How to implement flat shading in OpenGL without duplicate vertices? - opengl

Question

I am trying to render 3D prisms in LWJGL OpenGL with flat shading. For example, I have a cube indexed as following:
I only have 8 vertices in the vertex buffer, which I have indexed as above. Is there any way to implement flat normal shading on the cube such as below? I don't want to rewrite my vertex and index buffers to include duplicate vertices if possible.

Answer 1

If you don't need any other attributes (e.g. texture coordinates), then there is an option to create a cube mesh with face normal vectors, by 8 vertices only. Use the flat Interpolation qualifier for the normal vector.
Vertex shader:
flat out vec3 surfaceNormal;
Fragment sahder:
flat out vec3 surfaceNormal;
When the flat qualifier is used, then the output of the vertex shader will not be interpolated. The value given to the fragment shader is one of the attributes associated to one vertex of the primitive, the Provoking vertex.
For a GL_TRINANGLE primitive this is either the last or the first vertex. That can be chosen by glProvokingVertex.
Choose the first vertex:
glProvokingVertex(GL_FIRST_VERTEX_CONVENTION);
For the order of the points of your cube mesh (image in the question)
front back
1 3 7 5
+---+ +---+
| | | |
+---+ +---+
0 2 6 4
you have to setup the following vertex coordinates and normal vectors:
// x y z nx, ny, nz
-1, -1, -1, 0, -1, 0, // 0, nv front
-1, -1, 1, 0, 0, 1, // 1, nv top
1, -1, -1, 0, 0, 0, // 2
1, -1, 1, 1, 0, 0, // 3, nv right
1, 1, -1, 0, 1, 0, // 4, nv back
1, 1, 1, 0, 0, 0, // 5
-1, 1, -1, 0, 0, -1, // 6, nv bottom
-1, 1, 1, -1, 0, 0, // 7, nv left
Define the indices in that way, that the vertices 7, 3, 0, 4, 6, 1 are the first vertex for both triangles of the left, right, front, back, bottom and top of the cube:
0, 2, 3, 0, 3, 1, // front
4, 6, 7, 4, 7, 5, // back
3, 2, 4, 3, 4, 5, // right
7, 6, 0, 7, 0, 1, // left
6, 4, 2, 6, 2, 0, // bottom
1, 3, 5, 1, 5, 7 // top
Draw 12 triangle primitives. e.g:
glDrawElements(GL_TRIANGLES, 36, GL_UNSIGNED_INT, 0);

Answer 2

For flat shading, it is better to use a geometry shader to compute the normals for each of the primitives. Although you can use the provoking-vertex method when rendering a cube, you cannot use it for certain geometric objects where the number of faces is more than that of the vertices: e.g. consider the polyhedron obtained by gluing two tetrahedra at their base triangle. Such an object will have 6 triangles but only 5 vertices (note that Euler's formula still holds: v-e+f = 5-9+6 = 2), so there are not enough vertices to send the face-normals via the vertices. Even if you can do that, another reason not to use provokig-vertex method is that it is not convenient to do so, because you would have to find a way to enumare the vertices in a way such that each vertex uniquely 'represents' a single face, so that you can associate the face-normal with it.
In a nutshell, just use a geometry shader, it is much simpler and more importantly much more robust. Not to mention that the normal calculations are done on the fly inside the GPU, rather than you having to set them up on CPU, creating & binding the necessary buffers and defining attributes which increases both the set-up costs and eats up the memory bandwith between the CPU and the GPU.