programming with opengl part 2: complete...
TRANSCRIPT
291.427 Computer Graphics I, Fall 2008 2
Objectives
•Refine first program Alter default values
Introduce standard program structure
•Simple viewing 2-D viewing as special case of 3-D viewing
•Fundamental OpenGL primitives
•Attributes
391.427 Computer Graphics I, Fall 2008 3
Program Structure
•Most OpenGL programs have similarstructure
•Consists of following functions-main():
• defines callback functions• opens one or more windows with required properties• enters event loop (last executable statement)
-init(): sets state variables• Viewing• Attributes
callbacks• Display function• Input and window functions
491.427 Computer Graphics I, Fall 2008 4
simple.c revisited
•In this version, see same output, but
•defined all relevant state valuesthrough function calls using defaultvalues
•In particular, we set Colors
Viewing conditions
Window properties
591.427 Computer Graphics I, Fall 2008 5
main.c
#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();}
includes gl.h
define window properties
set OpenGL state
enter event loop
display callback
691.427 Computer Graphics I, Fall 2008 6
GLUT functions
•glutInit allows application to get command linearguments and initializes system
•gluInitDisplayMode requests properties for window(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
791.427 Computer Graphics I, Fall 2008 7
init.c
void init(){glClearColor (0.0, 0.0, 0.0, 1.0);
glColor3f(1.0, 1.0, 1.0);
glMatrixMode (GL_PROJECTION);glLoadIdentity ();glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
}
black clear coloropaque window
fill/draw with white
viewing volume
891.427 Computer Graphics I, Fall 2008 8
Coordinate Systems
•Units in glVertex determined by application
•object or problem coordinates
•Viewing specifications also in object coordinates
•size of viewing volume determines what will appearin image
•Internally, OpenGL convert to camera (eye)coordinates
•and later to screen coordinates
•OpenGL also uses some internal representations
•usually not visible to application
991.427 Computer Graphics I, Fall 2008 9
OpenGL Camera
•OpenGL places cameraat origin in objectspace pointing innegative z direction
•default viewing volume
= box centered at
origin with side of
length 2
1091.427 Computer Graphics I, Fall 2008 10
Orthographic Viewing
z=0
z=0
In default orthographic view, points areprojected forward along z axis ontoplane z = 0
1191.427 Computer Graphics I, Fall 2008 11
Transformations and Viewing
•In OpenGL, projection carried out by projectionmatrix (transformation)
•only one set of transformation functions ==> mustset matrix mode first
glMatrixMode (GL_PROJECTION)
• Transformation functions incremental ==>•start with identity matrix•alter it with projection matrix that gives view
volume glLoadIdentity();glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
1291.427 Computer Graphics I, Fall 2008 12
Two- and three-dim viewing
•In glOrtho(left, right, bottom, top,near, far) near and far distances measuredfrom camera
•2-D vertex commands place all vertices in planez = 0
•If application is in 2-D, can use function gluOrtho2D(left, right,bottom,top)
•In 2-D view or clipping volume becomes clippingwindow
1391.427 Computer Graphics I, Fall 2008 13
mydisplay.c
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();
}
1491.427 Computer Graphics I, Fall 2008 14
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
1591.427 Computer Graphics I, Fall 2008 15
Polygon Issues
•OpenGL only display correctly polygons thatare Simple: edges cannot cross
Convex: All points on segment also in polygon
Flat: all vertices in same plane (planar)
•User program can check if above true OpenGL produce output if conditions violated
• but may not be what desired
•Triangles satisfy all conditions
nonsimple polygon nonconvex polygon
1691.427 Computer Graphics I, Fall 2008 16
Attributes
•Attributes are part of OpenGL state
•determine appearance of objects Color (points, lines, polygons)
Size and width (points, lines)
Stipple pattern (lines, polygons)
Polygon mode•Display as filled: solid color or stipplepattern
•Display edges
•Display vertices
1791.427 Computer Graphics I, Fall 2008 17
RGB color
•Each color component stored separately inframe buffer
•Usually 8 bits per component in buffer•Note in glColor3f color values range from0.0 (none) to 1.0 (all), whereas inglColor3ub values range from 0 to 255
1891.427 Computer Graphics I, Fall 2008 18
Indexed Color
•Colors = indices into tables of RGB values
•Requires less memory indices usually 8 bits
not as important now•Memory inexpensive
•Need more colors for shading
1991.427 Computer Graphics I, Fall 2008 19
Color and State
•color as set by glColor becomes part of state•will be used until changed Colors and other attributes not part of object but assigned when object rendered
•can create conceptual vertex colors by code such as glColor glVertex glColor glVertex
2091.427 Computer Graphics I, Fall 2008 20
Smooth Color
•Default is smooth shading OpenGL interpolates vertex colors acrossvisible polygons
•Alternative is flat shading Color of first vertex
determines fill color
•glShadeModel(GL_SMOOTH)or GL_FLAT