Computer Graphics: Overview of Graphics Systems

Transcription

1 Computer Graphics: Overview of Graphics Systems By: A. H. Abdul Hafez 1

2 Outlines 1. Video Display Devices 2. Flat-panel displays 3. Video controller and Raster-Scan System 4. Coordinate Representation 5. Graphics functions 6. End 2

3 Video Display Devices Cathode-Ray Tubes Electrostatic deflection of the electron beam in a CRT. 3

4 Video Display Devices Raster scan display: from TV technology The electron beam is swept across the screen one row a time. Each row is referred to as scan line As the beam moves across the scan line, the beam intensity s value varies from on to off via an intermediate values. Picture definition is stored in memory called frame buffer. Each screen spot that can be illuminated by the electron beam is called pixel. Resolution is the number of pixel positions that can be plotted. Aspect ratio is the number of pixel columns divided by the number of scan lines. The number of bits per pixel in a frame buffer is referred to as depth or bit planes. A raster-scan system displays an object as a set of discrete points across each scan line. 4

5 Video Display Devices Random scan display: called also vector display The electron beam is directed only to those parts of the screen where a picture is to be displayed. The picture is generated as line drawing. The beam trace out the component lines one after another. For this it is referred to as vector displays. Picture definition is stored as a set of line drawing commands in a memory called display list or vector file. The concepts of resolution and aspect ratio are the same as raster scan. A random-scan system draws the component lines of an object in any specified order. 5

6 Video Display Devices Color CRT Monitors: using phosphors that emit different colored light. Shadow mask uses 3 color phosphor dots at each pixel position. They are RGB colors. This monitors type has 3 electron guns, one for each color dot, and shadow mask grid. Operation of a delta-delta, shadowmask CRT. Three electron guns, aligned with the triangular color-dot patterns on the screen, are directed to each dot triangle by a shadow mask. 6

7 Flat-panel displays Plasma panel: Filling the region between two glass plates using a mixture of gaz, mainly neon. A series of vertical conducting ribbons is placed on one glass panel, and a set of horizontal ribbons is placed on the other glass panel. Applying voltages on the intersecting conductors, cause the gaz to break down into a glowing plasma of electrons and ions. Thin-film display: They are similar to plasma. The difference is that the region is filled with a phosphor, such as zinc sulfide doped by manganese. Applying voltage on the conductors, phosphor becomes conductor. Energy is absorbed by the manganese atoms and then released as a spot of light. 7

8 Video controller in Raster-Scan System A fixed area of the system memory is reserved for the frame buffer, and the video controller is given direct access to the frame buffer. Frame buffer locations and screen positions are referenced in 2D Cartesian coordinates. The origin is at the lower-left screen corner. The screen surface is represented as the first quadrant with positive values for x and y coordinates. Pixel positions are assigned integers x values from 0 to xmax, left to right and y values from 0 to ymax top to left. 8

9 Video controller in Raster-Scan System Two registers are used to store the coordinates. Initially x register is set to 0 and y register is set to the value of the top scan line. The content of the frame buffer at this position is retrieved. After that the x register is incremented by 1 moving to the next pixel. Repeating the same steps till reaching xmax, then x value is reset to 0, and y value is set to the next scan line. The above are repeated for all scan lines till the bottom scan line. After that the controller resets the registers to the first pixel position of the top scan lines. 9

10 Raster-Scan Display Processor The purpose of the display processor is to free the CPU from the graphics chores. In addition to the system memory, a separate display-processor memory area can be provided. A major task of the display processor is digitizing a picture definition given in an application program into a set of pixel values for storage in the frame buffer. This digitization process is called scan conversion. Display processors are also designed to perform a number of additional operations. These functions include: Generating various line styles (dashed, dotted, or solid), Displaying color areas, and Applying transformations to the objects in a scene. Also, display processors are typically designed to interface with interactive input devices, such as a mouse. 10

11 The end of the Lecture Thanks for your time Questions are welcome 11