03. Graphics Systems 2
Dated: 27-03-2025
Raster Scan Systems
In these systems, there are several processing units.
In addition to CPU, a special purpose processor, called the video controller or display controller is used to control the operation of the display device.
Architecture of a simple raster graphics system
The video controller can access the frame buffer which can be anywhere in the memory.
Video Controller
Frame-buffer locations, and the corresponding screen positions, are referenced in Cartesian coordinates.
Initially, \(x\) is set to \(0\) and is incremented by \(1\) for the horizontal movements.
And \(y\) is set to \(y_\max\) and is decremented by \(1\) for the vertical movements.
To reduce slow cycle times in video controllers, multiple pixel values are retrieved from the refresh buffer per pass.
These values are stored and used to control CRT beam intensity for adjacent pixels, speeding up pixel processing.
Raster Scan Display Processors
A major task of the display processor is digitizing a picture definition given in an application program into a set of pixel-intensity values for storage in the frame buffer.
This digitization process is called scan conversion.
Raster Scan Characters
Graphics commands for geometric objects are scan-converted into pixel intensities for display.
Lines and curves are approximated by pixel positions in the frame buffer.
Characters are rendered either as rectangular grid patterns or by scan-converting curved outlines into pixel data.
Random-scan Systems
In a random scan system, an application program and graphics package reside in system memory.
The graphics package translates commands into a display file,1 also in memory.
The display processor then refreshes the screen by cycling through the display file1's commands.
The display processor is also known as a display processing unit or graphics controller.
Architecture of a simple random scan system
Graphics Card or Display Adapters
A video display adapter, or video card, is a circuit board that plugs into a computer's motherboard, or is integrated directly onto it (as in some PCs and laptops), to handle display output.
Intel systems support following adapters:
- Monochrome Adapter (MA)
- Hercules Adapter (HA)
- Color Graphics Adapter (CGA)
- Enhanced Graphics Adapter (EGA)
- Multicolor Graphics Adapter (MCGA)
- Video Graphics Adapter (VGA)
- Super Video Graphics Adapter (SVGA)
- Extended Graphics Adapter (XGA)
Monochrome Adapter
The Monochrome Adapter (MA), the earliest video adapter, only displayed single-color text and lacked graphics capability.
While offering high resolution text, its lack of graphics makes it unsuitable for modern software.
Hercules Adapter (HA)
The Hercules card extended the Monochrome Adapter (MA) by adding graphics capabilities, making it a standard for monochrome systems.
Color Graphics Adapter (CGA)
This adapter can display text as well as graphics.
Text Mode
- 16 colors
- \(25 \times 80\) resolution
Graphics Mode
- \(320 \times 200\) with \(4\) colors.
- \(640 \times 200\) with \(2\) colors.
One drawback is that this card produces flickers (text flashing when it moves up and down) and snow (flurry of bright spots that can appear anywhere on the screen.)
Enhanced Graphics Adapter (EGA)
The Enhanced Graphics Adapter (EGA), introduced in \(1984\), emulated CGA and MA, offering higher resolution (\(640 \times 350\), \(16\) colors from \(64\)) and eliminating CGA's display issues.
However, its lack of read access to internal registers hindered memory-resident and multitasking applications.
Multicolor Graphics Adapter (MCGA)
The MCGA was designed to emulate the CGA card and to maintain compatibility with all CGA modes, with introduction to 2 more graphics modes.
- \(640 \times 480\) with \(2\) colors
- \(320 \times 200\) with \(256\) colors
Video Graphics Adapter (VGA)
VGA supports all modes of MA, CGA, MCGA and supports a graphics mode of
- \(640 \times 480\) with \(16\) colors
Super Video Graphics Adapter (SVGA)
Super VGA (SVGA) refers to a group of video cards that enhance the VGA standard with varying capabilities, such as higher resolutions and color depths.
Unlike previous standards, SVGA lacks a single specification, requiring unique device drivers for each card.
Extended Graphics Adapter (XGA)
The Extended Graphics Array (XGA) improved upon VGA with higher resolution, more colors, and better performance.
Its bus mastering capability allows it to control the system like the motherboard.
XGA supports two new modes:
- \(640 \times 480\) with \(16\)-bit color (\(65536\) colors)
- \(1024 \times 768\) with \(8\)-bit color (\(256\) colors).
Video Card Supports the Cpu
The video card provides a support function for the CPU.
It is a processor like the CPU.
Ram on Video Card
Video cards always has certain amount of RAM called the frame buffer.2
Video card RAM is necessary to keep the entire screen image in memory.
The CPU sends its data to the video card.
The video processor forms a picture of the screen image and stores it in the frame buffer.
This picture is a large bit map.
It is used to continually update the screen image.
3D - Lots of Ram
Modern 3D graphics cards utilize 16 or 32 MB of RAM and the AGP interface to provide high-quality 3D performance through increased bandwidth and memory access.
Vram
In principle, a VRAM cell is made up of two ordinary RAM cells, which are "glued" together.
Therefore, you use twice as much RAM than otherwise.
Video RAM (VRAM) is more expensive but offers faster performance due to its dual-port design, allowing simultaneous read and write operations at the same memory address.
Increasing VRAM capacity does not further enhance speed, as its inherent dual-port architecture already maximizes data throughput.
Uma and Dvmt
Older motherboards integrated video controllers using Shared Memory Buffer Architecture (SMBA) or Unified Memory Architecture (UMA), allocating system RAM as a frame buffer.
These methods were slow and unpopular.
Intel's 810 and 815 chipsets introduced Dynamic Video Memory Technology (DVMT), a newer approach that also integrates the graphics controller and uses system RAM.
The Ramdac
Traditional graphics cards use a RAMDAC chip to convert digital signals from the PC into analog signals for CRT3 monitors.
- Should be an external chip, not internal to a
video card - Clock speeds: \(250\) to \(360\) MHz.
Heavy Data Transport
Original VGA cards were "flat," relying on the CPU for all image calculations and data transfer, which became a bottleneck with graphical interfaces like Windows.
High-resolution, high-color images required significant data movement (e.g., 1.5 MB for \(1024 \times 768\), \(16\)-bit color), straining early CPUs and the limited ISA bus bandwidth, especially with high refresh rates.
Accelerator Cards
Early 1990s introduced accelerated video cards, now standard, offloading image processing from the CPU.
These cards, connected via high-speed buses like PCI and AGP, receive instructions to draw elements, minimizing CPU workload.
Modern 3D gaming demands high bandwidth, requiring high-end RAM such as
Graphics Libraries
Graphics developers some time use 2D or 3D libraries to create graphics rapidly and efficiently.
These developers include game developers, animators, designers etc.
The following libraries are commonly used among developers:
- FastGL
- OpenGL
- DirectX
- Others
Advantages of Graphics Libraries
These libraries help developers to create fast and optimized animations and also help to access features that are available in video hardware.
Hardware manufacturers give support in hardware for libraries.
Famous manufacturers include
- SIS
- ATI
- NVIDIA
- INTEL
- AMD
Graphics Software
A wide range of 2D and 3D software tools, including Flash, Maya, 3D Studio Max, Adobe Photoshop, CorelDraw, and others, are available for creating animations, models, and images, catering to movie makers, animators, and designers.
References
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Read more about display files. ↩↩
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Read more about frame buffers. ↩