How do I check why is no output on screen?

Question

I'm using OpenGL 3.3+, and I'm trying to create an orthographic projection to draw on screen. I have the model data in a buffer:

X    Y
0,   0
50,  100
100, 0

And I create my Model-View-Projection matrix as follows:

glm::mat4 projection = glm::ortho(0.f, 100.f, 0.f, 100.f);
    glm::mat4 view = glm::lookAt(
        glm::vec3(0, 0, 5),
        glm::vec3(0, 0, 0),
        glm::vec3(0, 1, 0)
    );
    glm::mat4 model = glm::mat4(1);

    glm::mat4 mvp = projection * view * model;

In my debugging efforts, I used the values from the model buffer and did the same operations the shader was doing, but on the CPU using GLM:

glm::vec4 pos(x, y, 1.0, 1.0);
posicao = mvp * posicao;

And got these results:

X    Y
-1, -1
0,   1
1,  -1

Which were as expected, as the Vertex Shader's code is plain simply multiplying the matrix by the incoming vec4(vec2(x, y), 1, 1). In my understanding, this should've outputted a triangle on my screen, but it's blank. What are some ways I can debug this?

Answer 1

Some of the things I typically try when nothing appears to be drawing on the screen:

• Disable backface culling (in case your geometry winding is wrong)
• Change the glClearColor() to something other than black (texturing problems can cause your geometry to be drawn solid black, in which case you won't see it)
• Change your fragment shader to output a specific solid color
• If you use framebuffers anywhere, try explicitly calling glBindFramebuffer(GL_FRAMEBUFFER, 0) before drawing your triangle, to rule out binding leaks
• Continuously rotate your camera angle in a loop (in case the geometry is accidentally behind you)
• Make sure your geometry is not within your near clip distance (try setting the near clip distance as small as possible, and moving the geometry further away)