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1//========================================================================
2// Offscreen rendering example
3// Copyright (c) Camilla Löwy <[email protected]>
4//
5// This software is provided 'as-is', without any express or implied
6// warranty. In no event will the authors be held liable for any damages
7// arising from the use of this software.
8//
9// Permission is granted to anyone to use this software for any purpose,
10// including commercial applications, and to alter it and redistribute it
11// freely, subject to the following restrictions:
12//
13// 1. The origin of this software must not be misrepresented; you must not
14//    claim that you wrote the original software. If you use this software
15//    in a product, an acknowledgment in the product documentation would
16//    be appreciated but is not required.
17//
18// 2. Altered source versions must be plainly marked as such, and must not
19//    be misrepresented as being the original software.
20//
21// 3. This notice may not be removed or altered from any source
22//    distribution.
23//
24//========================================================================
25
26#define GLAD_GL_IMPLEMENTATION
27#include <glad/gl.h>
28#define GLFW_INCLUDE_NONE
29#include <GLFW/glfw3.h>
30
31#include "linmath.h"
32
33#include <stdlib.h>
34#include <stdio.h>
35
36#define STB_IMAGE_WRITE_IMPLEMENTATION
37#include <stb_image_write.h>
38
39static const struct
40{
41    float x, y;
42    float r, g, b;
43} vertices[3] =
44{
45    { -0.6f, -0.4f, 1.f, 0.f, 0.f },
46    {  0.6f, -0.4f, 0.f, 1.f, 0.f },
47    {   0.f,  0.6f, 0.f, 0.f, 1.f }
48};
49
50static const char* vertex_shader_text =
51"#version 110\n"
52"uniform mat4 MVP;\n"
53"attribute vec3 vCol;\n"
54"attribute vec2 vPos;\n"
55"varying vec3 color;\n"
56"void main()\n"
57"{\n"
58"    gl_Position = MVP * vec4(vPos, 0.0, 1.0);\n"
59"    color = vCol;\n"
60"}\n";
61
62static const char* fragment_shader_text =
63"#version 110\n"
64"varying vec3 color;\n"
65"void main()\n"
66"{\n"
67"    gl_FragColor = vec4(color, 1.0);\n"
68"}\n";
69
70static void error_callback(int error, const char* description)
71{
72    fprintf(stderr, "Error: %s\n", description);
73}
74
75int main(void)
76{
77    GLFWwindow* window;
78    GLuint vertex_buffer, vertex_shader, fragment_shader, program;
79    GLint mvp_location, vpos_location, vcol_location;
80    float ratio;
81    int width, height;
82    mat4x4 mvp;
83    char* buffer;
84
85    glfwSetErrorCallback(error_callback);
86
87    glfwInitHint(GLFW_COCOA_MENUBAR, GLFW_FALSE);
88
89    if (!glfwInit())
90        exit(EXIT_FAILURE);
91
92    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2);
93    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
94    glfwWindowHint(GLFW_VISIBLE, GLFW_FALSE);
95
96    window = glfwCreateWindow(640, 480, "Simple example", NULL, NULL);
97    if (!window)
98    {
99        glfwTerminate();
100        exit(EXIT_FAILURE);
101    }
102
103    glfwMakeContextCurrent(window);
104    gladLoadGL(glfwGetProcAddress);
105
106    // NOTE: OpenGL error checks have been omitted for brevity
107
108    glGenBuffers(1, &vertex_buffer);
109    glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer);
110    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
111
112    vertex_shader = glCreateShader(GL_VERTEX_SHADER);
113    glShaderSource(vertex_shader, 1, &vertex_shader_text, NULL);
114    glCompileShader(vertex_shader);
115
116    fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
117    glShaderSource(fragment_shader, 1, &fragment_shader_text, NULL);
118    glCompileShader(fragment_shader);
119
120    program = glCreateProgram();
121    glAttachShader(program, vertex_shader);
122    glAttachShader(program, fragment_shader);
123    glLinkProgram(program);
124
125    mvp_location = glGetUniformLocation(program, "MVP");
126    vpos_location = glGetAttribLocation(program, "vPos");
127    vcol_location = glGetAttribLocation(program, "vCol");
128
129    glEnableVertexAttribArray(vpos_location);
130    glVertexAttribPointer(vpos_location, 2, GL_FLOAT, GL_FALSE,
131                          sizeof(vertices[0]), (void*) 0);
132    glEnableVertexAttribArray(vcol_location);
133    glVertexAttribPointer(vcol_location, 3, GL_FLOAT, GL_FALSE,
134                          sizeof(vertices[0]), (void*) (sizeof(float) * 2));
135
136    glfwGetFramebufferSize(window, &width, &height);
137    ratio = width / (float) height;
138
139    glViewport(0, 0, width, height);
140    glClear(GL_COLOR_BUFFER_BIT);
141
142    mat4x4_ortho(mvp, -ratio, ratio, -1.f, 1.f, 1.f, -1.f);
143
144    glUseProgram(program);
145    glUniformMatrix4fv(mvp_location, 1, GL_FALSE, (const GLfloat*) mvp);
146    glDrawArrays(GL_TRIANGLES, 0, 3);
147    glFinish();
148
149    buffer = calloc(4, width * height);
150    glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
151
152    // Write image Y-flipped because OpenGL
153    stbi_write_png("offscreen.png",
154                   width, height, 4,
155                   buffer + (width * 4 * (height - 1)),
156                   -width * 4);
157
158    free(buffer);
159
160    glfwDestroyWindow(window);
161
162    glfwTerminate();
163    exit(EXIT_SUCCESS);
164}
165
166
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