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I'm trying to render an object (say cube) with OpenGL 1.1 (I know that doesn't makes sense nowadays, but I've to use this). Everything works fine until I try some lighting.

Here's the problem:

Screenshot

The Global variable set are:

static GLfloat light_position[] = {1.0, 1.0, 2*cZ.x , 0.0};   
// cZ.x is the minimum z of the mesh.  I know 
// this is at infinity, but don't work also with w=1.0

In the main function:

...
    glMatrixMode(GL_MODELVIEW);     // Select The Modelview Matrix
    glLoadIdentity();               // Reset The Modelview Matrix
    glEnable(GL_LIGHT0);
    glEnable(GL_LIGHTING);
    glShadeModel(GL_SMOOTH);        // Enable Smooth Shading

....

Drawing a mesh k

// While drawing mesh k 

        GLfloat light_ambient[] = {COLOUR[k][0], COLOUR[k][1], COLOUR[k][2], 1.0}; 
        GLfloat light_diffuse[] = {COLOUR[k][0], COLOUR[k][1], COLOUR[k][2], 1.0};  
        glLightfv(GL_LIGHT0, GL_POSITION, light_position);
                glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
        glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);


....
        //This is a mesh, so will be drawn using triangles
        glBegin(GL_TRIANGLES);
        //Triangles will be defined by vertex indices in faces
        for (unsigned int i = 0; i<mesh->faces.size(); i++){

            int index1 = mesh->faces.at(i).x;
            int index2 = mesh->faces.at(i).y;
            int index3 = mesh->faces.at(i).z;
            glNormal3f(mesh->normals.at(i).x,mesh->normals.at(i).y,mesh->normals.at(i).z);
            glVertex3f(mesh->vertices.at(index1).x, mesh->vertices.at(index1).y, mesh->vertices.at(index1).z);
            glVertex3f(mesh->vertices.at(index2).x, mesh->vertices.at(index2).y, mesh->vertices.at(index2).z);
            glVertex3f(mesh->vertices.at(index3).x, mesh->vertices.at(index3).y, mesh->vertices.at(index3).z);
        }
        glEnd();

....

Whereas the normal are computed as: 

    glm::vec3 currFace = m->faces.at(faceIndex);
    glm::vec3 vert1 = m->vertices.at(currFace.x);
    glm::vec3 vert2 = m->vertices.at(currFace.y);
    glm::vec3 vert3 = m->vertices.at(currFace.z);

    glm::vec3 side1 = (vert2 - vert1);
    glm::vec3 side2 = (vert3 - vert1);

    glm::vec3 normal = glm::cross(side1, side2);

    normal = glm::normalize(normal);

I'm really struggling to understand what's wrong, can you point me in the right direction?

EDIT: This happens similarly with the standford bunny (taken from standford repo, so it's well formed)

https://i.stack.imgur.com/y1sCU.jpg

JTulip
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  • most probably your normal computations are wrong – Abhishek Bansal Mar 27 '14 at 18:34
  • @AbhishekBansal I've included my normal calculations. Are they wrong? – JTulip Mar 27 '14 at 19:23
  • Also, plotting them they seems alright – JTulip Mar 27 '14 at 19:28
  • I cant go through whole code but there seem to be two possibilities. Either triangles themselves are wrong or normals are wrong. – Abhishek Bansal Mar 28 '14 at 04:13
  • Your normal calculation code is correct only as long as all triangles maintain the same vertex order (counterclockwise, in your case). If vertex order isn't the same for all triangles - you'll get flipped normal (normal that points in backward direction). It is impossible to answer without actual mesh data. Your mesh is simple, just get different sides separately and check that normal is the same. – keltar Mar 28 '14 at 13:06
  • @keltar The same issue (less evident) is showing when using a model from standford repository (bunny). Shouldn't it be with all the faces in same order? – JTulip Mar 29 '14 at 01:21
  • @AbhishekBansal I've plotted the normal here: http://imgur.com/FVjRLpd seems alright (some of them seem skewed, but it's because the mesh is transparent-like so what seems deviated are normals for other faces.) – JTulip Mar 29 '14 at 01:23
  • @keltar As I said, this happens with a ply from standford repo (added a picture in the post). Shouldn't the vertices be all in same order? – JTulip Mar 29 '14 at 01:41

1 Answers1

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Looking at your normals picture it looks like that more than not being shaded, some of your object faces are transparent.

I used to have a similar problem while learning OpenGL, in my case I forgot to enable DEPTH_TEST. You can do it simply adding this line to your GL init function function:

 glEnable(GL_DEPTH_TEST);

Give it a try

cifz
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