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COMPUTER GRAPHICS

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COMPUTER GRAPHICS

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  1. COMPUTER GRAPHICS • Computer graphics are pictures and films created using computers. • The term refers to computer-generated image data created with the help of specialized graphical hardware and software. • It involves computations, creation, and manipulation of data. • We can say that computer graphics is a rendering tool for the generation and manipulation of images.

  2. IMAGE PROCESSING Vs COMPUTER GRAPHICS • computer graphics referred to the process of creating images from an abstract image. • A computer game, for example, might keep track of image as a large list of points, where each point has its (x, y, z) coordinates. Then, given the coordinates of the camera, and the direction its facing, the computer will calculate the color at each row and column in the final image that you see on your screen.

  3. IMAGE PROCESSING Vs COMPUTER GRAPHICS continued • Image processing refers to the process of starting with an existing image and refining it in some way to obtain another image. • For example, starting with a picture, then an image processing algorithm can be used to make the colors more vibrant, or remove the blur, or increase the resolution. • The output of an image processing algorithm is another image.

  4. COMPUTER VISION • computer vision, that refers to the process of computing an abstract model given an input image. • For example, from a picture, a vision algorithm would infer the list of (x, y, z) coordinates of the points that make up the image from the colors at each row and column in the input image. • Computer Graphics: Convert Model to Image • Image Processing: Refine Image to Image • Computer Vision: Convert Image to Model

  5. TYPES OF COMPUTER GRAPHICS • There are two kinds of computer graphics  raster (composed of pixels) and vector (composed of paths). • Raster images are more commonly called bitmap images. • A bitmap image uses a grid (matrix) of individual pixels where each pixel can be a different color or shade. • Bitmaps are composed of pixels. • Vector graphics uses mathematical relationships between points and the paths connecting them to describe an image.  • Vector graphics are composed of paths.

  6. Examples of bitmap and vector images

  7. APPLICATIONS • Computational biology • Computational photography • Computational physics • Computer-aided design • Computer simulation • Design • Digital art • Education • Graphic design • Information visualization • Rational drug design • Scientific visualization • Special Effects for cinema • Video Games • Virtual reality • Web design

  8. GRAPHIC FILE FORMATS • There are many graphic image file formats. The most frequently used ones are listed below: • For Web Pages: • GIF - Graphics Interchange Format - Images using a fixed color palette (limited to only 256 colors - not the  full spectrum of colors available to your monitor). The GIF format uses compression (not “lossy”) for smaller files and faster downloads. This format is best for images with solid colors or areas of uniform color such as illustrations and logos.JPEG - Joint Photographic Experts Group - Used for photographic (continuous tone) images. Unlike GIF files the JPEG format can take advantage of the full spectrum of colors available to your monitor. The JPEG format also uses compression for smaller files and faster downloads. JPEG compression is "lossy" which means it discards data in the process. Once a file is saved in JPEG format the data is permanently lost. If you want all the image data available for future use, save the image using no compression or "lossless" compression and make JPEG copies from it.

  9. GRAPHIC FILE FORMATS • For Printed Documents:TIFF - Tagged-Image File Format - Used for bitmaps only. The TIFF format is supported by all graphics applications.EPS - Encapsulated PostScript- A file format used for both vector graphics and bitmaps. EPS files contain a PostScript description of the graphic data within them.

  10. IMAGE REPRESENTATION

  11. PIXELS • Digital image or image composed of discrete pixels. • Pixels are smallest unit of picture element. • Pixels are arranged as matrix to form a rectangular picture area. • Number of pixel per unit length is called its resolution.

  12. PIXELS CONTINUED • The ratio of image’s width to its height is called as its aspect ratio. • Composing an image in computer is setting the pixel value. (color, intensity) • Total number of pixel is a function of size of image and number of pixel per unit length, both in horizontal and vertical direction. • Image size is given as total number of pixel in horizontal direction times total no. of pixel in vertical direction. e.g. 512 × 512, 640 × 480, 1024 × 768 etc.

  13. PIXELS CONTINUED • e.g. 512 × 512 image’s aspect ratio is 1/1. • 1024 × 768 image’s aspect ratio is 4/3. • Individual pixel is referred by their coordinates. Typically pixel at lower left corner of an image is considered to be at origin (0, 0). • e.g. for 640 × 480 image, the lower right corner would have coordinates (639, 0) and pixel of upper right corner would have coordinates (639, 479).

  14. PROBLEMS • Compute the size of 640 × 480 image at 240 pixel per inch. • Compute the resolution of 2 × 2 inch image that has 512 × 512 pixels. • If an image has height of 2 inches and aspect ratio of 1.5, calculate its width. • Find the size of image 640 × 480 in inches, for a) 96 pixel per inch, 6.667 inch by 5 inch b) 400 pixel per inch , 1.6 inch by 1.2 inch • 5. Calculate total number of pixels for a 3 × 2 inch image at resolution of 300 pixels per inch.

  15. RGB COLOR MODEL

  16. RGB COLOR MODEL • The fig. shows color coordinate system with three primary colors: R (Red), G (Green), B (Blue). Each primary color can take any intensity value ranging from 0 (lowest) to 1 (highest). • The collection of all possible colors obtainable by RGB forms cube shaped RGB color space. • Given this model any arbitrary color within this space can be specified by its color coordinates (r, g, b). • RGB model is an additive process. We start from black (0, 0, 0) and add on primary components to get desired color. • The complementary color model is CMY.

  17. CMY COLOR MODEL

  18. PROBLEMS • Resize a 1024 x 768 image to one that is 640 pixels wide with the same aspect ratio, find its height. • Find CMY coordinates of a color at (0.2, 1, 0.5) in RGB space. • Find RGB coordinates of a color at (0.15, 0.75, 0) in the CMY space.

  19. DIRECT CODING

  20. DIRECT CODING • Using direct coding we allocate some amount of storage space for each pixel to code its color • A widely accepted industry standard uses 3 bytes (24 bits) per pixel, called TRUE COLOR REPRESENTATION. • With one byte for each primary color.

  21. DIRECT CODING • Hence each primary color can have 256 different intensity levels. Total 16.7 million • Difference between two colors that differ by one intensity level is undetectable under normal viewing condition. • Special case of direct coding is the representation of black- and- white and gray- scale images, where the three primary colors have the same value.

  22. LOOK UP TABLE

  23. LOOK UP TABLE • The color of a pixel whose value is i, is determined by the color value in the table entry whose address is i.

  24. PROBLEMS

  25. DISPLAY MONITOR

  26. CRT • These devices convert digital images to visually perceivable pictures. • The CRT is a vacuum glass tube with display screen, coated with phosphor, at one end which is positively charged. • The color of light and the time period vary from one type of phosphor to another.

  27. Components of Electron Gun • HEATED METAL CATHODE • Accelerates electron toward the screen through focusing anode and accelerating anode. • CONTROL GRID • This metal cylinder controls beam intensity (brightness). • A high negative voltage at it shut off the beam by repelling electrons. • Smaller negative voltage just decreases number of electron passing through it.

  28. Deflection system of CRT • Consists of two or more metal plates at positive potential. • They are horizontal and vertical deflection plates. • Collision of electron beam with phosphor releases energy. • One part of it is converted to heat energy • Other part moves the phosphor atom to higher quantum energy level. • After short time they return back to original and releasing light energy.

  29. CRT • The light given off by the phosphor during exposure to electron beam is known as fluorescence. • The continuing glow given off after the beam is removed is known as phosphorescence. • The duration of phosphorescence is known as phosphor’s persistence. • The persistence of the phosphor need to be long enough for a frame to remain visible, but short enough for it to fade before the next frame is displayed.

  30. REFRESH BUFFER AND REFRESHING RATE • The image to be displayed is stored in an area of system memory called frame buffer or refresh buffer. • The frequency at which the content of frame buffer is sent to the display monitor is called refreshing rate. • Typical refreshing rate is 60 frames per second (60 Hz) or higher. • Lower persistence phosphor requires higher refresh rate to maintain a picture with out FLICKER.

  31. INTERLACING • Interlacing is used to double the refreshing rate. • Here half of the scan line is refreshed at a time, first the odd numbered lines, then the even numbered lines. • Thus the screen is refreshed from top to bottom in half the time it would have taken to sweep across all the scan lines. • It is effective in reducing flicker. • But this method does not increase the rate at which the entire screen is refreshed.

  32. Consider three different Raster systems with Resolutions of 640 x 480, 1280 x 1024, 2560 x 2048. Find the size of frame buffer (in bytes) is needed for each of these systems to store 12 bits/ pixel.

  33. COLOR DISPLAY

  34. Color CRT

  35. Color CRT • Here there are three guns instead of one inside CRT, with one electron gun for each primary color. • The phosphor coating inside the screen consists of dot pattern for three different types of phosphor. • The distance between centers of the dot patterns is called PITCH of color CRT. • The pitch places upper limit on the number of addressable positions on the display area. • A thin metal screen called shadow mask is placed between phosphor screen and electron guns. Tiny holes on the shadow mask constrain each electron beam to hit its corresponding phosphor dots.

  36. Raster Scan In a raster scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots.

  37. Random Scan or Vector Scan In this technique, the electron beam is directed only to the part of the screen where the picture is to be drawn rather than scanning from left to right and top to bottom as in raster scan. It is also called vector display, stroke-writing display, or calligraphic display.

  38. Problems • How long would it take to load a 640 x 480 image with 12 bits per pixel, if 10oooo bits can be transferred per second?