Question

How to draw a rectangle using OpenGL with vertex shader and fragment shader from file in C

Answer 1

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

#include <stdio.h>
#include <stdlib.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;
void main() {
  color = vec4(1.0, 1.0, 0.0, 1.0);
}
*/

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

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;
    }

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

    glfwMakeContextCurrent(window);

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

    float positions[] = {
        -0.4f, -0.4f,
         0.4f, -0.4f,
         0.4f,  0.4f,

         0.4f,  0.4f,
        -0.4f,  0.4f,
        -0.4f, -0.4f
    };

    unsigned int buffer;
    glGenBuffers(1, &buffer);
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
    glBufferData(GL_ARRAY_BUFFER, 6 * 2 * sizeof(float), positions, GL_STATIC_DRAW);

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

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

    glClearColor(0.752941f, 0.752941f, 0.752941f, 1.0f);

    while (!glfwWindowShouldClose(window)) {
        glClear(GL_COLOR_BUFFER_BIT);

        glDrawArrays(GL_TRIANGLES, 0, 6);

        glfwSwapBuffers(window);

        glfwPollEvents();
    }

    glDeleteProgram(shader);

    glfwTerminate();

    return 0;
}



/*
run:


*/

 

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