Prepared by / Nael Alian University Of Palestine

1. digital image representation Prepared by / Nael Alian University Of Palestine

2. Basic Concepts (Digital Image Representation) • An image is a spatial representation of an object, a 2D or 3D scene etc. • Abstractly, an image is a continuous function defining a rectangular region of a plane • intensity image - proportional to radiant energy received by a sensor/detector • range image - line of sight distance from sensor position. • An image can be thought of as a function with resulting values of the light intensity at each point over a planar region.

3. Basic Concepts (Digital Image Representation) To acquire good quality images we’ll examine several important concepts as we go along: • Pixels • Resolution • File sizes • File formats- TIF, JPEG, GIF, BMP… • Compression • Color Depth • Color palettes • Web color issues

4. Digital Image Representation • For computer representation, function (e.g. intensity) must be sampled at discrete intervals. • Sampling quantizes the intensity values into discrete intervals. • Points at which an image is sampled are called picture elements or pixels. • Resolution specifies the distance between points - accuracy. • A digital image is represented by a matrix of numeric values each representing a quantized intensity value. • I(r,c) - intensity value at position corresponding to row r and column c of the matrix. • Intensity value can be represented by bits for black and white images (binary valued images), 8 bits for monochrome imagery to encode color or grayscale levels, 24 bit (color-RGB).

5. 100% 800% 400% Pixels • Most digital images are a grid of pixels, a checkerboard of colored squares, where each square is a single color • Each pixel is described by a number

6. 36 ppi 18 ppi 72 ppi Resolution • The resolution of an image tells you how small and densely packed the pixels are. It tells you how many pixels you have per inch.

7. Resolution • The higher the resolution, the sharper the images look, to a point. Monitors only have about 72 screen pixels per inch so any resolution above 72 won’t look any sharper. • Most web graphics have a resolution of 72 pixels per inch. 72 ppi 150 ppi

8. Color Depth • Number of colors each pixel can be. • 1 bit black or white • 8 bit 256 colors • 16 bit 65,536 colors • 24 bit 16.7 million colors • 32 bit Millions plus extra information • The more colors per pixel, the larger the file size

9. Image Formats • Captured Image Format • format obtained from an image frame grabber • Important parameters • Spatial resolution (pixels X pixels) • Color encoding (quantization level of a pixel - 8-bit, 24-bit) • e.g. “SunVideo” Video digitizer board allows pictures of 320 by 240 pixels with 8-bit grayscale or color resolution. Parallax-X video includes resolution of 640X480 pixels and 24-bit frame buffer.

10. Image Formats • Stored Image Format - format when images are stored • Images are stored as 2D array of values where each value represents the data associated with a pixel in the image. • Bitmap - this value is a binary digit • For a color image - this value may be a collection of • 3 values that represent intensities of RGB component at that pixel, 3 numbers that are indices to table of RGB intensities, index to some color data structure etc. • Image file formats include - GIF (Graphical Interchange Format) , X11 bitmap, Postscript, JPEG, TIFF

11. Image Formats • Graphics Format - • specifies graphics images through graphics primitives and attributes. • Graphics primitives - line, rectangle, circles, ellipses, specifications of 2D and 3D objects • Graphics attributes - line style, line width, color • Graphics formats represent a higher level of image representation, i.e., they are not represented by a pixel matrix initially. • Advantage - less storage space per graphical image • Disadvantage - more overhead during display time; must convert from a graphical image to the image format which may be a bitmap or pixmap. • E.g PHIGS (programmer’s hierarchical interactive graphics system), GKS (graphical kernel system).

12. For printed output • Newsletters, flyers, posters, business cards • Page Layout programs: Adobe PageMaker, InDesign, Quark XPress, Microsoft Publisher • Illustration: Adobe Illustrator, Macromedia Freehand, Corel Draw • Image editing: Adobe Photoshop • Document Delivery: Adobe Acrobat- creates PDF files

13. Common File Types • For Print • TIF: Tagged Image File Format : High color quality, large file • EPS: Encapsulated Postscript : Illustrations, logos. Lrg. File • For General Use • BMP: Bitmap: Not great for color printing, very generic • WMF: Windows Meta File, used in some clip art. • For Web, Onscreen Use • GIF: Graphic Interchange Format WEB: Only 256 colors. Good for flat color art. Allows transparency. Small files. • JPEG: Joint Photographic Experts Group WEB: Millions of colors. Good for compressing photos. Small files, but “lossy.” No transparency. • PNG: Portable Network Graphics WEB: 256 or Millions of colors, allows transparency. Not widely supported yet.

14. x - File Size = Number of pixels Color depth of pixels any compression (Millions of colors,256 colors,1 bit color) (Physical dimension,Resolution) (GIF, JPEG, LZW compression) File Size File size is determined by the number of pixels, the color depth of each pixel and any compression used in the file format.

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