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Unformatted text preview: Introduction to
Computer Graphics Lecture 4
OpenGL Programming Guide Introduction to OpenGL OpenGL and Related APIs WGL: glue between OpenGL and Microsoft Windows
GLU: OpenGL utility library – rendering quadric surfaces, curves
GLUT: OpenGL utility toolkit – creating windows, animation Intro to OpenGLPreliminaries
Header Files
•#include <GL/glut.h>
Include this header file in your codes for correct
compiling.
The glut header file includes “glu.h”, and the glu
header file includes “gl.h”. Intro to OpenGLPreliminaries
Libraries
opengl32.lib
glu32.lib
glut32.lib
Append these library files in your library modules for
correct linking. Intro to OpenGLPreliminaries
DLL
glut32.dll
glut32.dll
Copy this file into your system32 directory for
Copy
correct program performance.
correct Intro to OpenGLApplication Structure
• Configure and open window
• Initialize OpenGL state
• Register input callback functions:
•render
•resize
•input: keyboard, mouse, etc.
• Enter event processing loop Intro to OpenGLEvent Processing Loop
OpenGL operates as an infinite loop
• Put things in the scene (points, colored lines, textured polys)
• Describe the camera (location, orientation, field of view)
• Render – draw the scene
• Listen for input (keyboard, mouse, etc.) events OpenGL Basics
Window Configuration Callback Functions Onetime Initialization:
Initialize any OpenGL state and
program variables GLUT Callback Functions
Event Processing: callbacks define what actions are
Event
supported and automatically handling the user events.
supported
glutDisplayFunc() – called for refreshing pixels in the
glutDisplayFunc()
window
window
glutReshapeFunc() – called for changing the window size
glutKeyboardFunc() – called for keyboard input GLUT Callback Functions
glutMouseFunc() – called for mouse input
glutMotionFunc() – called for moving the mouse
glutMotionFunc()
while a button is pressed
while
glutPassiveMouseFunc() – called for moving the
glutPassiveMouseFunc()
mouse regardless of mouse button state
mouse
glutIdleFunc() – called for nothing (useful for
glutIdleFunc()
animation)
animation) Rendering Callback
Clear the window
Render a triangle strip for smooth animation The callback, glutDisplayFunc(display) is called when the window
needs to be refreshed. Idle Callback – for animation
global variable for
controlling the moving
activate glutDisplayFunc()
as soon as possible glutIdleFunc(idle) is called for updating the motion variables and
then the scene is updated and refreshed User Input Callback – for interactive use glutKeyboardFunc(keyboard) is called for receiving keyboard input
Other input devices include the mouse and track ball Intro to OpenGL
OpenGL has a “state”
• There are a lot of ways your OpenGL program can be
There
configured
configured
• The current configuration is stored in OpenGL’s state
• Be aware that OpenGL commands affect the program’s state
Be
rather than redirect its logical execution
rather Light Source Placement
Setup lighting, if any
Simplest option: change the current color between
Simplest
current
polygons or vertices
polygons
• glColor() sets the current color Or OpenGL provides a simple lighting model:
• Set parameters for light(s)
– Intensity, position, direction & falloff (if applicable)
Intensity,
• Set material parameters to describe how light reflects from the
Set material
surface
surface Won’t go into details now; Model (Object) Placement
Geometry in OpenGL consists of a list of vertices in
Geometry
between calls to glBegin() and glEnd()
glBegin()
glEnd()
• A simple example: telling GL to render a triangle
glBegin(GL_POLYGON);
glVertex3f(x1, y1, z1);
glVertex3f(x2, y2, z2);
glVertex3f(x3, y3, z3);
glEnd();
• Usage: glBegin(geomtype) where geomtype is:
Usage: glBegin(
– Points, lines, polygons, triangles, quadrilaterals, etc... Model (Object) Placement  Primitive
Types
GL_POINTS
GL_LINE
• {S  _STRIP  _LOOP} GL_TRIANGLE
• {S  _STRIP  _FAN} GL_QUAD
• {S  _STRIP} GL_POLYGON Model (Object) Placement  Drawing
Triangles
You can draw multiple triangles between
You
glBegin(GL_TRIANGLES) and glEnd():
glBegin(GL_TRIANGLES)
glEnd()
float v1[3], v2[3], v3[3], v4[3];
...
glBegin(GL_TRIANGLES);
glVertex3fv(v1); glVertex3fv(v2); glVertex3fv(v3);
glVertex3fv(v1); glVertex3fv(v3); glVertex3fv(v4);
glEnd(); Each set of 3 vertices forms a triangle
• What do the triangles drawn above look like?
• How much redundant computation is happening? Model (Object) Placement  Triangle
Strips
An OpenGL triangle strip primitive reduces this redundancy by sharing vertices:
An
triangle
glBegin(GL_TRIANGLE_STRIP);
glVertex3fv(v0);
glVertex3fv(v1);
glVertex3fv(v2);
glVertex3fv(v3); v2 v0 v4 glVertex3fv(v4); v5 glVertex3fv(v5);
glEnd();
• triangle 0 is v0, v1, v2 • triangle 1 is v2, v1, v3 (why not v1, v2, v3?) • triangle 2 is v2, v3, v4 • triangle 3 is v4, v3, v5 (again, not v3, v4, v5) v1 v3 Intro to OpenGLFour Coordinate Systems
(1) Local Coordinate System
(2) World Coordinate System
(3) Viewing (Camera) Coordinate System
(4) Viewport Coordinate System (Screen or
Viewport
Window Client Region)
Window Intro to OpenGL Coordinate System
Transformation
(1) Local to World (Modeling Transformation)
We consider it as a vector transformation, and
we need programming for its configuration.
we
(2)World to Viewing (Viewing Transformation)
We consider it as a coordinate system
transformation, and we need programming for
its configuration.
its Intro to OpenGL Coordinate System
Transformation
(3) Viewing to Viewport (Projection
(3)
Transformation)
Transformation)
1) The perspective projection is a dimension
The perspective
degrading transformation. (3D to 2D)
degrading
2) Because the Aspect Ratios of the camera
Because
retinal plane and the viewport are different,
retinal
viewport
there is a scaling transformation.
scaling
Programming is needed for its configuration. Intro to OpenGL Coordinate System
Transformation
(4) Rasterization in Viewport
1)This transformation is not a typical Coordinate
1)This
system transformation. It is a sampling procedure.
system
2)This procedure is performed automatically when
2)This
projection is performed. No programming is
needed.
needed. Synthetic Camera Model and Transformation Modeling transformations – moving the model
Viewing transformations – tripod position and orientation of the
viewing volume in the world
Projection transformations – adjust the retinal plane of the camera
Viewport transformations – enlarge or reduce the physical photograph Intro to OpenGLMatrix
OpenGL uses matrices • Matrix describes current configuration of the 3D
Matrix
space (Modeling Transformation, Viewing
space Modeling
Transformation)
Transformation
• Matrix describes camera type (Projection
Projection
Transformation)
Transformation Intro to OpenGLTwo Matrix Type
MODELVIEW_MATRIX
• Use this matrix type to decide the model (object) and camera
Use
placement. (Why consider these two placement as the
placement. Why
same matrix type?)
same
• The matrix is actually 4x4 because of the homogeneous
The
coordinates.
coordinates.
• OpenGL maintains a stack of these matrices. You can control
OpenGL
this stack with the glPushMatrix and glPopMatrix commands
this Intro to OpenGLTwo Matrix Type
OpenGL preserves a similar matrix to describe
OpenGL
the camera type and this is called the
PROJECTION_MATRIX.
PROJECTION_MATRIX.
This matrix is also 4x4 matrix because of the
This homogeneous coordinate is used. Operate MODELVIEW_MATRIX
• When you decide or modify a MODELVIEW_MATRIX,
When
the following function must be used.
the
• glMatrixMode(GL_MODELVIEW);
glMatrixMode(GL_MODELVIEW); Operate MODELVIEW_MATRIX
• (1) Modeling Transformation
• glMatrixMode(GL_MODELVIEW);
glMatrixMode(GL_MODELVIEW); • glLoadIdentity(); // used before any transformation
used • glTranslatef(GLfloat x, y, z);
glTranslatef(GLfloat • glRotatef(GLfloat degrees, x, y, z);
glRotatef(GLfloat • glScalef (GLfloat x, y, z);
glScalef Modeling Transformations
glTranslate (x, y, z)
• Postmultiplies the current matrix by a matrix that moves the object by
Postmultiplies
the given x, y, and zvalues
the glRotate (theta, x, y, z)
• Postmultiplies the current matrix by a matrix that rotates the object in
Postmultiplies
a counterclockwise direction about the ray from the origin through the
point (x, y, z)
point glScale (x, y, z)
• Postmultiplies the current matrix by a matrix that stretches, shrinks,
Postmultiplies
or reflects an object along the axes
or Modeling Transformations
It is important that you understand the order in which
It
OpenGL concatenates matrices
OpenGL
glMatrixMode (MODELVIEW);
glLoadIdentity();
glMultMatrix(N);
glMultMatrix(M);
glMultMatrix(L);
glBegin(POINTS);
glVertex3f(v);
glEnd(); Modelview matrix successively contains:
I(dentity), N, NM, NML
The transformed vertex is:
NMLv = N(M(Lv)) Operate MODELVIEW_MATRIX
• (2) Viewing Transformation (Setting up camera)
• glMatrixMode(GL_MODELVIEW);
glMatrixMode(GL_MODELVIEW); • glLoadIdentity(); // used before camera placement
used • gluLookAt( eyex, eyey, eyez, aimx, aimy, aimz, upx, •
• upy, upz); // Camera Placement; Define the
// Viewing Coordinate System • eye[XYZ]: camera position in world coordinates
• look[XYZ]: a point centered in camera’s view
• up[XYZ]: a vector defining the camera’s vertical
vector Operate MODELVIEW_MATRIX
• (3)Management of the matrix
(3)Management
• glPushMatrix(), • glPopMatrix();
glPopMatrix(); • Why a matrix stack is used in opengl?
• Because a complex model may be composed of many subobjects, and some subobjects may control the movement of
objects,
the others. (Example: Robot Arm)
the Manipulating Matrix Stacks
Observation: Certain model transformations are shared
Observation:
among many models
among
We want to avoid continuously reloading the same
We
sequence of transformations
sequence
glPushMatrix ( )
• push all matrices in current stack down one level and copy topmost
push
matrix of stack
matrix glPopMatrix ( )
• pop the top matrix off the stack Matrix Manipulation  Example
Drawing a car with wheels and lugnuts
draw_wheel( );
for (j=0; j<5; j++) {
glPushMatrix ();
glRotatef(72.0*j, 0.0, 0.0, 1.0);
glTranslatef (3.0, 0.0, 0.0);
draw_bolt ( );
glPopMatrix ( );
} Matrix Manipulation  Example
draw_wheel( );
for (j=0; j<5; j++) {
glPushMatrix ();
glRotatef(72.0*j, 0.0, 0.0, 1.0);
glTranslatef (3.0, 0.0, 0.0);
draw_bolt ( );
glPopMatrix ( );} R
RT
RTv Vector Transformation (Local to World) – Bottom Up
Start Trans Rot Perspective Projection
Set projection parameters (e.g., field of view)
Typically, we use a perspective projection
perspective
• Distant objects appear smaller than near objects
Distant
• Defined by a view frustum
Defined
view Other projections: orthographic, isometric
Other
orthographic isometric Perspective Projection
In OpenGL:
In
• Projections implemented by projection matrix
Projections
projection
• gluPerspective() creates a perspective
projection matrix:
projection glSetMatrix(GL_PROJECTION);
glLoadIdentity(); //load an identity
load
matrix
matrix
gluPerspective(vfov, aspect, near,
gluPerspective(vfov,
far);
far); Perspective Projection
Parameters to
Parameters
gluPerspective():
gluPerspective()
• vfov: vertical field of view
• aspect: window width/height
• near, far: distance to near &
far distance
far clipping planes
far +Y Projection: Front/Back Rendering
Each polygon has two sides, front and back
OpenGL can render the two differently
The ordering of vertices in the list determines
The
which is the front side:
which
• When looking at the front side, the vertices go
When
front
counterclockwise
counterclockwise
– This is basically the righthand rule
– Note that this still holds after perspective projection Rasterization: Double Buffering
Avoids displaying partially rendered frame buffer
OpenGL generates one raster image while another
OpenGL
raster image is displayed on monitor
raster
glxSwapBuffers (Display *dpy, Window w)
glutSwapBuffers (void) 3D Geometric Primitives Pipeline (for direct illumination)
Rendering
3D
Modeling
Transformation
Lighting Transform into 3D world coordinate system
Illuminate according to lighting and reflectance Viewing
Transformation Transform into 3D camera coordinate system Projection
Transformation Transform into 2D screen coordinate system Clipping
Scan
Conversion
Image Clip primitives outside camera’s view
Draw pixels (includes texturing, hidden surface, ...) OpenGL Example – Complete Camera
Setting up
void SetUpViewing()
{
// The viewport transformation
glViewport( 0, 0, window_width, window_height );
// Set up camera>screen transformation
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
gluPerspective( 60, 1, 1, 1000 ); // fov, aspect, near, far
// Set up the model>camera transformation
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
gluLookAt( 3, 3, 2, // eye point
0, 0, 0, // look at point
0, 0, 1 ); // up vector
glRotatef( theta, 0, 0, 1 ); // rotate the model
glScalef( zoom, zoom, zoom ); // scale the model
glBegin();
……
glEnd();
} Learn OpenGL by example
robot.c from the OpenGL Programming Guide Learn OpenGL by example
Two bodies
• Upper arm
• Lower arm Major tasks
• Position
• Orientation Learn OpenGL by example
Body Placement:
Both bodies originally at origin (World
Both
Coordinate)
Coordinate) Learn OpenGL by example
Headers
#include <GL/gl.h>
#include
#include <GL/glu.h>
#include <GL/glut.h> Learn OpenGL by example
int main(int argc, char** argv) {
int
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE  GLUT_RGB);
glutInitDisplayMode
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutInitWindowPosition
glutCreateWindow (argv[0]);
init ();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutKeyboardFunc(keyboard);
glutMainLoop();
return 0; }
return Learn OpenGL by example
void init(void) {
void
glClearColor (0.0, 0.0, 0.0, 0.0);
glShadeModel (GL_FLAT);
} Learn OpenGL by example
void display(void){
void
glClear (GL_COLOR_BUFFER_BIT);
glPushMatrix();
glTranslatef (1.0, 0.0, 0.0);
glRotatef ((GLfloat) shoulder, 0.0, 0.0, 1.0);
glTranslatef (1.0, 0.0, 0.0);
glPushMatrix();
glScalef (2.0, 0.4, 1.0);
glutWireCube (1.0);
glPopMatrix();
Continued…
Continued… Upper Arm Placement
Y
Translate Rotate
Translate
X
Scaling Learn OpenGL by example
glTranslatef (1.0, 0.0, 0.0);
glTranslatef
glRotatef ((GLfloat) elbow, 0.0, 0.0, 1.0);
glTranslatef (1.0, 0.0, 0.0);
glPushMatrix();
glScalef (2.0, 0.4, 1.0);
glutWireCube (1.0);
glPopMatrix();
glPopMatrix();
glutSwapBuffers();
glutSwapBuffers();
} Lower Arm Placement
Translate
Y Rotate
Rotate Upper Arm Translate
Translate Translate
X
Scaling OpenGL: Conventions
Functions in OpenGL start with gl
gl
Functions
• Most functions just gl (e.g., glColor())
Most
glColor()
• Functions starting with glu are utility functions (e.g.,
Functions
glu
gluLookAt())
gluLookAt()
• Functions starting with glx are for interfacing with the X
Functions
glx
Windows system (e.g., in gfx.c)
Windows OpenGL: Conventions
Function names indicate argument type and number
• Functions ending with f take floats
Functions
• Functions ending with i take ints
Functions
• Functions ending with b take bytes
Functions
• Functions ending with ub take unsigned bytes
Functions
ub
• Functions that end with v take an array.
Functions Examples
• glColor3f() takes 3 floats
• glColor4fv() takes an array of 4 floats
glColor4fv() takes OpenGL: Conventions
Variables written in CAPITAL letters
• Example: GLUT_SINGLE, GLUT_RGB
• usually constants
• use the bitwise or command (x  y) to combine constants ...
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 Spring '10
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