OpenGL ignores glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

Question

I'm learning OpenGL for a personal project. I need to draw a triangle mesh as a wireframe. From what I've read, I can do this using glPolygonMode. However, the program always draws my polygons filled. Is is a bad driver? I'm using the linux subsystem on a chromebook. It's basically debian in a container. How do I get an wireframe triangle?

Here is my program, with all the windowing functions and the shader compilation pulled into their own .cpp file.

#include <glad/glad.h>
#include <GLFW/glfw3.h>
#include <glm/glm.hpp>
#include <iostream>
#include "ah_graphics.h"
#include "ah_window.h"

const char *vertexShaderSource();

const char *fragmentShaderSource();


// set up vertex data (and buffer(s)) and configure vertex attributes
// ------------------------------------------------------------------
float positions[] = {
        -1.0f, 1.0f, 0.0f, // left
        -1.0f, 0.0f, 0.0f, // right
        0.0f, 1.0f, 0.0f // top
};


int main() {
    GLFWwindow *window = ahCreateWindow(800, 600);
    ahLoadGlFunctionPointers();
    GLuint program = ahCreateGlProgram(vertexShaderSource(), fragmentShaderSource());
    unsigned int VBO_pos, VAO;
    glGenVertexArrays(1, &VAO);
    glBindVertexArray(VAO);
    glGenBuffers(1, &VBO_pos);
    glBindBuffer(GL_ARRAY_BUFFER, VBO_pos);
    glBufferData(GL_ARRAY_BUFFER, sizeof(positions), positions, GL_STATIC_DRAW);
    int numComponents = 3; // (x,y,z) and (r,g,b)
    std::size_t stride = numComponents * sizeof(float);
    glVertexAttribPointer(0, numComponents, GL_FLOAT, GL_FALSE, stride, (void *) 0);
    glEnableVertexAttribArray(0);

    while (!glfwWindowShouldClose(window)) {
        // input
        ahProcessInput(window);
        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glUseProgram(program);
        glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
        glDrawArrays(GL_TRIANGLES, 0, 3);
        glfwSwapBuffers(window);
        glfwPollEvents();
    }
    return 0;
}

const char *vertexShaderSource() {
    return "#version 330 core\n"
           "layout (location = 0) in vec3 aPos;\n"
           "void main()\n"
           "{\n"
           "  gl_Position = vec4(aPos, 1.0);\n"
           "}\0";
}

const char *fragmentShaderSource() {
    return "#version 330 core\n"
           "void main()\n"
           "{\n"
           " gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);\n"
           "}\n\0";
}

And this is the scene it renders:

Answer 1

I've ran into the same problem in the same exact situation. How I fixed it was to draw using GL_LINE_STRIP inside the render loop and discard the glPolygonMode code.

glDrawArrays(GL_LINE_STRIP, 0, 3);

This workaround does work, but it would be very appreciated if someone smarter than me could figure out why glPolygonMode doesn't work in such cases. I'm aware that this post is a year old, but I hope this helps the people out there!