original data to allow greater compression. Lossy compression is applicable to digitized voice and images where calculated loss is not perceived by our senses. Compresion ratios of 1/100 are possible JPEG and MPEG are lossy techniques. Ali Mustafa Zaidi: Give example of MP3 files and WAV files, lossy used when perceiving analog(in converting analog to digital, there is alteady an inherent loss) data such as images, video, because for minor/negligible losses, compression ratio is boosted from 1/3 to 1/100

Need for compression/decompression in hardware Ali Mustafa Zaidi: Explain briefly about multimedia TV, HDTV. These technologies provide high quality audio/video and allow user interaction as well. Need for compression/decompression in hardware Applications such as video-teleconferencing, interactive/multimedia TV, HDTV etc. require high bandwidth communication. Very high speed VLSI implementations of image/video/audio compression/ decompression make possible real-time media applications. Ali Mustafa Zaidi: Explain what is bandwidth limitation and how 1GB/s is not feasible. Even fast networks need compression to efficiently utilize available bandwidth Ali Mustafa Zaidi: Explain about real-time applications. Real time applications give performance comparable to traditional analog media.

Hardware compression/decompression examples Modems use MNP-5 and V.42bis data compression modes to effectively utilize available bandwidth. MPEG-1 produces full motion video at a data rate of only 1.5 Mbits/s. Cellular phones use voice compression techniques Dolby Digital AC-3 sound employs 5 channel audio with only 384 kbits/s rate.

JPEG Still Image Compression Standard Ali Mustafa Zaidi: Explain how it makes feasible...give an example : images on web pages would take 10 to 100 times longer to download if uncompressed. A .bmp file of size 1MB is 40kB large only when stored as a .jpg JPEG Still Image Compression Standard Ali Mustafa Zaidi: JPEG compression should be unaffected by image characteristics, e.g. it should not produce the opposite results. Ali Mustafa Zaidi: JPEG standard can produce remarkable quality results even with a high compression ratio Co-developed by ISO, IEC and CCITT Aimed at making the use of color images feasible by being state-of-the-art w.r.t. compression ratio & image quality applicable to all kinds of images computationally tractable implementable in both hardware & software. JPEG standard allows interoperability b/w different platforms using image applications. Ali Mustafa Zaidi: explain the importance of interoperability: To be widespread, an image standard should be flexible enough to a support a large variety of applications on different platforms. E.g. The compression standard used in fax machines allows interoperability of equipment from different manufactures.

JPEG Overview JPEG compression is a combination of lossy and lossless techniques designed for high compression ratios on image data. JPEG relies on a mathematical transform similar to the Fourier transform to convert pixel values to “Frequency domain” representation.

JPEG Overview Ali Mustafa Zaidi: Refer RLE, Huffman and quantization to the upcoming attraction. Frequency domain representation tells which data may be discarded. The lossy step which discards original data is called Quantization. After quantization the data is further compressed using Zero Run-Length Encoding and Huffman compression. Ali Mustafa Zaidi: give example of a frequency spectrum resulting from Fourier series which contains higher frequencies of smaller magnitudes which can be neglected.

2D-DCT Properties DCT is computationally intensive Calculating DCT for one pixel requires 128 multiplication and 112 additions for a block size of 8X8 pixels. Use of DCT in MPEG I & II has resulted in extensive research on fast implementations.

Optimizing DCT Calculation Flow of computations in DCT lends itself to pipelining and parallel implementations. Very fast architectures for computing the DCT have been proposed which for example require only 5 multiplications, 29 additions and 16 two’s complement operations.

JPEG Codec in hardware Very high speed operation allowing 1024X1024 images to be compressed at 30 fps. This is called “Motion JPEG” which encodes video as a sequence of individual frames.

Single Chip JPEG Applications Applications include Color printers Digital Still Cameras Scanners Video Conferencing Image archiving Desktop video editing Digital surveillance systems

JPEG 2000 Standard Highlights Support for both lossy and lossless compression. Resistance to transmission errors Access to region of interest without decoding the complete image. Greater compression than current standard. Compatible with color facsimile.