OpenGL: How to resolve the lighting problem of Cube? - c++

Question

// Vertices information
GLfloat vertices[] = { 1, 1, 1, -1, 1, 1, -1,-1, 1, 1,-1, 1, // (front)
1, 1, 1, 1,-1, 1, 1,-1,-1, 1, 1,-1, // (right)
1, 1, 1, 1, 1,-1, -1, 1,-1, -1, 1, 1, // (top)
-1, 1, 1, -1, 1,-1, -1,-1,-1, -1,-1, 1, // (left)
-1,-1,-1, 1,-1,-1, 1,-1, 1, -1,-1, 1, // (bottom)
1,-1,-1, -1,-1,-1, -1, 1,-1, 1, 1,-1 }; // (back)
// Normal information
GLfloat normals[72] = { 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 1, // (front)
1, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, // (right)
0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, // (top)
-1, 0, 0, -1, 0, 0, -1, 0, 0, -1, 0, 0, // (left)
0,-1, 0, 0,-1, 0, 0,-1, 0, 0,-1, 0, // (bottom)
0, 0,-1, 0, 0,-1, 0, 0,-1, 0, 0,-1 }; // (back)
// drawing cube
glEnableClientState(GL_NORMAL_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glNormalPointer(GL_FLOAT, 0, normals);
glVertexPointer(3, GL_FLOAT, 0, vertices);
glDrawElements(GL_TRIANGLES, 36, GL_UNSIGNED_BYTE, indices);
glPopMatrix();
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_NORMAL_ARRAY);
I want to make sure there is no color difference between the two sides.
Is there a solution for this?

Answer 1

The issue is causes, because you've activated the flat shading mode (see Detecting legacy OpenGL and glShadeModel).
Thus the light is computed for the vertices of the triangle primitive, but the color of the triangle is determined by the Provoking vertex.
Switch to the the smooth (GL_SMOOTH) shading model and ensure that the light is diffuse (GL_DIFFUSE parameter - see glLight):
glShadeModel(GL_SMOOTH);
See the difference between shading mode GL_FLAT (left) and GL_SMOOTH (right):
Another option would be to render GL_QUADS primitives instead of GL_TRIANGLES primitives:
glDrawElements(GL_TRIANGLES, 36, GL_UNSIGNED_INT, indices)
glDrawArrays(GL_QUADS, 0, 24);
Again, GL_FLAT (left) and GL_SMOOTH (right):