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02. Graphics Systems 1

Dated: 27-03-2025

Introduction to Graphics Systems

Even general purpose computers are capable of 2D and 3D graphics rendering.
For higher quality applications, special purpose graphics hardware systems are and technologies employed.

Video Display Devices

Primarily, a monitor is used for the graphical output. Most of them follow the standard CRT (Cathode ray tube) design.

Refresh Cathode Ray Tubes

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Cathode ray tube

  • The heater is the source, it causes electrons to escape.
  • The control grid controls the intensity.
  • The accelerating anode attracts the electrons.
  • The focusing anode focuses the electrons on a single spot.
  • The plates determine the position of the focused beam of electrons.
  • The phosphorus screen is the target where beam will hit.
\[\text{Persistence} = \frac{\text{Time of light decay}}{\text{1 / 10 intensity}}\]

Monitors come with range of 10 to 60 microseconds.
Resolution is defined to be maximum number of points on the screen.

\[\text{Aspect ratio} = \frac{\text{number of vertical points}}{\text{number of horizontal points}}\]

Raster Scan Systems

In raster scan systems, the picture is stored in refresh buffer or frame buffer.

Pixel

Frame buffer is a 2D matrix, storing pixels(picture elements).

Bitmap and Pixmap

If the color data for each pixel uses only \(1\) bit (\(0\) for black and \(1\) for white, for black and white image) then the frame buffer is called a bitmap. Otherwise if it uses multiple bits (ideally, \(8\) bits for \(256\) colors) then the frame buffer is called pixmap.

Horizontal Retrace

We start drawing the image from left to right, after the image reaches to the right end, we reset the beam to left. This is called horizontal retrace.

Vertical Retrace

After the beam reaches the bottom, we reset it to the top, this is called vertical retrace.
Normally, each vertical retrace takes \(1 / 60^{\text{th}}\) second to avoid flickering.

Interlacing

This is another method where we first draw the odd indexed lines and then even indexed lines. It provides effect of doubling the refresh rate.

Random Scan Displays

These are also called vector displays or stroke writing or calligraphic displays because they draw 30 to 60 lines (through line drawing commands called refresh display file).
The lines drawn in vector displays are smoother whereas in raster-scan lines often become jagged.

Color Crt Monitor

These are displays which use combination of phosphors to emit different lights.
There are 2 techniques used by these displays

Beam Penetration

Uses two layers and displayed color depends on how far the electron beam penetrates the phosphor layers.

Shadow Mask

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Shadow Masking

Each pixel position is made up of 3 phosphorus dots.

Advantages

  • Fast response (high resolution possible)
  • Full color (large modulation depth of E-beam)
  • Saturated and natural colours
  • Inexpensive, matured technology
  • Wide angle, high contrast and brightness

Disadvantages

  • Large and heavy (typ. 70x70 cm, 15 kg)
  • High power consumption (typ. 140W)
  • Harmful DC and AC electric and magnetic fields
  • Flickering at 50-80 Hz (no memory effect)
  • Geometrical errors at edges

Direct view Storage Devices

A direct view storage tube stores the picture information as a charge distribution just behind the phosphor-coated screen.
Primary gun is used to store the picture pattern whereas flood gun maintains the picture display.

Advantage

There is no refresh rate requirement and a complex high quality image can be maintained without flickering.

Disadvantage

Image cannot be modified by only editing a section and the whole picture needs to be redrawn which can take seconds.

Flat Panel Displays

They have following properties

  • Little Volume
  • Light Weight
  • Lesser Power consumption

These are used in

  • Calculators
  • Pocket Video Games
  • Laptops

Categories

Emissive

These are called emitters. These convert electrical energy to light.

Examples
  • Plasma panels
  • Thin film electro-luminescent displays
  • Light-emitting diodes

Non Emissive

These use optical effects to convert sunlight or light from some other source into graphics patterns.

Example

liquid-crystal device

Plasma Panel Displays

These are also called gas discharged displays.
There are 2 glass panels (one with vertical and other with horizontal conducting ribbons).
The intersection of a vertical and horizontal ribbon results into the gas at the spot to break down and start glowing.

Advantages

  • Large viewing angle
  • Good for large-format displays
  • Fairly bright

Disadvantages

  • Expensive
  • Large pixels (\(1\) mm versus \(0.2\) mm)
  • Phosphors gradually deplete
  • Less bright as compared to CRTs, using more power

Liquid Crystal Display

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Liquid crystal displays

Liquid crystal refers to the fact that these compounds have a crystalline arrangement of molecules, yet they flow like a liquid.
Flat panel displays use nematic liquid crystal.

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One glass panel has horizontal and other has vertical conductors.
When we need to turn off the pixel, we apply an electric field such that the polarizer blocks the light.

Advantages

  • Small footprint (approx 1/6 of CRT)
  • Light weight (typ. 1/5 of CRT)
  • Low power consumption (typ. 1/4 of CRT)
  • Completely flat screen - no geometrical errors
  • Crisp pictures - digital and uniform colours
  • No electromagnetic emission
  • Fully digital signal processing possible
  • Large screens (>20 inch) on desktops

Disadvantages

  • High price (presently 3x CRT)
  • Poor viewing angle (typ. +/- 50 degrees)
  • Low contrast and luminance (typ. l: 100)
  • Low luminance (Natural light) (typ. 200 cd/m2)

Three Dimensional Viewing Devices

A 3D graphics monitor uses a vibrating, flexible mirror to reflect a CRT image.
The mirror's changing focal length, synchronized with the CRT display, projects object points into their correct spatial distances.
This enables users to view the 3D scene from different angles by moving around it.

Virtual Reality Devices

Virtual reality (VR) immerses users in computer-simulated environments through specialized interface devices.
Head-mounted displays (HMDs) provide visual and auditory input, while position trackers and data gloves enable interaction.
The HMD tracks head movements to adjust the displayed scene, and data gloves allow object manipulation, though haptic feedback is limited.
Interactive walkthroughs further enhance immersion by dynamically updating the scene based on the user's movement.

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Head mounted display and data glove