glfw/examples/offscreen.c

//========================================================================
// Offscreen rendering example
// Copyright (c) Camilla Löwy <elmindreda@glfw.org>
//
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
//
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
//
// 1. The origin of this software must not be misrepresented; you must not
//    claim that you wrote the original software. If you use this software
//    in a product, an acknowledgment in the product documentation would
//    be appreciated but is not required.
//
// 2. Altered source versions must be plainly marked as such, and must not
//    be misrepresented as being the original software.
//
// 3. This notice may not be removed or altered from any source
//    distribution.
//
//========================================================================

#define GLAD_GL_IMPLEMENTATION
#include <glad/gl.h>
#define GLFW_INCLUDE_NONE
#include <GLFW/glfw3.h>

#include "linmath.h"

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

#define STB_IMAGE_WRITE_IMPLEMENTATION
#include <stb_image_write.h>

static const struct
{
    float x, y;
    float r, g, b;
} vertices[3] =
{
    { -0.6f, -0.4f, 1.f, 0.f, 0.f },
    {  0.6f, -0.4f, 0.f, 1.f, 0.f },
    {   0.f,  0.6f, 0.f, 0.f, 1.f }
};

static const char* vertex_shader_text =
"#version 110\n"
"uniform mat4 MVP;\n"
"attribute vec3 vCol;\n"
"attribute vec2 vPos;\n"
"varying vec3 color;\n"
"void main()\n"
"{\n"
"    gl_Position = MVP * vec4(vPos, 0.0, 1.0);\n"
"    color = vCol;\n"
"}\n";

static const char* fragment_shader_text =
"#version 110\n"
"varying vec3 color;\n"
"void main()\n"
"{\n"
"    gl_FragColor = vec4(color, 1.0);\n"
"}\n";

static void error_callback(int error, const char* description)
{
    fprintf(stderr, "Error: %s\n", description);
}

int main(void)
{
    GLFWwindow* window;
    GLuint vertex_buffer, vertex_shader, fragment_shader, program;
    GLint mvp_location, vpos_location, vcol_location;
    float ratio;
    int width, height;
    mat4x4 mvp;
    char* buffer;

    glfwSetErrorCallback(error_callback);

    glfwInitHint(GLFW_COCOA_MENUBAR, GLFW_FALSE);

    if (!glfwInit())
        exit(EXIT_FAILURE);

    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
    glfwWindowHint(GLFW_VISIBLE, GLFW_FALSE);

    window = glfwCreateWindow(640, 480, "Simple example", NULL, NULL);
    if (!window)
    {
        glfwTerminate();
        exit(EXIT_FAILURE);
    }

    glfwMakeContextCurrent(window);
    gladLoadGL(glfwGetProcAddress);

    // NOTE: OpenGL error checks have been omitted for brevity

    glGenBuffers(1, &vertex_buffer);
    glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    vertex_shader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertex_shader, 1, &vertex_shader_text, NULL);
    glCompileShader(vertex_shader);

    fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragment_shader, 1, &fragment_shader_text, NULL);
    glCompileShader(fragment_shader);

    program = glCreateProgram();
    glAttachShader(program, vertex_shader);
    glAttachShader(program, fragment_shader);
    glLinkProgram(program);

    mvp_location = glGetUniformLocation(program, "MVP");
    vpos_location = glGetAttribLocation(program, "vPos");
    vcol_location = glGetAttribLocation(program, "vCol");

    glEnableVertexAttribArray(vpos_location);
    glVertexAttribPointer(vpos_location, 2, GL_FLOAT, GL_FALSE,
                          sizeof(vertices[0]), (void*) 0);
    glEnableVertexAttribArray(vcol_location);
    glVertexAttribPointer(vcol_location, 3, GL_FLOAT, GL_FALSE,
                          sizeof(vertices[0]), (void*) (sizeof(float) * 2));

    glfwGetFramebufferSize(window, &width, &height);
    ratio = width / (float) height;

    glViewport(0, 0, width, height);
    glClear(GL_COLOR_BUFFER_BIT);

    mat4x4_ortho(mvp, -ratio, ratio, -1.f, 1.f, 1.f, -1.f);

    glUseProgram(program);
    glUniformMatrix4fv(mvp_location, 1, GL_FALSE, (const GLfloat*) mvp);
    glDrawArrays(GL_TRIANGLES, 0, 3);
    glFinish();

    buffer = calloc(4, width * height);
    glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

    // Write image Y-flipped because OpenGL
    stbi_write_png("offscreen.png",
                   width, height, 4,
                   buffer + (width * 4 * (height - 1)),
                   -width * 4);

    free(buffer);

    glfwDestroyWindow(window);

    glfwTerminate();
    exit(EXIT_SUCCESS);
}