Simple opengl triangle function not drawing anything to screen

user_19560631 0
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I am just getting started with OpenGL, and have already hit a pretty frustrating bug with it. I've followed the learnopengl tutorial, encapsulating most stuff into a renderer class, which has uints for buffers and such. Here is the main code that does everything:

#include <gfx/gfx.h>
#include <gfx/gl.h>

#include <gfx/shaders.h>

#include <iostream>

void Renderer::init() {

    vertex_shader_id = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertex_shader_id, 1, &vertex_shader, nullptr);
    glCompileShader(vertex_shader_id);

    GLint vertex_shader_status;
    glGetShaderiv(vertex_shader_id, GL_COMPILE_STATUS, &vertex_shader_status);
    
    if (vertex_shader_status == false) {
        std::cout << "vsh compilation failed due to";
        char vertex_fail_info_log[1024];
        glGetShaderInfoLog(vertex_shader_id, 1024, nullptr, vertex_fail_info_log);
        std::cout << vertex_fail_info_log << std::endl;
        abort();
    }
    
    fragment_shader_id = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragment_shader_id, 1, &fragment_shader, nullptr);
    glCompileShader(fragment_shader_id);

    GLint fragment_shader_status;
    glGetShaderiv(fragment_shader_id, GL_COMPILE_STATUS, &fragment_shader_status);
    
    if (fragment_shader_status == false) {
        std::cout << "fsh compilation failed due to";
        char fragment_fail_info_log[1024];
        glGetShaderInfoLog(fragment_shader_id, 1024, nullptr, fragment_fail_info_log);
        std::cout << fragment_fail_info_log << std::endl;
        abort();
    }
    
    shader_program = glCreateProgram();
    glAttachShader(shader_program, vertex_shader_id);
    glAttachShader(shader_program, fragment_shader_id);
    glLinkProgram(shader_program);

    GLint shader_program_status;
    glGetProgramiv(shader_program, GL_LINK_STATUS, &shader_program_status);
    
    if (shader_program_status == false) {
        std::cout << "shprogram compilation failed due to";
        char shader_program_fail_info_log[1024];
        glGetShaderInfoLog(shader_program, 1024, nullptr, shader_program_fail_info_log);
        std::cout << shader_program_fail_info_log << std::endl;
        abort();
    }
    
    glUseProgram(shader_program);

    glDeleteShader(vertex_shader_id);
    glDeleteShader(fragment_shader_id);

    
}
 
void Renderer::draw(f32 verts[]) {
    
    glUseProgram(shader_program);
    
    glClearColor(1, 0, 0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);

    glCreateVertexArrays(1, &vertex_array);
    glBindVertexArray(vertex_array);
    
    glCreateBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(verts), verts, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(f32), (void*)0);
    glEnableVertexAttribArray(0);

    glBindVertexArray(vertex_array);
    glUseProgram(shader_program);
    glDrawArrays(GL_TRIANGLES, 0, 3);
}

Here is shaders.h :

#ifndef SHADERS_H
#define SHADERS_H

const char* vertex_shader =
"#version 460 core\n"
"layout (location = 0) in vec3 aPos;\n"

"void main() {\n"
"gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n"
"}\n\0";


const char* fragment_shader =
"#version 460 core\n"
"out vec4 FragColor;\n"

"void main() {\n"
"FragColor = vec4(0.0f, 1.0f, 0.0f, 1.0f);\n"
"}\n\0";

#endif

I cannot figure out for life of me, what is wrong. The red clear color shows up, but nothing else.

opengl c++
1 Answers

Looks like there's a problem with your draw method. The signature is void Renderer::draw(f32 verts[]) { Then later on you call glBufferData(GL_ARRAY_BUFFER, sizeof(verts), verts, GL_STATIC_DRAW); . The thing is, when you pass an array to a function, it decays to a pointer (Even though the declaration can make that look like it's not happening, which is very confusing). So in a function parameter, draw(f32 verts[]) is equivalent to draw(f32* verts). This question has some explanations on what's happening there.

Anyways, when you call sizeof(verts), you're just getting the number of bytes of a float pointer, not the number of bytes owned by verts. So you will not be specifying enough bytes when you call glBufferData() to create the triangle you are going for. The simple fix is to pass a length into your draw function, and then you would have something like

    void Renderer::draw(f32* verts, int length) {
        //...
        glBufferData(GL_ARRAY_BUFFER, sizeof(float) * length, verts, GL_STATIC_DRAW);
        //...
    }

Here is some docs on this particular function. It is possible there are other errors, but since you aren't blackscreening, and generally the code looks right, it is unlikely there's a bunch of invalid operations or anything.

To continue debugging after this, add the following to your code

#define GL_ERROR_CHECK() (log_error(__FILE__, __LINE__))

void log_error(const* file, int line) {
    GLenum err;
    while((err = glGetError()) != GL_NO_ERROR) {
        std::cout << "GL error " << err << " in " << file << "at line " << line << std::endl;
    }
}

and sprinkle GL_ERROR_CHECK() all over the place to see if any of the OpenGL calls were invalid.

user_91789740
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