Faces missing when drawing icosahedron in OpenGL following code in redBook

问题

I am attempting to draw an icosahedron following this popular OpenGl tutorial in the redBook.

I am using GLUT to handle windowing.

Here is my complete code. It is mostly the code from the tutorial plus some clerical work using GLUT

#include <stdio.h>
#include <GL/glut.h>
#define X .525731112119133606
#define Z .850650808352039932

void mouseEventHandler(int button, int state, int x, int y){
}

void display() {
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    static GLfloat vdata[12][3] = {
        {-X,0.0,Z}, {X,0.0,Z}, {-X,0.0,-Z}, {X,0.0,-Z},
        {0.0,Z,X}, {0.0,Z,-X}, {0.0,-Z,X}, {0.0,-Z,-X},
        {Z,X,0.0}, {-Z,X,0.0}, {Z,-X,0.0}, {-Z,-X,0.0},
    };

    static GLuint tindices[20][3] = { 
        {0,4,1}, {0,9,4}, {9,5,4}, {4,5,8}, {4,8,1},    
        {8,10,1}, {8,3,10}, {5,3,8}, {5,2,3}, {2,7,3},    
        {7,10,3}, {7,6,10}, {7,11,6}, {11,0,6}, {0,1,6}, 
        {6,1,10}, {9,0,11}, {9,11,2}, {9,2,5}, {7,2,11} };

    int i;

    glBegin(GL_TRIANGLES);
    for (i = 0; i < 20; i++){
        glNormal3fv(&vdata[tindices[i][0]][0]);
        glVertex3fv(&vdata[tindices[i][0]][0]);
        glNormal3fv(&vdata[tindices[i][1]][0]);
        glVertex3fv(&vdata[tindices[i][1]][0]);
        glNormal3fv(&vdata[tindices[i][2]][0]);
        glVertex3fv(&vdata[tindices[i][2]][0]);
    }
    glEnd();
    glFlush ( );
}

void windowSetup(){
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);

    glutInitWindowPosition(80, 80);
    glutInitWindowSize(1000,1000);

    glutCreateWindow("OpenGL Ico");

    glClear(GL_COLOR_BUFFER_BIT);
    glMatrixMode( GL_MODELVIEW);
    glLoadIdentity();           
    gluOrtho2D( -2.0, 2.0, -2.0, 2.0 );
}

int main(int argc, char** argv) {

    glutInit(&argc, argv);
    windowSetup();

    glutDisplayFunc(display);
    glutMouseFunc(&mouseEventHandler);
    glutMainLoop();
}

This is my output: This is very different from the expected output:

Does someone know why these differ so much?

The differences seem to be:

My icosahedron is missing faces
My icosahedron is being viewed from a different angle
My icosahedron is lit differently

The first one is the most pressing. I have noticed when I change glMatrixMode( GL_MODELVIEW); to glMatrixMode( GL_PROJECTION); the faces that aren't showing up appear and those that are currenty appearing disappear. Does anybody know why this could be?

回答1:

missing faces

most likely you just have wrong order of indices. In such case Reversing them will solve the issue. To check this you can try:
```
glDisable(GL_CULL_FACE);
```
if problem disappears I am right. If not it is different thing (like too close to camera cutting by Z_NEAR but that would look a bit different).

To identify the correct face you can use glColor based on i for exaple
```
if (i==5) glColor3f(1.0,0.0,0.0); else glColor3f(1.0,1.0,1.0);
```
the red face would be the 6th in this case {8,3,10}
lighting

You are using vertex coordinates as normals so do not expect FLAT shading. Also I do not see you are setting any lights here (but that can be hidden in GLUT somewhere I do not use it). To remedy this use just single normal per triangle. so average the 3 normals you got and make an unit vector from that and use that (before first glVertex call of each triangle).
orientation

just rotate your GL_MODELVIEW to desired orientation. Standard perspective GL_PROJECTION has z axis as viewing direction and x,y axises matches the screen (while GL_MODELVIEW is unit)

[Edit1] I tried your code

So the problem is you got reverse order of indices then default polygon winding in OpenGL (at least in my environment) and wrong normals here fixed code:

glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
const GLfloat vdata[12][3] =
    {
    {-X,0.0,Z}, {X,0.0,Z}, {-X,0.0,-Z}, {X,0.0,-Z},
    {0.0,Z,X}, {0.0,Z,-X}, {0.0,-Z,X}, {0.0,-Z,-X},
    {Z,X,0.0}, {-Z,X,0.0}, {Z,-X,0.0}, {-Z,-X,0.0},
    };

const GLuint tindices[20][3] =
    {
    {0,4,1}, {0,9,4}, {9,5,4}, {4,5,8}, {4,8,1},
    {8,10,1}, {8,3,10}, {5,3,8}, {5,2,3}, {2,7,3},
    {7,10,3}, {7,6,10}, {7,11,6}, {11,0,6}, {0,1,6},
    {6,1,10}, {9,0,11}, {9,11,2}, {9,2,5}, {7,2,11}
    };

int i;
GLfloat nx,ny,nz;

glEnable(GL_CULL_FACE);
glFrontFace(GL_CW);
glBegin(GL_TRIANGLES);
for (i = 0; i < 20; i++)
    {
    nx =vdata[tindices[i][0]][0];
    ny =vdata[tindices[i][0]][1];
    nz =vdata[tindices[i][0]][2];
    nx+=vdata[tindices[i][1]][0];
    ny+=vdata[tindices[i][1]][1];
    nz+=vdata[tindices[i][1]][2];
    nx+=vdata[tindices[i][2]][0]; nx/=3.0;
    ny+=vdata[tindices[i][2]][1]; ny/=3.0;
    nz+=vdata[tindices[i][2]][2]; nz/=3.0;
    glNormal3f(nx,ny,nz);
    glVertex3fv(vdata[tindices[i][0]]);
    glVertex3fv(vdata[tindices[i][1]]);
    glVertex3fv(vdata[tindices[i][2]]);
    }
glEnd();

And preview:

it is a screenshot and my object is rotating so do not expect correct orientation you expect.

来源：https://stackoverflow.com/questions/43107006/faces-missing-when-drawing-icosahedron-in-opengl-following-code-in-redbook

标签

opengl

glut