Computer Graphic Opengl

Question Description


In this assignment, you are going to create a cube object with applying transparency effect. To initiatetransparency, you need to define the following codes.

glEnable(GL_BLEND); //Enable alphablend blendingglBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA); //Set the blend function

Your cube object needs to be rotated when the user presses ‘+’ or ‘-‘ keyboard button ( ‘+’: counter-clock-wise rotation, ‘-‘ clock-wise rotation.) Also, you need to support two keyboard buttons ‘<’ and ‘>’.‘<’ is for decreasing the alpha blending value by 0.01 and ‘>’ is for increasing it by 0.01.

The default value for the alpha blending should be 0.6. When the user presses the button ‘>’, the valueis continuously increased by 0.01 up to 1.0.
(HINT: use assignment 3-2 and reference Chapter 5 alpha blending)

Figure 1 Rotation examples when the user presses ‘‘ button.

Figure 2. When the alpha blending is 1.0, you are not able to see lines (left). But, if the value is changed closer to 0.0, you can seebackground lines (right).



#ifdef __APPLE_CC__ #include <GLUT/glut.h> #else #include <GL/freeglut.h> #endif int angledirection = 0; // Clears the window and draws the tetrahedron. The tetrahedron is easily // specified with a triangle strip, though the specification really isn’t very // easy to read. void display() { // We rotate 2 degree * angledirection around y so we can see a lot of the left side. glRotatef(2.0*angledirection, 0, 1, 0); // clear frame buffer glClear(GL_COLOR_BUFFER_BIT); // Draw a white grid “floor” for the tetrahedron to sit on. glColor3f(1.0, 1.0, 1.0); glBegin(GL_LINES); for (GLfloat i = -2.5; i <= 2.5; i += 0.25) { glVertex3f(i, 0, 2.5); glVertex3f(i, 0, -2.5); glVertex3f(2.5, 0, i); glVertex3f(-2.5, 0, i); }


// Draw the tetrahedron. It is a four sided figure, so when defining it // with a triangle strip we have to repeat the last two vertices. glBegin(GL_TRIANGLE_STRIP); glColor3f(1, 1, 1); glVertex3f(0, 2, 0); glColor3f(1, 0, 0); glVertex3f(-1, 0, 1); glColor3f(0, 1, 0); glVertex3f(1, 0, 1); glColor3f(0, 0, 1); glVertex3f(0, 0, -1); glColor3f(1, 1, 1); glVertex3f(0, 2, 0); glColor3f(1, 0, 0); glVertex3f(-1, 0, 1);



// Sets up global attributes like clear color and drawing color, enables and // initializes any needed modes (in this case we want backfaces culled), and // sets up the desired projection and modelview matrices. It is cleaner to // define these operations in a function separate from main().

void init() {

// Set the current clear color to sky blue and the current drawing color to// white.
glClearColor(0.1, 0.39, 0.88, 1.0);
glColor3f(1.0, 1.0, 1.0);

// Tell the rendering engine not to draw backfaces. Without this code, // all four faces of the tetrahedron would be drawn and it is possible // that faces farther away could be drawn after nearer to the viewer. // Since there is only one closed polyhedron in the whole scene, // eliminating the drawing of backfaces gives us the realism we need. // THIS DOES NOT WORK IN GENERAL. glEnable(GL_CULL_FACE); glCullFace(GL_BACK); // Set the camera lens so that we have a perspective viewing volume whose // horizontal bounds at the near clipping plane are -2..2 and vertical // bounds are -1.5..1.5. The near clipping plane is 1 unit from the camera // and the far clipping plane is 40 units away. glMatrixMode(GL_PROJECTION); glLoadIdentity(); glFrustum(-2, 2, -1.5, 1.5, 1, 40); // Set up transforms so that the tetrahedron which is defined right at // the origin will be rotated and moved into the view volume. // We rotate 45 degrees around x to “drop” the top of the pyramid // down a bit. Then we move the object back 3 units “into the screen”. glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glTranslatef(0, 0, -3); glRotatef(45, 1, 0, 0);


void keyboard(unsigned char key, int x, int y) { switch(key) { } glutPostRedisplay();


// Initializes GLUT, the display mode, and main window; registers callbacks; // does application initialization; enters the main event loop. int main(int argc, char** argv) { glutInit(&argc, argv); glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); glutInitWindowPosition(80, 80); glutInitWindowSize(800, 600); glutCreateWindow(“A Simple Tetrahedron”); glutDisplayFunc(display); glutKeyboardFunc(keyboard); init(); glutMainLoop();


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