ch 2 graphics programming page 1 csc 367 coordinate systems (2.1.2) device coordinates, or screen...

Ch 2 Graphics Programming page 1CSC 367

Coordinate Systems (2.1.2)

• Device coordinates, or screen coordinates (pixels)

put limitations on programmers and the code is not portable

generally expressed in integers

• World coordinates

an effort was made to hide the actual device coordinates from the programmer and allow ANY coordinate to be used

generally expressed in floats

programmer defines world space

• Mapping

the mathematical transformation from one coordinate system to another

involves relatively simple algebra

• Example

Device coordinates of 640 x 480

World coordinates of 5000 x 4000

Map (2500, 2000) to ___________

Map (1000, 1000) to ___________


OpenGL

• Originally SGIs GL

modified to be more portable

• Collection of functions that interface to graphics hardware

• Works well on a network

• Protocol (format of instructions) is the same regardless of the computer. This makes the code portable or system independent

• It does NOT do

windows

input

modeling

• It does do sophisticated rendering of 3D scenes


uisGL (3D object library)

• The book explains that OpenGL is not OO

• This API helps to hide some ugly details

• #include <uisGL.h>

• Vertex

uisVertex P1, P2, P3(10,10,0);

P1.set(20,20,0);

P2.set(40,40,0);

P3 = P1 + P2;

P3 = P1 / 2;

P3 = P2 * 2.0;

cout << P3;


Typical API Functions (2.2.1)

• Primitives

draw points, lines, polygons (maybe curves)

• Attributes

colors, fill patterns, fonts

• Viewing

position, orientation and lens for the virtual camera

• Input

handle events from keyboard and mouse

• Control

initialize programs, create windows

• Transformation

rotate, translate, and scale objects


Drawing Primitives (2.1.1)

• We will consider 2D to be a special case (x,y,0)

• Point or Vertex define geometric objects

glVertex2f (x, y);

glVertex3f (x,y,z);

• Array or Vector format

float vertex[3];

glVertex3fv(vertex);

• Drawing individual points

glBegin(GL_POINTS);

glVertex3f (x,y,z);

glVertex3f (x,y,z);

glEnd( ); <--- warning!

• Drawing a line

glBegin(GL_LINES);

glVertex3f(x,y,z);

glVertex3f(x,y,z);

glEnd( );


Polygons (2.3.1)

• a series of connected lines

• simple polygon edges do not cross

• the interior can be filled with a color

• convex polygons can be tested by connecting all vertices with each other. If all of the lines remain inside the polygon then it is covex.

• concave polygons are not convex

• Drawing a polygon

float P1[3] = {10, 10, 0};

glBegin(GL_POLYGON);

glVertex3fv (P1);

glVertex3fv (P2);

glVertex3fv (P3);

glVertex3fv (P4);

glEnd();

• Interior can be filled with a color

• Edges can be turned on or off


Attributes (2.3.5)

• the name for any property that determines how an object appears

• a solid red line is different that a dashed green line

• color, line thickness, fill pattern

• The general approach is to set current attributes and then display the object.

• Each geometric type has a set of attributes. A line has color, width and style.

• glPointSize (2.0) // 2 pixels wide


Color (2.4)

• Ignore color theory and mathematics

• Three colored spot lights analogy

• Different than mixing paints

• RGB color or true color

C = T1R + T2G + T3B

T is the intensity that ranges from 0 - 1

• glColor3f (0, 0, 0); // balck

• glColor3f (1, 0, 0); // red

• glClearColor (1, 1, 1, 0); // white background

• glClear (); // clear the screen

• The number of colors that can be displayed are determined by the number of bits available for each pixel

true color requires 24 bits - 8 bits for each


Indexed Color

• not often needed any longer

• when the number of bits per pixel is limited

• rather than limit the possible range of colors we limit the possible number of colors

• painter example that can mix an unlimited number of paints but can only fit so many on the cardboard plate

• Color-lookup Table

the 8-bit representation is used as an index into the color lookup table

contains a limited number of entries but each entry can represent any color

set current color by specifying an index

common configuration is 256 indices


Viewing 2.5

• Viewing rectangle or window

items within the window will be seen and others will be clipped

• 3D Viewing Volume

glOrtho(left,right,bottom,top,near,far)

• Be aware of right handed coordinate system

positive Z comes out from screen

• Viewport

a rectangular region within the window

glViewport(x,y,width,height)

• Aspect Ratio

the ratio of the viewing rectangle should be the same as the viewport


Matrix Mode (2.5.3)

• Primitives are multiplied by matrices before being displayed.

• OpenGL has two important matrices

MODELVIEW - for viewing parameters

PROJECTION - for display parameters

• obj’ = obj x MV

• obj’’ = obj’ x P

• Typical initialization

glMatrixMode(GL_PROJECTION);

glLoadIdentity();

glOrtho(0,10,0,10,0,10);

glMatrixMode(GL_MODELVIEW);

• Leave in MODELVIEW mode


GLUT (GL Untility Toolkit) 2.6

• Windowing system independent API for OpenGL applications

• Allows one to

window management

simple menus

event processing loop

input

• We will use an X11 implementation of GLUT. There could be a Windows version or Macintosh version.

• X11 is a protocal that allows device independent display of graphics over a network. Using real X11 commands would be a pain. Somewhat equivalent to programming in assembler.

• Client Server model. The client is the application and the computer that does the display is the server.


Event Processing 2.6

• What stops the image from disappearing after it has been drawn?

• Event Processing Loop

glutMainLoop()

this causes an infinite loop until closing the window by hand

• Callback Mechanism

a programmer defined function that glutMainLoop will call when GLUT determines an event has occurred.

this is called “registering a call back”

• Display function

glutDisplayFunc(*func(void))

• Additional Events

mouse button presses

keyboard

window movement


Sample Code

• look a source

• discuss Makefile


Sierpinski Gasket

• interesting example from the book

• Choose a random point within the triangle

• Loop

Find midpoint between current point and a random vertex

Make a mark

Update current point

• Extended to 3D requires a tetrahedron

ch 2 graphics programming page 1 csc 367 coordinate systems (2.1.2) device coordinates, or screen...

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