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Why am I getting alpha faces?

 
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Why am I getting alpha faces?
RicardoDiaz
12 Aug, 2007 - 09:30 PM
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Hi, I followed a tutorial and modified it a bit to display a rotating 3D triangle. The first face comes perfect, but the other three appear to have some transparency. Why is this? Here's the code (only the part that draws the pyramid):

CODE
glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    glTranslatef(0.0, 0.0, -6.0);
    glRotatef(rtri, 0.0, 1.0, 0.0);
    glBegin(GL_TRIANGLES);                        
        glColor3f(1.0, 0.0, 0.0);                
        glVertex3f(0.0, 1.0, 0.0);
                
        glColor3f(0.0, 1.0, 0.0);
        glVertex3f(-1.0, -1.0, 1.0);
                
        glColor3f(0.0, 0.0, 1.0);    
        glVertex3f(1.0, -1.0, 1.0);
        
        /*------------------------*/
        
        glColor3f(1.0, 0.0, 0.0);    
        glVertex3f(0.0, 1.0, 0.0);
        
        glColor3f(0.0, 0.0, 1.0);            
        glVertex3f( 1.0, -1.0, 1.0);
        
        glColor3f(0.0, 1.0, 0.0);
        glVertex3f(1.0, -1.0, -1.0);
        
        /*------------------------*/
        
        glColor3f(1.0, 0.0, 0.0);    
        glVertex3f(0.0, 1.0, 0.0);
        
        glColor3f(0.0, 1.0, 0.0);            
        glVertex3f( 1.0, -1.0, -1.0);
        
        glColor3f(0.0, 0.0, 1.0);
        glVertex3f(-1.0, -1.0, -1.0);
        
        /*------------------------*/
        
        glColor3f(1.0, 0.0, 0.0);    
        glVertex3f(0.0, 1.0, 0.0);
        
        glColor3f(0.0, 0.0, 1.0);            
        glVertex3f( -1.0, -1.0, -1.0);
        
        glColor3f(0.0, 1.0, 0.0);
        glVertex3f(-1.0, -1.0, 1.0);
    glEnd();
    glutSwapBuffers();
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RicardoDiaz
RE: Why Am I Getting Alpha Faces?
13 Aug, 2007 - 04:15 AM
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Oh, is it because of the depths? How can I fix this?
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1lacca
RE: Why Am I Getting Alpha Faces?
13 Aug, 2007 - 05:33 AM
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Could you post the OGL initialization codes as well (just to see if blending is enabled, and things like that)?
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RicardoDiaz
RE: Why Am I Getting Alpha Faces?
13 Aug, 2007 - 07:45 AM
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Here's the entire code:

CODE
#include <GLUT/glut.h>

#define windowWidth    550
#define windowHeight 400

GLvoid initGL(GLvoid);
GLvoid display(GLvoid);
GLvoid reshape(int width, int height);
GLvoid idle(GLvoid);

GLfloat    rtri;

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize(windowWidth, windowHeight);
    glutInitWindowPosition (0, 44);
    glutCreateWindow ("GLUT Display");
    
    initGL();

    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutIdleFunc(idle);
    
    glutMainLoop();
    
    rtri = 0;
    return 0;
}

GLvoid initGL(GLvoid)
{
    glClearColor(1.0, 1.0, 1.0, 0.0);
    glClearDepth(1.0);                    
    glDepthFunc(GL_LESS);
    glEnable(GL_DEPTH | GL_DOUBLE | GLUT_RGB);
    glShadeModel(GL_SMOOTH);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45.0, (GLfloat)windowWidth/(GLfloat)windowHeight, 0.1, 100.0);
    glMatrixMode(GL_MODELVIEW);
}

GLvoid idle(GLvoid)
{
    rtri += 0.1;
    glutPostRedisplay();
}

GLvoid display(GLvoid)
{    
    glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    glTranslatef(0.0, 0.0, -6.0);
    glRotatef(rtri, 0.0, 1.0, 0.0);
    glBegin(GL_TRIANGLES);                        
        glColor3f(1.0, 0.0, 0.0);                
        glVertex3f(0.0, 1.0, 0.0);
                
        glColor3f(0.0, 1.0, 0.0);
        glVertex3f(-1.0, -1.0, 1.0);
                
        glColor3f(0.0, 0.0, 1.0);    
        glVertex3f(1.0, -1.0, 1.0);
        
        /*------------------------*/
        
        glColor3f(1.0, 0.0, 0.0);    
        glVertex3f(0.0, 1.0, 0.0);
        
        glColor3f(0.0, 0.0, 1.0);            
        glVertex3f( 1.0, -1.0, 1.0);
        
        glColor3f(0.0, 1.0, 0.0);
        glVertex3f(1.0, -1.0, -1.0);
        
        /*------------------------*/
        
        glColor3f(1.0, 0.0, 0.0);    
        glVertex3f(0.0, 1.0, 0.0);
        
        glColor3f(0.0, 1.0, 0.0);    
        glVertex3f( 1.0, -1.0, -1.0);
        
        glColor3f(0.0, 0.0, 1.0);
        glVertex3f(-1.0, -1.0, -1.0);
        
        /*------------------------*/
        
        glColor3f(1.0, 0.0, 0.0);
        glVertex3f(0.0, 1.0, 0.0);
        
        glColor3f(0.0, 0.0, 1.0);            
        glVertex3f( -1.0, -1.0, -1.0);
        
        glColor3f(0.0, 1.0, 0.0);
        glVertex3f(-1.0, -1.0, 1.0);
    glEnd();
    glutSwapBuffers();
}

GLvoid reshape(int width, int height)
{
    glViewport (0, 0, width, height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45.0, (GLfloat)width/(GLfloat)height, 0.1, 100.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}
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Nova Dragoon
RE: Why Am I Getting Alpha Faces?
13 Aug, 2007 - 08:05 AM
Post #5

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I think you are drawing that transparent triangle in the wrong order, I think the correct way is drawing them clock wise, the visible face then points towards you.
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RicardoDiaz
RE: Why Am I Getting Alpha Faces?
13 Aug, 2007 - 08:23 AM
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Well, every face was drawn clockwise, I don't know if hthat's what you mean. But the triangle rotates, how can I change which face points twards me?
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serializer
RE: Why Am I Getting Alpha Faces?
15 Aug, 2007 - 08:17 PM
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QUOTE(RicardoDiaz @ 13 Aug, 2007 - 09:23 AM) *

Well, every face was drawn clockwise, I don't know if hthat's what you mean. But the triangle rotates, how can I change which face points twards me?


Draw it twice; once clockwise, once anticlockwise; that way, you will always be able to see one of the faces...

--serializer
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