Missing faces on cube OPENGL

Question

i tried everything, i know that my program is drawing the correct vertices, i basically pointed it down to indices, here is the mainloop

void mainLoop() {

    while (!glfwWindowShouldClose(window)) {

        glfwSwapBuffers(window);
        glEnable(GL_DEPTH_TEST);
        glClearColor(0.1, 0.1, 0.3, 1.0);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glfwPollEvents();
        glFrontFace(GL_CW);
        glDisable(GL_CULL_FACE);

        glfwGetFramebufferSize(window, &WIDTH, &HEIGHT);
        glViewport(1, 1, WIDTH, HEIGHT);

        glm::mat4 view = glm::lookAt(
            glm::vec3(1.2f, 1.2f, 1.2f),
            glm::vec3(0.0f, 0.0f, 0.0f),
            glm::vec3(0.0f, 0.0f, 1.0f)
        );


        glm::mat4 projectionmatrix = glm::perspective(glm::radians(45.0f), 800.0f / 800.0f, 1.0f, 10.0f);

        glm::mat4 identitymatrix = glm::mat4(1.0f);

        glm::mat4 modelmatrix = identitymatrix;

        MVPmatrix = projectionmatrix * view * modelmatrix;

        GLint loc = glGetUniformLocation(SHADERPROGRAM, "MVPmatrix");
        glUniformMatrix4fv(loc, 1, GL_FALSE, &MVPmatrix[0][0]);

        short vertex_index[] = { 2U, 4U, 1U, 8U, 6U, 5U, 5U, 2U, 1U, 6U, 3U, 2U, 3U, 8U, 4U, 1U, 8U, 5U, 2U, 3U, 4U, 8U, 7U, 6U, 5U, 6U, 2U, 6U, 7U, 3U, 3U, 7U, 8U, 1U, 4U, 8U };


        glEnableVertexAttribArray(0);

        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);



        glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(vertex_index) * sizeof(unsigned short), &vertex_index[0], GL_STATIC_DRAW);


        glBufferData(GL_ARRAY_BUFFER, temp_vertices.size() * sizeof(glm::vec3), &temp_vertices[0], GL_STATIC_DRAW); 

        glUseProgram(SHADERPROGRAM);
        glDrawElements(GL_TRIANGLES, sizeof(vertex_index) ,  GL_UNSIGNED_SHORT, (void*)0);


    }

}

i really dont know at this point and i've been trying for ages. The indices are straight from a obj file so are the vertices. Also i tried turning off culling without success, also i tried adding manual indices amounts and even tried to offset them.

importing the verts for a cube works perfectly fine, but as soon as i try to make a indexed cube oops.

Answer 1

I have found 2 issues in your code:

1.) Element indices start at 0 rather than 1.

Either decrement all the the vertex indies in the vertex_index array or add a "dummy" vertex coordinate at begin of temp_vertices , to solve the issue.

2.) The 2nd parameter or glDrawElements is the number of the indices rather than the size of the index buffer in bytes:

glDrawElements(GL_TRIANGLES, sizeof(vertex_index), GL_UNSIGNED_SHORT, (void*)0);
glDrawElements(GL_TRIANGLES, sizeof(vertex_index)/sizeof(short), GL_UNSIGNED_SHORT, (void*)0);

But I recommend to put the indices in std::vector<short> and to use std::vector::size() .

---

Further use the WIDTH and HEIGHT when setting up the projection matrix, for calculating the aspect ratio of the viewport.

Except of that your code works fine. I used the following projection and view matrix

glm::mat4 view = glm::lookAt(
    glm::vec3(2.0f, 2.0f, 2.0f), glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3(0.0f, 0.0f, 1.0f));
glm::mat4 projectionmatrix = glm::perspective(
    glm::radians(90.0f), (float)WIDTH / (float)HEIGHT, 0.1f, 10.0f);

And glPolygonMode

glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

To generate the following image

Answer 2

In addition to the answer of Rabbid76 :

I took the values provided by OP and made a sketch:

Then I created the vertex indices for it and got:

GLushort vertex_index[] = {
  0, 1, 2, 2, 3, 0, // front
  0, 4, 5, 5, 1, 0, // right
  4, 7, 6, 6, 5, 4, // back
  7, 3, 2, 2, 6, 7, // left
  0, 3, 7, 7, 4, 0, // bottom
  1, 5, 6, 6, 2, 1 // top
};

If I didn't make any mistake (I carefully kept orientation of triangles in mind), this should even work with back face culling (rendering only CCW oriented triangles) to show outside of cube only.