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Image Sampling and Quantization

Image Sampling and Quantization. Actual image is continuous. Digital image has a finite number of pixels and levels. RR R. Contd………. Digital Image Representation. Contd……………. An Image Capturing Format is specified by:

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Image Sampling and Quantization

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  1. Image Sampling and Quantization Actual image is continuous. Digital image has a finite number of pixels and levels. RR R

  2. Contd………

  3. Digital Image Representation

  4. Contd……………. • An Image Capturing Format is specified by: • spatial resolution (pixel x pixel) and color encoding (bits per pixel) • Example: captured image of a DVD video with 4:3 picture size: • spatial resolution: 768 x 576 pixel • color encoding: 1-bit (binary image), 8-bit (color or grayscale), • 24-bit (color-RGB)

  5. Contd……………. • An Image Storing Format is a 2-dimensional array of values representing the image in a bitmap or pixmap, respectively. Also called raster graphics. The data of the fields of a bitmap is a binary digit, data in a pixmap may be a collection of: • 3 numbers representing the intensities of red, green, and blue components of the color • 3 numbers representing indices to tables of red, green and blue intensities • Single numbers as index to a table of color triples • Single numbers as index to any other data structures that represents a color / color system • Further properties can be assigned with the whole image: width, height, depth, version, etc.

  6. Image Formats Lots of different image formats are in use today, e.g. • GIF (Graphics Interchange Format) Compressed with some basic lossless compression techniques to 20 – 25% of original picture without loss. Supports 24-bit colors. • BMP (Bitmap) Devide-independent representation of an image: uses RGB color model, without compression. Color depth up to 24-bit, additional option of specifying a color table to use. • TIFF (Tagged Image File Format) Supports grey levels, RGB, and CYMK color model. Also supports lots of different compression methods. Additionally contains a descriptive part with properties a display should provide to show the image. • PostScript Images are described without reference to special properties as e.g. resolution. Nice feature for printers, but hard to include into documents where you have to know the image size...

  7. JPEG (Joint Photo graphics Expert Group) Implementation Independent of image size Applicable to any image and pixel aspect ratio Color representation JPEG applies to color and grey-scaled still images Image content Of any complexity, with any statistical characteristics Properties of JPEG State-of-the-art regarding compression factor and image quality Run on as many available standard processors as possible Compression mechanisms are available as software-only packages or together with specific hardware support - use of specialized hardware should speed up image decompression Encoded data stream has a fixed interchange format Fast coding is also used for video sequences: Motion JPEG

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