使用GLSL着色器后，第一个三角形不会渲染

Question

I'm currently learning OpenGL (and GLSL) from this tutorial . So far I made it to Tutorial 2: The First Triangle. My code is a bit different since I'm more comfortable programming C, instead of the more common C++. Also, I used glfw3 to open my window, where the tutorial uses glfw, I believe. As for now I've been able to write everything in C, and everything worked til the point where I used my first shaders. Whenever i use them, my triangle wont render. I'm on a Mac and I haven't had the time to upgrade to OSX 10.9 (Mavericks) and therefore I'm forced to use OpenGL 3.2 and GLSL 1.2.

tl;dr: problem: My triangle will not render whenever I use the shaders.

Here's my code:

int main(void) {
GLFWwindow* window;

/* Initialize the library */
if(!glfwInit()) {
    return -1;
}

/* Create a windowed mode window and its OpenGL context */
window = glfwCreateWindow(640, 480, "Tutorial 1", NULL, NULL);
if (!window) {
    glfwTerminate();
    return -1;
}

/* Make the window's context current */
glfwMakeContextCurrent(window);

/* Initialize GLEW */
glewExperimental = GL_TRUE; // Needed in core profile
if(glewInit() != GLEW_OK) {
    fprintf(stderr, "Failed to initialize GLEW\n");
    return -1;
}

GLuint VertexArrayID;
glGenVertexArrays(1, &VertexArrayID);
glBindVertexArray(VertexArrayID);

/* Triangle's vertices */
static const GLfloat g_vertex_buffer_data[] = {
    -1.0f, -1.0f, 0.0f,
    1.0f, -1.0f, 0.0f,
    0.0f,  1.0f, 0.0f,
};

// This will identify our vertex buffer
GLuint vertexbuffer;
// Generate 1 buffer, put the resulting identifier in vertexbuffer
glGenBuffers(1, &vertexbuffer);
// The following commands will talk about our 'vertexbuffer' buffer
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
// Give our vertices to OpenGL.
glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);  

// Create and compile our GLSL program from the shaders
GLuint programID = LoadShaders("VertexShader.txt", "FragmentShader.txt");

glClearColor(0.0f, 0.0f, 0.4f, 0.0f);

/* Loop until the user closes the window */
while (!glfwWindowShouldClose(window)) {
    /* Render here */
    // 1rst attribute buffer : vertices
    glClear(GL_COLOR_BUFFER_BIT);

    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
    glVertexAttribPointer(
        0,                  // attribute 0. No particular reason for 0, but must match the layout in the shader.
        3,                  // size
        GL_FLOAT,           // type
        GL_FALSE,           // normalized?
        0,                  // stride
        (void*)0            // array buffer offset
    );

    // Use our shader
    glUseProgram(programID);

    // Draw the triangle !
    glDrawArrays(GL_TRIANGLES, 0, 3); // Starting from vertex 0; 3 vertices total -> 1 triangle
    glDisableVertexAttribArray(0);

    /* Swap front and back buffers */
    glfwSwapBuffers(window);
    /* Poll for and process events */
    glfwPollEvents();
}

glfwTerminate();
return 0;
}

And here is my VertexShader:

#version 120
vec3 vertexPosition_modelspace;

void main() {
    gl_Position.xyz = vertexPosition_modelspace;
    gl_Position.w = 1.0;
}

My FragmentShader:

#version 120
vec3 color;

void main() {
    color = vec3(1, 0, 0);
}

I think it might have something to do with shadersfiles, I think I might have translated them wrong from the tutorial (he uses GLSL 3.3).

Answer 1

There are two main things that are wrong with your shaders and the crux of the problem boils down to how you are declaring the variables intended for shader input and output.

In your vertex shader, you have an input: vec3 vertexPosition_modelspace

• Using GLSL 1.20 syntax this should be attribute vec3 vertexPosition_modelspace .

In your fragment shader you have an output: vec3 color _{(which would be out vec3 color in a GLSL 3.30 tutorial)}

• You cannot declare arbitrary variables as fragment shader outputs in GLSL 1.20, you have to use gl_FragColor or if you want to output to a specific draw buffer: gl_FragData [n] .

  ^{Side note: Never mix and match the two.}

---

Corrected Vertex Shader:

 #version 120
 attribute vec3 vertexPosition_modelspace;

 //in vec3 vertexPosition_modelspace;
 //^^ It was probably written this way in the GLSL 3.30 tutorial

 void main() {
   gl_Position.xyz = vertexPosition_modelspace;
   gl_Position.w = 1.0;
 }

Corrected Fragment Shader:

 #version 120
 //out vec3 color; -- DO NOT DO THIS IN GLSL 1.20

 void main() {
   //color = vec3(1, 0, 0);
   gl_FragColor = vec4 (1.0, 0.0, 0.0, 0.0);
 }

---

On a final note, you have to match the attribute binding location to your attribute pointer location. Since GLSL 1.20 does not support layout qualifiers the best way to do this would be to add an additional line of code to your program:

glBindAttribLocation (programID, 0, "vertexPosition_modelspace");