Input and Interaction
Lecture No. 4
Introduction
 Interactive computer graphics opens up a myriad of
applications, ranging from interactive design of buildings, to
control of large systems through graphical interfaces, to
virtual-reality systems, to computer games.
Interaction
 One of the most important advances in computer technology
was enabling users to interact with computer displays.
 Ivan Sutherland’s Project Sketchpad launched the present era
of interactive computer graphics.
Interaction
 OpenGL does not support interaction directly. The major
reason for this omission is that the system architects who
designed OpenGL wanted to increase its portability by
allowing the system to work in a variety of environment
 Windowing and input functions were left out of the
Application Programming Interface (API.s).
Interaction
 We can avoid such potential difficulties by using a simple
library, or toolkit.
 The toolkit can provide the minimal functionality, such as
opening of windows, use of the keyboard and mouse, and
creation of pop-up menus through the toolkit’s API.
Input Devices
 Physical vs. logical
 Physical
 mouse, keyboard, etc.
 perspective of how they interact with system
 Logical
 function
 perspective of what they send to the application
 familiar to all writers of high-level programs
Input Devices
 For example, data input and output in C are done through
functions such as printf, scanf, getchar, and putchar, whose
arguments use the standard C data types, and through input
(cin) and output (cout) streams in C++.
Input Devices
 In computer graphics, the use of logical devices is more
complex , because the forms that input can take are more
varied than the strings of bits or characters to which we are
usually restricted in non-graphical applications.
Input Devices
 For example, we can use the mouse—a physical device—
either to select a location on the screen, or to indicate which
item in a menu we wish to select.
 In the first case, an x, y pair (in some coordinate system) is returned
to the user program; in the second, the application program
may receive an integer as the identifier of an entry in the
menu.
Physical Input Devices
 Two categories:
 pointing devices: allows the user to indicate a position on the
screen.
 keyboard devices: a physical keyboard, or generally, include
any device that returns character codes to a program.
Physical Input Devices
 Absolute-positioning ( some fixed position)
 pen returns position (settable)
 data glove always returns 3D position coords
 Relative-positioning ( current position)

mouse pos. always begins where cursor is
Measure and Trigger
 The manner by which physical and logical input devices
provide input to an application program can be described in
terms of two entities:
 a measure process:
is what the device returns to the user program.
 a device trigger:
is a physical input on the device with which the
user can signal the computer.
Input modes
 Three distinct mode for a measure of a device.
 Each mode is defined by the relationship
between the measure process and the trigger
Request mode:
 the measure of the device is not returned to the program
until the device is triggered.
 This input mode is standard in non graphical applications,
such as a typical C program that requires character input.
Request mode
Trigger
Trigger
Process
Request
Measure
Process
Measure
Program
The relationship between measure and trigger for
request mode
Inputs modes
 Sample-mode:
 input is immediate , as soon as the function call in the user
program is encountered, the measure is returned.
 No trigger is needed
Measure
Process
Measure
Program
Inputs modes
 In both request- and sample-mode the user must identify
which device is to provide the input.
 We usually interface with the devices through functions such
as
 request_locator(device_id, &measure);
 sample_locator(device_id, &measure);
Inputs modes
 Both request and sample modes are useful for situations
where the program guides the user, but are not useful in
applications where the user controls the flow of the program.
 For example , a flight simulator might have multiple input
devices—such as a joystick, buttons, and switches.
 Sample- and request-mode input are not sufficient for
handling the variety of possible human–computer
interactions that arise in a modern computing environment.
Inputs modes
 Event mode: the most flexible input mode
 triggers generate events (events store data)
 measures are sent to
1) an event queue
program checks queue
events are examined and acted upon
2) a special-purpose function
program associates function called a callback
 used with the major windowing systems because it has been
proved to work well in client–server environments.
Event - Mode Model


Trigger
Process

Trigger
Measure
Measure
Process
A wait
Measure
Process
Event
Program
Event-driven input
 Callback for display
 glutDisplayFunc(display)
 Callback for mouse
 glutMouseFunc(mouse)
 Callback for keyboard
 glutKeyboardFunc(keyboard);
Clients and servers
• In general:
Servers perform tasks for clients
 In OpenGL
 assumes the OpenGL program is the client
 assumes workstations are graphics servers that provide display and
input services to the OpenGL program
 for example, the client can display its output on any networked
server
Display lists
 Display list is a group of OpenGL commands that have been
stored (compiled) for later execution. Once a display list is
created, all vertex and pixel data are evaluated and copied
into the display list memory on the server machine. It is only
one time process. After the display list has been prepared
(compiled), you can reuse it repeatedly without re-evaluating
and re-transmitting data over and over again to draw each
frame.
Display lists
 Display list is one of the fastest methods to draw static data
because vertex data and OpenGL commands are cached in
the display list and minimize data transmissions from the
client to the server side. It means that it reduces CPU cycles
to perform the actual data transfer.
 Another important capability of display list is that display list
can be shared with many clients, since it is server state.
Display lists
 We can send graphical entities to a display in one of two
ways:
 immediate mode:
 send the complete description of our objects (vertices,
attributes, and primitive types )to the graphics server.
 No memory ,re-compute to redisplay.
Display lists
 retained-mode:
 define the object once, then put its description in a display
list.
 The display list is stored in the server and redisplayed by a
simple function call issued from the client to the server.
Display lists
 Advantage:
 reduced network traffic.
 allows the client to take advantage of any special-purpose
graphics hardware that might be available in the graphics
server.
 Disadvantage:
 require memory on the server.
 overhead of creating a display list.
Display lists
 Create multiple lists
 glGenLists(number) generates multiple lists
 glCallLists displays multiple lists
Menus
 GLUT provides pop-up menus, that we can use with the
mouse to create sophisticated interactive applications.
 Using menus involve taking a few simple steps:
- define entries
-link menu to mouse button
- define callback function for each menu entry
Picking
 Picking is the logical input operation that allows the user to
identify an object on the display.
 Although the action of picking uses the pointing device, the
information that the user wants returned to the application
program is not a position.
 Old display processors could accomplish picking
easily by means of a lightpen.
Picking
 One reason for the difficulty of picking in modern systems is
the forward nature of their rendering pipelines.
 Primitives are defined in an application program and move
forward through a sequence of transformations and clippers
until they are rasterized into the frame buffer.
Picking
 three ways to deal with this difficulty:
- selection:
 involves adjusting the clipping region and viewport such that
we can keep track of which primitives in a small clipping
region are rendered into a region near the cursor. These
primitives go into a hit list that can be examined later by the
user program.
Picking
- bounding rectangles:
 or extents, for objects of interest. The extent of an object is
the smallest rectangle, aligned with the coordinates axes, that
contains the object.
- back buffer:
 Recall that the back buffer is not displayed and thus we can
use it for purposes other than forming the image we will
display when we swap the front and back buffers.
Buffering
 Single buffering
 objects are always rendered into the same frame buffer, which is
always being displayed
 delay of clearing of and re-drawing into frame-buffer will cause flicker if
re-draw takes longer than refresh or refresh and animation not synced
 The higher the refresh rate, the less "flicker" you will see.
 Double buffering
 keep two buffers (front and back)
 always display front, always render into back*
 swap front and back when rendering complete