Raster scan systems with video controller and display processor

Computer Graphics -
Raster scan systems with video controller and display processor

1. RASTER SCAN DISPLAY SYSTEM WITH VIDEO CONTOLLER 1

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3. In raster scan displays a special area of memory is dedicated to Graphics only. This memory area is called Frame Buffer. It holds the set of intensity values for all the screen points. The video controller retrieves the stored intensity values from frame buffer and displays them on the screen one row ( scan line) at a time , typically 50 times per second. 3

4. Raster display system Architecture 4

5. Raster Scan Systems In addition to the central processing unit (CPU), a special processor, called the video controller or display controller , is used to control the operation of the display device .

6. Video Controller 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 memory .

7. Video Controller Frame buffer location, and the corresponding screen positions, are referenced in Cartesian coordinates. x y

8. Video Controller Scan lines are then labeled from y max at the top of the screen to 0 at the bottom. Along each scan line, screen pixel positions are labeled from 0 to x max . Line Scan y max x max x y Line Scan y max x max

9. Simple Organization of the Video Controller Two registers are used to store the coordinates of the screen pixels. Raster Scan Generator x Register y Register Memory Address Frame Buffer Intensity The Basic refresh operation of the video controller.

10. In color displays, 24 bits per pixel are commonly used, where 8 bits represent 256 levels for each color. It is necessary to read 24-bits for each pixel from frame buffer. This is very time consuming. To avoid this video controller uses Look Up Table (LUT) to store many entries of pixel values in RGB format. With this facility , now it is necessary to only read index to the Look Up Table from the frame buffer for each pixel. The specified entry in the Look Up Table is then used to control the intensity or color of the CRT. 10

11. 11 Color Map Look-Up Tables Extends the number of colors that can be displayed by a given number of bit-planes. 1 1 1 67 100110100001 67 255 1001 1010 0001 R G B RED GREEN BLUE Pixel displayed at x', y' Pixel in bit map at x', y' x y x max max y Bit map Look-up table Display Video look-up table organization: each table entry is a 12 bit per entry. A pixel with value 67 is displayed on the screen with the red electron gun at 9/15 (binary 1001) of maximum, green at 10/15, and the blue is 1/15.

12. Raster Scan Display System with Display Controller

13. Raster Scan Display Controller/Processor A raster system containing a separate display processor (graphics controller, display coprocessor) The purpose of the DP is to free the CPU from the graphics chores.

14. Display processor Generation various line styles (dashed, dotted, or solid) Displaying color areas Performing certain transformation and manipulation on display objects.

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16. Random Scan Systems

17. Random Scan System Graphic commands are translated by the graphics package into a display file stored in the system memory. This file is then accessed by the d isplay p rocessor u nit ( DPU )(graphic controller) to refresh the screen .