CSC Graphics Programming Budditha Hettige Department of Statistics and Computer Science

Graphics Programming OpenGL 3D Drawing 2

3D Graphics Projections – Getting 3D to 2D 3

Projections Orthographic Projection Perspective Projection 4 Orthographic Perspective

Orthographic Projections glOrtho ( double left, double right, double bottom, double top, double near, double far); 5

Perspective Projection gluPerspective ( double angle, double aspect, double near, double far); 6

Draw a 3D Model Object Composition 7 3D Model Polygon

Object Composition Use Transformation – glPushMatrix ( ); – glPopMatrix ( ); –Save the current transformation state and then restore it after some objects have been placed

The Matrix Stack Draw a car use transformation – The car body and four wheeles The Car Body Wheele 1 Wheele 2 Wheele 3 Wheele 4 Transformation state

Hidden Surface Removal Enable Depth Testing – glEnable (GL_DEPTH_TEST); 10 Hidden Surface

Drawing 3D Objects Cube –void glutWireCube(GLdouble size); –void glutSolidCube(GLdouble size); Sphere –void glutWireSphere(GLdouble radius, GLint slices, GLint stacks); –void glutSolidSphere(GLdouble radius, GLint slices, GLint stacks); Cone 11

12 Three-dimensional Applications In OpenGL, two-dimensional applications are a special case of three-dimensional graphics –Not much changes –Use glVertex3*( ) –Have to worry about the order in which polygons are drawn or use hidden-surface removal –Polygons should be simple, convex, flat

13 Sierpinski Gasket (2D) Start with a triangle Connect bisectors of sides and remove central triangle Repeat Example : Gasket.cpp

14 Moving to 3D We can easily make the program three- dimensional by using typedef Glfloat point3[3] glVertex3f glOrtho But that would not be very interesting Instead, we can start with a tetrahedron

15 3D Gasket We can subdivide each of the four faces We can easily make the program three-dimensional by using typedef Glfloat point3[3] glVertex3f glOrtho

16 Example After 4 interations

17 triangle code void triangle( point a, point b, point c) {// right-hand rule glBegin(GL_POLYGON); glVertex3fv(a); glVertex3fv(b); glVertex3fv(c); glEnd(); }

18 subdivision code void divide_triangle(point a, point b, point c, int m) { point v1, v2, v3; int j; if(m>0) { for(j=0; j<3; j++) v1[j]=(a[j]+b[j])/2; for(j=0; j<3; j++) v2[j]=(a[j]+c[j])/2; for(j=0; j<3; j++) v3[j]=(b[j]+c[j])/2; divide_triangle(a, v1, v2, m-1); divide_triangle(c, v2, v3, m-1); divide_triangle(b, v3, v1, m-1); } else(triangle(a,b,c)); }

19 tetrahedron code void tetrahedron( int m) { glColor3f(1.0,0.0,0.0); divide_triangle(v[0], v[1], v[2], m); glColor3f(0.0,1.0,0.0); divide_triangle(v[3], v[2], v[1], m); glColor3f(0.0,0.0,1.0); divide_triangle(v[0], v[3], v[1], m); glColor3f(0.0,0.0,0.0); divide_triangle(v[0], v[2], v[3], m); }

20 Problem Because the triangles are drawn in the order they are defined in the program, the front triangles are not always rendered in front of triangles behind them get this want this

21 Solution: Hidden-Surface Removal We want to see only those surfaces in front of other surfaces OpenGL uses a hidden-surface method called the z-buffer algorithm that saves depth information as objects are rendered so that only the front objects appear in the image

Locating the camera 22 Position and orient the camera – three inputs Eye, at, and direction – up Camera is on Model-View mode

Default Camera Settings With default, camera is located at the origin and oriented at the negative z-axes If place a cube at the origin, only one side of the cube is visible Look at objects from different angles –Move the objects –Move the camera Example – a cube Glu function void gluLookAt(eyex, eyey, eyez, atx, aty, atz, upx, upy, upz) gluLookAt(1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0); 23

24 Code: Display a cube void display() { glClear(GL_COLOR_BUFFER_BIT); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); gluLookAt(1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0); glutWireCube(1.0f); glFlush(); }