For beginners to understand basics of Graphics

1. Graphics System Basics & Models
Book:
Chapter 1 [Ed. Angel, Interactive Computer Graphics]

2. Computer Graphics
 Computer Graphics: Use of computer in generating
images.
 Computer graphics: concerned with all aspects of
producing pictures or images using a computer.

3. Applications of Computer Graphics
 Can be roughly divided into four major areas
 Display of Information
 Design
 Simulation and Animation
 User Interfaces

4. 1. Display of Information
 Classic graphics techniques
used as a medium of
conveying information
 Human written/spoken
language
 Historical era:
 Babylonians used to display
floor plans on stones
 Greeks used to display their
architectural plans and language
 Today graphical
representation are generated
by Architects, Designers using
computers.

5. Display of Information (Contd…)
 Statisticians
 Uses CG to display plots/graphs
of a data set
 Extract information from these
plots
 Very useful in extracting info from
large datasets.
 Medical Imaging
 Graphics used in Computed
Tomography (CT)
 Medical Resonance Imaging
(MRI)
 Ultrasound
 Data Visualization
 Understanding data by placing it
in visual context

6. 2. Design
 Many fields concerned
with design. (Engineering
& Arch)
 With set of specification, a
cost effective and esthetic
design is tried to achieve
using computer graphics.
 Starting 40 years ago,
today Computer Aided
Design (CAD) pervades
many fields.

7. 3. Simulation and Animation
 Simulation is the imitation of the
operation of a real-world process or
system over time.
 Flight simulators - train pilots.
 Safety and Cost reduction
 Architectural designs are tested in many
weather conditions
 Animation - illusion of motion
 Became famous: After successful
simulations
 Artistic effects are achieved.
 Complete movies are made using CG
 Photo-realistic images
 Virtual Reality-replicates an
environment that simulates physical
presence in places in the real world or
imagined worlds and lets the user
interact in that world.

8. 3. Simulation & Animation (Virtual Reality)

9. 4. User Interfaces
 Interaction with computers
increased.
 Desktops, Tablets, Smartphones.
 Use of GUI has overcome CLI
 Microsoft Windows, Mac OS, Linux
 Android, iOS,
 Internet usage increased
 Webpages, applications all are
graphical
 Resources are accessed through
graphical browsers.
 Interaction with UI is so often, that
we have almost forgot that we’re
working with computer graphics.

10. A Graphics System
 General view of a graphics system
 Generally contains,
1. Input Devices
2. CPU
3. GPU (Graphics Processing Unit)
4. Memory
5. Frame Buffer
6. Output Devices

11. Pixels, Frame Buffer & Basic Terms
 Pixel: Short for Picture element
 The smallest addressable element of a
display device.
 Basic unit of a digital image.
 Each pixel corresponds to a location or
small area in the image.
 Raster: Array of picture elements or
pixels
 Images seen on displays are raters
produced by graphics sys.
 A raster is a grid of x and y coordinates on
a display space.

12. Pixels, Frame Buffer & Basic Terms (Contd…)
 Frame Buffer: portion of memory where pixels are
stored
 Core element of graphics system.
 Contains bitmap that is driven to video display
 Resolution: No. of pixels in frame buffer
 Resolution determines the details that can be seen in image
 Higher the resolution  Sharpen the image
 Depth / Precision: No. of bits used per pixel to
determine its properties like color
 1-bit-deep frame buffer  allows only two colors
 8-bit-deep frame buffer  allows 28 (256) colors.
 16-bit (High color)  216 Colors
 24-bit (True color)  224 Colors

13.  In Simple Sys: Frame Buffer only hold colored pixels that
are displayed
 In most systems, The frame buffer holds far more
information,
 depth information needed for creating images from 3D data.
 In these systems, the frame buffer comprises multiple
buffers,
 one or more of which are color buffers that hold the colored
pixels that are displayed.
 Terms, frame buffer and color buffer can be used
synonymously.
Pixels, Frame Buffer & Basic Terms (Contd…)

14. CPU and GPU
 In simple system there may be only one CPU
 In early systems, frame buffer was the part of standard
memory
 CPU is responsible for both Normal and Graphical
Processing
 Main graphical processing of CPU is
 Take graphical primitives from application program
 Like (lines, polygons, circles)
 Assign values to the pixels in frame buffer that best
represent those entities.
 Rasterization: Conversion of geometrical entities to
pixel colors and locations in frame buffer. (a.k.a scan

15. CPU & GPU (Contd…)
 Today, all graphics system are characterized by
special purpose graphics processing unit (GPU).
 GPU: A processing unit custom-tailored to carry out
specific graphics functions
 GPU can either be on the same system board or on
separate graphics card
 Frame buffer usually resides on the same board as GPU
 Frame buffer is accessed through GPU

16. Output Devices
 Cathode Ray Tube (CRT)
 Most dominant type of display (until, recently)
 Basic Op: When electron strikes the phosphor coating,
light is emitted.
 Deflection plates: to control the direction of the beam
 Computer output is converted from digital(bits) to
analog(voltage) by converters across x and y deflection
plates.
 When sufficiently intense beam of electrons is directed at
the phosphor, light appears on CRT surface

17. Output Devices (Contd…)
 Refresh rate: No. of times per second the device
retrace the same path/image.
 CRT emits light for short time (few milli seconds)
 To see flickering-free image, same image must be retraced.
 Old systems: Refresh rate = frequency of power system
 50 Hz in US and 60 Hz in most of the world
 Raster System (Fundamental ways of displaying
pixels)
 Non-interlaced: Pixels are displayed row by row at refresh
rate
 Interlaced Display: Odd and even rows are refreshed,
alternatively. 30 Hz instead of 60 Hz.

18. Output Devices (Contd…)
 Colored CRTs
 Phosphors of three different colors (Red,
Green, Blue)
 Phosphors arranged in small groups.
 Phosphors in triangular groups are called
triads
 Have three electron beams.
 Shadow Mask: a metal sheet with small
holes
 Used to ensure the excitation of proper color
phosphor.

19. Output Devices (Contd…)
 Flat-panel Technology.
 Flat-panels are inherently raster based.
 Mostly used flat panels are LCD, LED and Plasma
 Generic flat-panel display have
 2-outside plate: containing parallel
grids of wires, oriented
perpendicular to each other.
 Middle plate contains different
material depending upon the
technology.

20. Output Devices (Contd…)
 Flat-panel Display
 By sending electrical signal to proper wire on both plates.
 Electric field is produced at the point of intersection of two
wires
 Electric field is used to control the corresponding element on
the middle plate.
 Electric field produced can be used in,
 LED, to turn corresponding led on or off
 LCD, to control polarization of liquid crystals to pass light
 Plasma, to energize gases in order to glow or not.

21. Input Devices
 Input Devices: Devices used for input purposes.
 Common input device Keyboard, mouse
 Other input devices include joy stick, track ball, space ball
 Input Devices (Perspective)
 Physical Device
 Logical Device (application / programmer perspective)
 Their properties are specified in terms of “what they do” from
application perspective.
 For example: cout in C++ outputs the string, the output device
could be printer, display/terminal or a disk file.
 Even the “cout” output could be input for another program.

22. Input Devices (Physical)
 Two primary types of Input Devices
 Keyboard Devices
 Pointing Devices
 Keyboard generally include physical keyboards or
devices that return character codes.
 ASCII code is used to represent characters.
 ASCII assigns a single unsigned byte to each character.
 Internet application use multiple bytes to represent each
char.
 Mouse & Trackball:
 A mechanical mouse and trackball works on same
principal.
 Motion of the ball is converted to signals by converters.

23. Input Devices (Physical)
 Signals from encoders might be interpreted as
position (Not necessarily)
 Driver/Program can interpret the signals as two
velocities.
 The Computer can integrate velocities to obtain
position.
 When ball rotates position
changes, otherwise not.
 In this mode, positioning is
relative.
 Motion sensing devices are known as Relative
positioning devices

24. Input Devices (Physical)
 Data Tablets:
 Absolute positioning
 Position is determined using electromagnetic
interactions between signals traveling through the
wires and sensors in the stylus
 Position sensing devices are known as absolute
positioning devices
 READING: Space ball and Joy stick

25. Input Devices (Logical)
 Logical Input Devices:
 Addressing of physical input devices as abstract data
types
 ADT: data type defined by its behavior from user view
 Two major characteristics describe logical
behavior of input device:
1. The measurements that the device returns to the
user program
2. The time when the device returns those
measurements.

26. Input Devices (Logical)
 Logical Input Devices:
 String: Return string of characters from Keyboard, File,
etc
 Locator: Returns a position (in x, y coordinates)
 Pick: Returns a segment name & pick identifier of object
pointed by the user.
 Choice: Represents a choice from a selection of several
possibilities.
 Valuator: Returns a real/analogue value, for example, to
control some sort of analogue device.
 Stroke: series of locations. (Tablets/Touch inputs)

27. Input Modes
 Input is provided in terms of two entities
 Measure: The returned data from input devices.
 Ex: Data stream from keyboard OR location of pointer from
mouse
 Trigger: Physical input to signal the computer.
 Ex: Pressing of Return (Enter) Key / Esc Key OR clicking the
mouse button.
 Measure of device can be obtained in three (03)
distinct modes
 Each mode is defined by: Relationship b/w measure &
trigger.

28. Request Mode – Input Modes
 The measure of the device is not returned to the
program until the device is triggered.
 Ex: cin / scanf in C++/C language. (Input Statements)
 Program waits for trigger when input statement is
encountered.
 Take as long as the user wants.
 The measure is only returned upon trigger. (e.g upon
pressing enter/return)

29. Sample Mode (Input Modes)
 Sample-Mode: Input is immediate. Measure is
returned as the function is encountered in App.
 Position the device or Enter data before the function call.
 Program retrieves “measure” immediately from the
buffer/file/location.
 For example, your application can obtain the location
of the screen cursor at any point in time, through the
use of SAMPLE mode input.

30. Event Mode – Input Modes
 Case: Multi input devices each with its own measure &
trigger.
 For Example: Flight simulators with multiple inputs.
 Event Mode:
 Application Program & Devices work independently of each other.
 Each time listed device is triggered: measure + id is stored in event
queue
 App. Prog. Retrieves input from event queue whenever required.
 Event Mode (Callback Approach):
 Associate a function call (callback) with events specifically.
 OS queries event queue and calls the associated function.
 Efficient approach in client-server scenarios.

31. Images – Physically & Synthetic
Elements of Image Formation
 Basic entities of Image Formation
 Objects
 Viewers
 Light

32. Object(s)
 The object exists in space independent of any
image-formation process and of any viewer.
 Computer Graphics: deals with synthetic objects
 Objects are formed by specifying positions of
geometric primitives(basic shapes) in space like
triangle, polygons etc
 Mostly, set of spatial positions (vertices) are used to
define objects
 For Ex: line can be defined by two vertices
 Triangle can be defined by three vertices.

33. Viewer(s)
 To form an image, there must be someone or
something that is viewing our objects. Like human, a
camera, etc.
 It is the viewer that forms the image of our objects.
 Human Visual System: Image is formed at back of eye
 Camera: Image is formed in the film plane
 Objects are usually seen from different perspectives.

34. Light
 Be it physical or synthetic images are in complete
without light.
 No light = dark objects = no image formation
 Light is electromagnetic radiation.
 Light Spectrum: Radio + Infrared + Visual spectrum
 Visual spectrum: 350 – 780 nm
 Around 520 nm: Green
 Near 450 nm: Blue
 Near 650 nm: Red
 Except from recognizing that which frequency
is for which color CG doesn’t deal with light

35. Light Spectrum

36. Imaging Systems
 Physical imaging systems to understand imaging in
computers.
 Pin-hole Camera
 Human Visual System
 Pin-hole: To understand basic working principles of
camera.
 Human visual system is complex but obeys the
physical principles of other imaging systems

37. Pin-hole Camera
 A pinhole camera is a box with a small hole in the
center of one side of the box
 film is on the side opposite the pinhole.
 Hole is so small that only a single ray of light can
enter (assumption)
 For Example: We have point in scene (x, y, z)
 At Image: z = -d
 y = yp
 x = xp (In top view)
 (xp, yp, -d) is called
projection of (x,y,z)

38. Pin-hole Camera
 Color: In idealized mode, color of the image will be
same as in scene
 Field/Angle of View: is the extent of the observable
world that is seen at any given moment.
 If h is the height of camera (film) then
 Angle is formulated using basic
Trigonometry.

39. Pin-hole Camera
 Depth of Field: Every object in angle of view is in
focus i.e appears sharply.
 In Ideal pinhole camera depth of field is infinite.
(Assumed)
 Disadvantages of Pinhole camera
 It admits only single ray – almost no light.
 Camera cannot be adjusted to have different angle of
view.
 By replacing hole with lens; problems can be
eliminated
 With proper lens more light can be entered (larger
aperture)

40. Human Visual System
 Human Visual System is extremely complex.
 Light enters through cornea and lens
 Iris opens/shuts to adjust the amount of light
 Image is formed at retina (back of the eye)
 Cells (Rods and Cones) are sensors
 They excites/responds when light enters eye (350-780
nm)
 Rods: 1 type; Low light sensors; Night vision, not color
sensitive
 Cones: 3 types; responsible for color vision
 Resolution of Visual System
 Resolution: Measure of what size objects can we see.
 Technically: it is a measure of how close we can place

41. Human Visual System
 Brightness: Brightness is an overall measure of how
we react to the intensity of light
 HVS reacts differently to different wavelengths of light
 HVS is more sensitive to green and less sensitive to red &
blue
 HVS only reacts to three colors instead of whole
visual spectrum due to three types of cones.
 These colors are called primary colors.
 Primary colors are Red, Blue and Green.

42. Synthetic Camera Model
 The paradigm of emulating image formation by
optical system in computer is known as Synthetic
Camera Model
 Basic Principles:
 As Object & Viewer are independent of each, so CG (API)
should have separate functions for specifications.
 Images can be computed using simple geometric
calculation like in pin hole camera.

43. Synthetic Camera Model
 In pin hole camera image formed is
flipped.
 In computer, image is retain; by
moving the plane to front (Virtual
image plane)
 A line is drawn called projector from
center of lens/projection (COP) to
the object/point.
 All the light originates from COP
 This virtual image plane is called
projection plane

44. Synthetic Camera Model
 There is always limitation to the size of the image. In optical
system; field of view expresses the limitation
 Synthetic Camera Model places a Window/Rectangle in
projection plane to cope with the limitation.
 This window / rectangle through which a viewer at COP sees the
world called Clipping window/rectangle
 Given the following
 Location of COP
 Orientation of Projection plane
 Size of clipping window
 We can determine that which
objects will appear in image

45. Programmer’s Interface
 User interact with graphics system in different ways
like using CAD modules, paint programs etc.
 Programmers interact with GS using graphics library
(API)
 Graphics API: Interface b/w programmer and Graphics
system specified through set of functions.
 Programmers don’t see h/w related details.
 Software Drivers interpret the output of API to the
form that is understood by the specific hardware.

46. Three Dimensional API
 As per synthetic camera model, 3D API must provide
functions to specify,
 Objects
 Viewer
 Light Sources
 Material properties.
 Objects are specified using vertices.
 Objects are usually specified using geometric
primitives like lines, polygons, triangles etc
 Complex objects may involve multiple ways of
specification.

47. Three Dimensional API
 Camera can be defined in variety of ways.
 APIs differ in camera selection and methods
 Four types of specification for Camera
 Position: Camera location (COP)
 Orientation: Rotation of camera in axes.
 Focal length: Size of the image/ Angle of view
 Film plane: height & width
 Specifications can be satisfied in various ways
 Most used way is coordinate system transformation
 Transformations convert object positions
represented in coordinate system.

48. Three Dimensional API
 Light Sources
 Light sources are specified by their
 location, strength, color and direction
 These properties are specified for each light sources
used.
 Material Properties:
 Characteristics or attributes of the objects
 These attributes are specified when the objects are
defined
 Both light and material properties depend upon the
light-material model API supports.

49. The Modeling–Rendering Paradigm
 Model: Mathematical/Geometrical description of
shapes
 Rendering: Process of generating images from
models
 Modeling can be separated from Rendering.
 Helpful in generating complex images.
 The file/data produced by the modeler is used by the
Renderer.
 This File could be simple; containing info in specific
format

50. The Modeling–Rendering Paradigm
 Different hardware and software at both blocks.
 Modeler, as well as Rendered are both
customizable
 Use diff: modeler with same renderer
 Use same modeler with diff: renderer
 Most popular approach now a days.
 Models, lights, camera etc are place in special data
structure called scene graph
 Scene graph is then passed to renderer or game
engine.

51. Graphics Architectures
 Early graphics system: general-purpose computer of
von Neumann architecture
 Single processor system (Single instruction
Execution)
 Calligraphic CRT display
 Generate a line segment by connecting two points.
 Host used to Run app & compute end points and
send them to CRT
 Info needs to be sent at high speed (to avoid flicker)
 Refreshing was slow that even small image
would burden expensive computer.

52. Display Processors
 Earliest special purpose graphics system
 To separate the process of continuous display of
refreshing
 Display processor included instruction to display
CRT primitives
 Host generate image (Using instructions)
 Send to Display processor
 Display processor store program in memory
(as display file / display list)
 Display processor runs
program iteratively

53. Pipeline Architecture
 Process/Operation divided into several independent /
dependent segments.
 a + (b ∗ c)
 Pipelining increases throughput of the computer.

54. Graphics Pipeline
 Sets of Object  Objects (Set of primitives) 
vertices
 Complex objects may contain millions of vertices
 To make process of imaging fast we use pipeline
 Graphical pipeline consists of
 Vertex Processing
 Clipping and Primitive Assembly
 Rasterization
 Fragment Processing

55. 1. Vertex Processing
 Two major functions of this block
 Coordinate transformation
 Compute color of each vertex

56. Clipping and Primitive Assembly
 Vertices are assembled into primitives (shapes)
 No camera can see the whole world
 Clipping must be done.
 Clipping window/volume is considered
 Clipping is done primitive by primitive.
 Output: set of primitives which will appear in clipping
image.