SWE 423: Multimedia Systems Chapter 7: Data Compression (6)

Outline JPEG Introduction JPEG Requirements JPEG Modes and Procedure

Introduction JPEG (Joint Photographic Experts Group) is the result of a joint project between ISO and CCITT ISO (International Organization for Standardization) Founded in 1947 An international standard-setting body composed of representatives from national standards bodies. CCITT (Comité Consultatif International Téléphonique et Télégraphique) i.e. International Telegraph and Telephone Consultative Committee Since 1992 onwards known as ITU-T (International Telecommunication Union - ITU Telecommunication Standardization Sector) Under UN Developed many standards Group 3 and Group 4 protocols for sending faxes. V.34 and V.90 standards for sending and receiving data from full duplex fax modems JPEG became an ISO standard in 1992.

Introduction JPEG applies to color and gray-scaled still images. Motion JPEG handles video sequences through a fast coding and decoding of still images. Currently, implementations of parts of JPEG are available as s/w only packages or using special hardware support. Most products support the absolutely necessary algorithms. The commercially available JPEG includes the base mode with certain processing restrictions

JPEG Requirements These were put to ensure widespread distribution and application of JPEG. Independence from image size Applicability to any image aspect ratio and any pixel aspect ratio Independence of the color space and the number of colors used Unlimited complexity of image content Currency regarding the compression factor and image quality Platform independence of software solutions and major complexity reductions for h/w solutions Support for sequential and progressive decoding Support for lossless hierarchical coding with different resolutions

JPEG Modes

JPEG Modes JPEG defines four modes Lossy sequential DCT-based mode Must be supported by every JPEG decoder Expanded lossy DCT-based mode Provides a set of enhancements for the base mode Lossless mode Low compression ratio and perfect reconstruction of images Hierarchical mode Accommodates images of different resolutions by using algorithms defined for the other three modes

JPEG: Image Preparation JPEG specifies a general image model that can describe most commonly used still image representations The mapping between coded color values and the colors they represent is not coded Which requirements the above two properties satisfy? An image consists of at least one and at most N = 255 components or planes

JPEG: Image Preparation An image consists of at least one and at most N = 255 components or planes Planes: RGB, YIQ, YUV Gray-scale images will consist of ...... RGB color images will consist of ... YUV color images will consist of ...

JPEG: Image Preparation

JPEG: Image Preparation Each pixel is represented by p bits Values in the range of .... Lossy modes of JPEG use p = 8 or 12 bits/pixel Lossless modes can use 2 to 12 bits/pixel. Applications must conform to the standards above (if needed, it must transform the image to conform to the above)

JPEG: Image Preparation Compressed data includes values of X (maximum of all Xi’s) and Y (maximum of all Yi’s) as well as factors Hi and Vi for each plane representing the relative horizontal and vertical resolutions with respect to the minimal horizontal and vertical resolutions. Hi and Vi are integers ranging between 1 and 4 Example: 512  512 image consisting of 3 planes with the following factors: Plane 0: H0 = 4 and V0 = 1 Plane 1: H1 = 2 and V1 = 2 Plane 2: H2 = 1 and V2 = 1 leads to .... The image is divided into data units. Lossless mode: 1 pixel = 1 data unit Lossy mode: 8  8 pixels = 1 data unit (block) Consequence of DCT which always transforms connected blocks.

JPEG: Image Preparation Within each component, the data units are processed from left to right, as shown below (non-interleaved data ordering).

JPEG: Image Preparation Interleaved processing order of data units of different components

JPEG: Lossy Sequential DCT Mode

JPEG: Lossy Sequential DCT Mode After image preparation, the uncompressed image samples are grouped into data units of 8  8 pixels. The order is defined by the MCUs Each sample is encoded using p=8bit. Each pixel is an integer between 0 and 255 Image processing is carried out as follows DCT-based transformation coding is carried out Pixel values are shifted into (-128, 127) interval A forward DCT (FDCT) is applied to each transformed pixel value For later reconstruction, the decoder uses the IDCT Note that if the FDCT and IDCT computations were exact, it would be possible to reproduce the original 64 pixel values exactly. In practice, precision is limited, and therefore, the technique is lossy. JPEG does not specify a standard precision. Therefore, two different decoders may yield different images as output of the same compressed data.

JPEG: Lossy Sequential DCT Mode Image processing is followed by the quantization of all DCT coefficients Lossy process. Specific frequencies can be given more importance than others Tables are used for the quantization and dequantization Must use the same tables for both processes Image quality may decrease due to quantization

JPEG: Lossy Sequential DCT Mode

JPEG: Lossy Sequential DCT Mode Quantization is followed by Entropy Encoding (using Huffman Coding only) DC coefficients are encoded by subtracting the DC coefficient of the previous data unit Since changes are little in DC values of neighboring data units, the differences are stored instead of the values Huffman coding is chosen because it is free (not patented) However, coding tables must be provided by the application (one for DC and one for AC coefficients) AC-Coefficients are processed using the zig-zag sequence

JPEG: Lossy Sequential DCT Mode

JPEG: Lossy Sequential DCT Mode

JPEG: Expanded Lossy DCT Mode Image preparation here differs from that of lossy sequential using p = 12 instead of p = 8 bits per pixel. The image processing step is analogue to that of lossy sequential. JPEG also provides progressive coding, in addition to sequential coding, where the first decoding run produces a rough unsharp image that is refined during successive runs. Arithmetic entropy coding can be used in addition to Huffman coding in expanded lossy DCT-based mode.

JPEG: Expanded Lossy DCT Mode

JPEG: Lossless Mode Use predictive technique (as explained earlier) One of eight predictors is selected.

JPEG: Hierarchical Mode The main feature here is the encoding of an image at different resolutions. i.e. the compressed image contains images at several resolutions The process is done as follows The prepared image is reduced by a factor of 2n and compressed The original image is then reduced by a factor of 2n – 1 vertically and horizontally. The previously compressed image is subtracted from this one and the result is once again compressed This process repeats until the image with full resolution is compressed Can use both lossy DCT-based techniques or lossless compression techniques Computationally intensive and requires considerable storage space. The advantage of this mode is that applications working with lower resolution do not need to first decode the whole image and then reduce the resolution.