introduction to opengl
DESCRIPTION
INTRODUCTION TO OPENGL. INTRODUCTION TO OPENGL. What is OpenGL API Functions Event Driven Programming Using OpenGL. OpenGL. OpenGL is a platform-independent API that is Easy to use Close enough to the hardware to get excellent performance Focus on rendering - PowerPoint PPT PresentationTRANSCRIPT
INTRODUCTION TO OPENGL
INTRODUCTION TO OPENGL
What is OpenGL APIFunctions Event Driven Programming Using
OpenGL
OpenGL
OpenGL is a platform-independent API that is ◦Easy to use◦Close enough to the hardware to get
excellent performance◦Focus on rendering◦Omitted windowing and input to
avoid window system dependencies
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OpenGL InterfaceComponents of the OpenGL
Interface◦GL: core OpenGL functions◦GLU: graphics utility library
(helpers for creating common objects, eg. Spheres)
◦GLUT: GL Utility Toolkit(interface to windowing system)
◦GLX: low-level interface to X Windows
Basic OpenGL SyntaxFunction names from GL library
◦ prefixed with gl
Symbolic constants◦ Capital letters with underscore
OpenGL built-in data types
glBegin, glClear, glCopyPixelsglBegin, glClear, glCopyPixels
GL_2D, GL_RGB, GL_POLYGONGL_2D, GL_RGB, GL_POLYGON
GLbyte, GLshort, Gint, GLfloat, GLdouble, GLbooleanGLbyte, GLshort, Gint, GLfloat, GLdouble, GLboolean
GLUTOpenGL Utility Toolkit (GLUT)
◦Provides functionality common to all window systems Open a window Get input from mouse and keyboard Menus Event-driven
◦Code is portable but GLUT lacks the functionality of a good toolkit for a specific platform No slide bars
GLUT
Initializes GLUT and should be called before any OpenGL functions.
void glutInit (int argc, char ** argv)void glutInit (int argc, char ** argv)
Function names from GLUT library prefixed with glut
Creates a window on the screen with the title given by the argument.
void glutCreateWindow (*char title)void glutCreateWindow (*char title)
Event Loops & Callback Function
The function func() is called each time there is a display callback
void glutDisplayFunc (void (*func) (void))void glutDisplayFunc (void (*func) (void))
Events – mouse, keyboard, windows events.
Callback function – define how the program should react to specific events.
Event Loops & Callback Function
Causes the program to enter an event-processing loop.
Should be the last function in main().
void glutMainLoop()void glutMainLoop()
OpenGL function format
glVertex3f(x,y,z)
belongs to GL library
function name
x,y,z are floats
dimensions
A Simple Program#include <GL/glut.h>void mydisplay(){ glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_POLYGON); glVertex2f(-0.5, -0.5); glVertex2f(-0.5, 0.5); glVertex2f(0.5, 0.5); glVertex2f(0.5, -0.5);
glEnd();glFlush();
}int main(int argc, char** argv){
glutCreateWindow("simple"); glutDisplayFunc(mydisplay); glutMainLoop();
}
A Simple Program
Event LoopNote that the program defines a
display callback function named mydisplay◦Every glut program must have a
display callback◦The display callback is executed
whenever OpenGL decides the display must be refreshed, for example when the window is opened
◦The main function ends with the program entering an event loop
USING RGB COLOR#include <GL\glut.h>
void display(){
glClear (GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 0.0, 0.0); glBegin (GL_POLYGON); glVertex2f (-0.5, -0.5); glVertex2f (-0.5, 0.5);
glVertex2f (0.5, 0.5); glVertex2f (0.5, -0.5);
glEnd ();
glFlush ();}
int main(int argc, char** argv)
{
glutInit (&argc, argv);
glutCreateWindow (“Simple");
glClearColor(1.0, 1.0, 1.0, 0.0);
glutDisplayFunc (display);
glutMainLoop ();
}
USING RGB COLOR◦Set a particular color:◦ glColor3f(r, g, b);
◦Set a background color:◦ glClearColor(r, g, b, 1);
◦Clear the window to background color:
◦glClear(GL_COLOR_BUFFER_BIT);
USING RGB COLOR
Program StructureMost OpenGL programs have a similar
structure that consists of the following functions◦ main():
defines the callback functions opens one or more windows with the required
properties enters event loop (last executable statement)
◦ init(): sets the state variables Viewing Attributes
◦ callbacks Display function Input and window functions
main#include <GL/glut.h>
int main(int argc, char** argv){glutInit(&argc,argv); glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB); glutInitWindowSize(500,500); glutInitWindowPosition(0,0); glutCreateWindow("simple"); glutDisplayFunc(mydisplay);
init();
glutMainLoop();
}
GLUT functions glutInit allows application to get command line
arguments and initializes system gluInitDisplayMode requests properties for the
window (the rendering context)◦ RGB color◦ Single buffering◦ Properties logically ORed together
glutWindowSize in pixels glutWindowPosition from top-left corner of display glutCreateWindow create window with title “simple” glutDisplayFunc display callback glutMainLoop enter infinite event loop
Coordinate SystemsThe units in glVertex are determined by
the application and are called object or problem coordinates
The viewing specifications are also in object coordinates and it is the size of the viewing volume that determines what will appear in the image
Internally, OpenGL will convert to camera (eye) coordinates and later to screen coordinates
OpenGL also uses some internal representations that usually are not visible to the application
Transformations and ViewingIn OpenGL, projection is carried out by a
projection matrix (transformation)There is only one set of transformation
functions so we must set the matrix mode first
glMatrixMode (GL_PROJECTION)
Transformation functions are incremental so we start with an identity matrix and alter it with a projection matrix that gives the view volume
glLoadIdentity(); glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
Two- and three-dimensional viewingIn glOrtho(left, right, bottom, top, near, far) the near and far distances are measured from the camera
Two-dimensional vertex commands place all vertices in the plane z=0
If the application is in two dimensions, we can use the function
gluOrtho2D(left, right,bottom,top)In two dimensions, the view or clipping
volume becomes a clipping window
OpenGL Primitives
GL_QUAD_STRIP
GL_POLYGON
GL_TRIANGLE_STRIP GL_TRIANGLE_FAN
GL_POINTS
GL_LINES
GL_LINE_LOOP
GL_LINE_STRIP
GL_TRIANGLES
[Edward Angel, Interactive computer Graphics, 2009]
Event Driven ProgrammingMouse Event
◦Event that occurs when the mouse button is pressed or released.
Mouse Motion Event
◦Event that occurs when the mouse is moved while one of the buttons is pressed.
Mouse Event
Contains the following information:
◦Button: The mouse button that is pressed – left, middle, right.
◦State: The state of the button – up, down.
◦Position: The position of the mouse when the event occurs (x, y).
Mouse Event◦Registering with MouseEvent:◦ glutMouseFunc(myMouse);
◦Call-back function:◦ void myMouse(int button, int state, int x, int y)
◦Values for button:◦ GLUT_LEFT_BUTTON ◦ GLUT_MIDDLE_BUTTON◦ GLUT_RIGHT_BUTTON◦Values for state:◦ GLUT_UP◦ GLUT_DOWN◦x, y: ◦ screen coordinates of mouse position ◦ (origin at top-left corner)
Mouse Motion EventContains the following
information:
◦Position: The current position of the mouse as the mouse is being dragged holding one of the buttons pressed. The events are continuously generated as the mouse button is pressed and dragged.
Mouse Motion Event◦Registering with MouseMotionEvent:◦ glutMotionFunc(myMovedMouse);
◦Call-back function:◦ void myMovedMouse(int x, int y)
◦x, y: ◦ screen coordinates of mouse
position ◦ (origin at top-left corner)
Keyboard EventContains the following
information:
◦key: The ASCII value of the key pressed.
◦Position: The current position of the mouse when the key is pressed.
Keyboard Event◦Registering with KeyboardEvent:◦ glutKeyboardFunc(myKeyboard);
◦Call-back function:◦ void myKeyboard(unsigned int key, int x,
int y)
◦Values for keys:◦ ASCII values for normal keys ◦ GLUT_KEY_LEFT, GLUT_KEY_RIGHT, …
(arrow keys)◦x, y: ◦ screen coordinates of mouse position ◦ (origin at top-left corner)
Window EventsThe Window Redraw Event
occurs whenever the window needs to be redrawn. This happens when the window is first opened and when the window is exposed by moving another window off of it.
The Window Reshape Event is generated when the window is resized with the mouse.
Window Events◦Registering with
WindowRedrawEvent:◦ glutDisplayFunc(myDisplay);
◦Registering with WindowReshapeEvent:
◦ glutReshapeFunc(Reshape);