Question

How to draw a rectangle using OpenGL with vertex array in C

Answer 1

#include <GL/glew.h>
#include <GLFW/glfw3.h>

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>


// Draw rectangle:


/*
// vertex.shader

#version 410 core
layout(location = 0) in vec4 position;
void main() {
  gl_Position = position;
}
*/
/*
// fragment.shader

#version 410 core
layout(location = 0) out vec4 color;
uniform vec4 uColor;
void main() {
    color = uColor;
};
*/

// An old way to handle errors
#define ASSERT(x) if (!(x)) __debugbreak();

#define GLErr(s) GLClearErros();\
    s;\
    ASSERT(GLCheckError(#s, __FILE__, __LINE__))

char vertexShader[128] = "";
char fragmentShader[128] = "";

static void GLClearErros() {
    while (glGetError() != GL_NO_ERROR);
}

static bool GLCheckError(const char *function, const char *file, int line) {
    GLenum error;
    while (error = glGetError()) {
        printf("OpenGL error code: %d Function: %s File: %s Line: %d\n", error, function, file, line);
        return false;
    }
    return true;
}

static void ParseShaders() {
    FILE* fp = NULL;

    char vertex[32] = "vertex.shader";
    if (fopen_s(&fp, vertex, "r") != 0) {
        printf("Cannot open input file.\n");
    }
    else {
        int i = 0;
        while (!feof(fp)) {
            vertexShader[i++] = fgetc(fp);
        }
        vertexShader[--i] = '\0';
        fclose(fp);
    }

    char fragment[32] = "fragment.shader";
    if (fopen_s(&fp, fragment, "r") != 0) {
        printf("Cannot open input file.\n");
    }
    else {
        int i = 0;
        while (!feof(fp)) {
            fragmentShader[i++] = fgetc(fp);
        }
        fragmentShader[--i] = '\0';
        fclose(fp);
    }
}

static unsigned int CompileShader(unsigned int type, const char* source) {
    unsigned int id = glCreateShader(type);
    glShaderSource(id, 1, &source, NULL);
    glCompileShader(id);

    int ivresult;
    glGetShaderiv(id, GL_COMPILE_STATUS, &ivresult);
    if (ivresult == GL_FALSE) {
        int length;
        glGetShaderiv(id, GL_INFO_LOG_LENGTH, &length);
        char* errormessage = (char*)malloc(length * sizeof(char));
        glGetShaderInfoLog(id, length, &length, errormessage);
        printf("Failed to compile the %s shader!\n", (type == GL_VERTEX_SHADER ? "vertex" : "fragment"));
        printf("%s\n", errormessage);
        free(errormessage);
        glDeleteShader(id);

        return 0;
    }

    return id;
}

static unsigned int CreateShader(const char* vertexShader, const char* fragmentShader) {
    unsigned int program = glCreateProgram();
    unsigned int vs = CompileShader(GL_VERTEX_SHADER, vertexShader);
    unsigned int fs = CompileShader(GL_FRAGMENT_SHADER, fragmentShader);

    glAttachShader(program, vs);
    glAttachShader(program, fs);
    glLinkProgram(program);
    glValidateProgram(program);

    glDetachShader(program, vs);
    glDetachShader(program, fs);

    glDeleteShader(vs);
    glDeleteShader(fs);

    return program;
}

int main(void)
{
    //ParseShaders();

    if (!glfwInit()) {
        printf("glfwInit() Error!");
        return -1;
    }
    // Use OpenGL 4.4
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 4);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow* window = glfwCreateWindow(640, 480, "Window Title", NULL, NULL);
    if (!window) {
        glfwTerminate();
        return -1;
    }

    glfwMakeContextCurrent(window);

    glfwSwapInterval(1);

    if (glewInit() != GLEW_OK) {
        printf("glewInit() Error!");
        return -1;
    }

    float positions[] = {
        -0.4f, -0.4f, // 0
         0.4f, -0.4f, // 1
         0.4f,  0.4f, // 2
        -0.4f,  0.4f, // 3
    };
     
    unsigned int index_buffer[] = {
        0, 1, 2,
        2, 3, 0
    };

    unsigned int vertex_array;
    glGenVertexArrays(1, &vertex_array);
    glBindVertexArray(vertex_array);

    unsigned int buffer;
    glGenBuffers(1, &buffer);
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
    glBufferData(GL_ARRAY_BUFFER, 4 * 2 * sizeof(float), positions, GL_STATIC_DRAW);

    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, 0);

    unsigned int ibuffer;
    glGenBuffers(1, &ibuffer);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibuffer);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, 6 * sizeof(unsigned int), index_buffer, GL_STATIC_DRAW);

    ParseShaders();
    unsigned int shader = CreateShader(vertexShader, fragmentShader);
    glUseProgram(shader);

    GLErr(int location = glGetUniformLocation(shader, "uColor"));
    ASSERT(location != -1);
    GLErr(glUniform4f(location, 2.9f, 1.7f, 0.5f, 1.0f));
 
    glClearColor(0.752941f, 0.752941f, 0.752941f, 1.0f);

    glBindVertexArray(0);
    glUseProgram(0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

    float red = 0.0f, i = 0.07f;
    while (!glfwWindowShouldClose(window)) {
        GLErr(glClear(GL_COLOR_BUFFER_BIT));

        glUseProgram(shader);
        GLErr(glUniform4f(location, red, 1.7f, 0.5f, 1.0f));

        glBindVertexArray(vertex_array);

        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibuffer);

        GLErr(glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, NULL));
        if (red > 1.4f)
            i = -0.07f;
        else if (red < 0.0f)
            i = 0.07f;

        red += i;

        GLErr(glfwSwapBuffers(window));

        GLErr(glfwPollEvents());
    }

    glDeleteProgram(shader);

    glfwTerminate();

    return 0;
}



/*
run:


*/

 

70+ SQL courses for beginners and professionals