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Digital Image Processing: Sampling, Quantization, and Computer Representation, Slides of Multimedia Applications

Amity University - BiharMultimedia Applications

An overview of digital image processing, focusing on sampling, quantization, and computer representation of images. It covers topics such as picture elements, image resolution, bit-mapped displays, dithering, and picture operations. The document also discusses various image formats like gif, jpeg, tiff, exif, and postscript.

Typology: Slides

2012/2013

Uploaded on 04/23/2013

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Computer Representation of Images
•A picture function f(x,y) is a real-valued
function of two variables, having values that
are nonnegative and bounded,
• 0 ≤ f(x,y) ≤ L-1 for all (x,y)
•When a picture is digitized, a sampling process
is used to extract from the picture a discrete set
of samples, and then a quantization process is
applied to these samples
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Download Digital Image Processing: Sampling, Quantization, and Computer Representation and more Slides Multimedia Applications in PDF only on Docsity!

Computer Representation of Images

  • A picture function f(x,y) is a real-valued

function of two variables, having values that

are nonnegative and bounded,

  • 0 ≤ f(x,y) ≤ L-1 for all (x,y)
  • When a picture is digitized, a sampling process

is used to extract from the picture a discrete set

of samples, and then a quantization process is

applied to these samples

Sampling

f

t A sampled function

Computer Representation of Images

  • The resultant digital picture function f, or digital picture , can be represented by a two-dimensional array of picture elements, or pixels
  • The digital picture function f can be regarded as a mapping from {0, ... , M-1}X{0, ... , N-1} to {0, ... , L-1}
  • The set {0, ... , L-1} is called the gray level set , and L is the number of distinct gray levels - For example, if we use 3 bits to represent each pixel, then L is 8 - If 8 bits are used, L is 256.

Computer Representation of Images

  • Picture Elements: Pixel
    • Color,
    • gray-value images and
    • binary images (e.g., values 0 for black, 1 for white)
  • Example
    • gray-value images contain different number of brightness levels:

Computer Representation of Images

  • Even if two images have the same number of pixels, the quality of the images may differ in quality due to differences in how the images are captured
  • More expensive digital cameras will have larger digital sensors than less expensive ones (larger sensors cost more) - So if the two cameras produce images with the same number of pixels, the pixels in the larger array will represent a larger area – so more information is packed into each pixel

Sensor Arrays of Differing Size

Image Resolution Test Pattern

1-bit Images

  • Each pixel is stored as a single bit (0 or 1), so also referred to as a binary (or bilevel) image.
  • Such an image is also called a 1-bit monochrome image since it contains no color.
  • Fig. 3.1 shows a 1-bit monochrome image (called “Lena” by multimedia scientists - this is a standard image used to illustrate many algorithms).

Grey-scale image

Raster Display of Digital Images

• The special-purpose high-speed memory for

storing the image frames is called the frame

buffer

• The frame buffer is considered a component

of the graphics card which are used to drive

bit-mapped displays

Bit-mapped Displays

  • To lessen this requirement, some computers have used 8- bits to indicate the desired color. This number is then used as an index into a hardware table called the color palette that contains 256 entries, each holding a 24-bit RGB value. This is called indexed color. It reduces the required RAM by 2/3, but allows only 256 colors.
  • This technique is also called pseudocolor
  • Sometimes each window on the screen has its own mapping. The palette is changed when a new window gains focus.

Bit-mapped Displays

  • To display full-screen full-color multimedia on a

1024x768 display requires copying 2.3 MB of data to the video RAM for every frame. For full-motion video, 25 frame/sec is needed for a total data rate of 57.6 MB/sec.

  • In liquid crystal displays (LCDs) as well as plasma

display panels (PDPs), and digital micromirror displays (DMDs as in projectors) discrete pixels are constructed on the display device

  • CRTs don’t have this characteristic

Dithering

  • When an image is printed, the basic strategy of dithering is used, which trades intensity resolution for spatial resolution to provide ability to print multi-level images on 2-level (1-bit) printers.
  • Dithering is used to calculate patterns of dots such that values from 0 to 255 correspond to patterns that are more and more filled at darker pixel values, for printing on a 1-bit printer.

Dithering

  • The main strategy is to replace a pixel value by a larger pattern, say 2×2 or 4×4, such that the number of printed dots approximates the varying-sized disks of ink used in analog, in halftone printing (e.g., for newspaper photos). - 1. Half-tone printing is an analog process that uses smaller or larger filled circles of black ink to represent shading, for newspaper printing. - 2. For example, if we use a 2× 2 dither matrix