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Getting started

Table of Contents

This guide will show how to write simple OpenGL applications using GLFW 3. It will introduce a few of the most commonly used functions, but there are many others. To see detailed documentation on any GLFW function, just click on its name.

This guide assumes no experience with earlier versions of GLFW. If you have used GLFW 2.x in the past, you should also read the transition guide.

Including the GLFW header

In the files of your program where you use OpenGL or GLFW, you need to include the GLFW 3 header file.

#include <GLFW/glfw3.h>

This defines all the constants, types and function prototypes of the GLFW API. It also includes the OpenGL header, and defines all the constants and types necessary for it to work on your platform.

For example, under Windows you are normally required to include windows.h before including GL/gl.h. This would make your source file tied to Windows and pollute your code's namespace with the whole Win32 API.

Instead, the GLFW header takes care of this for you, not by including windows.h, but rather by itself duplicating only the necessary parts of it. It does this only where needed, so if windows.h is included, the GLFW header does not try to redefine those symbols.

In other words:

Starting with version 3.0, the GLU header glu.h is no longer included by default. If you wish to include it, define GLFW_INCLUDE_GLU before the inclusion of the GLFW header.

#define GLFW_INCLUDE_GLU
#include <GLFW/glfw3.h>

Initializing and terminating GLFW

Before you can use most GLFW functions, the library must be initialized. This is done with glfwInit, which returns non-zero if successful, or zero if an error occurred.

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

When you are done using GLFW, typically at the very end of the program, you need to call glfwTerminate.

This destroys any remaining windows and releases any other resources allocated by GLFW. After this call, you must call glfwInit again before using any GLFW functions that require it.

Setting an error callback

Most events are reported through callbacks, whether it's a key being pressed, a GLFW window being moved, or an error occurring. Callbacks are simply C functions (or C++ static methods) that are called by GLFW with arguments describing the event.

In case glfwInit or any other GLFW function fails, an error is reported to the GLFW error callback. You can receive these reports by setting the error callback. The callback function itself should match the signature of GLFWerrorfun. Here is a simple error callback that just prints the error description to stderr.

void error_callback(int error, const char* description)
{
fputs(description, stderr);
}

Setting the callback, so GLFW knows to call it, is done with glfwSetErrorCallback. This is one of the few GLFW functions that may be called before glfwInit, which lets you be notified of errors during initialization, so you should set it before you do anything else with GLFW.

glfwSetErrorCallback(error_callback);

Creating a window and context

The window (and its context) is created with glfwCreateWindow, which returns a handle to the created window. For example, this creates a 640 by 480 windowed mode window:

GLFWwindow* window = glfwCreateWindow(640, 480, "My Title", NULL, NULL);

If window creation fails, NULL will be returned, so you need to check whether it did.

if (!window)
{
exit(EXIT_FAILURE);
}

This handle is then passed to all window related functions, and is provided to you along with input events, so you know which window received the input.

To create a full screen window, you need to specify which monitor the window should use. In most cases, the user's primary monitor is a good choice. You can get this with glfwGetPrimaryMonitor. To make the above window full screen, just pass along the monitor handle:

GLFWwindow* window = glfwCreateWindow(640, 480, "My Title", glfwGetPrimaryMonitor(), NULL);

Full screen windows cover the entire display area of a monitor, have no border or decorations, and change the monitor's resolution to the one most closely matching the requested window size.

When you are done with the window, destroy it with the glfwDestroyWindow function.

Once this function is called, no more events will be delivered for that window and its handle becomes invalid.

Making the OpenGL context current

Before you can use the OpenGL API, it must have a current OpenGL context. You make a window's context current with glfwMakeContextCurrent. It will then remain as the current context until you make another context current or until the window owning it is destroyed.

Checking the window close flag

Each window has a flag indicating whether the window should be closed. This can be checked with glfwWindowShouldClose.

When the user attempts to close the window, either by pressing the close widget in the title bar or using a key combination like Alt+F4, this flag is set to 1. Note that the window isn't actually closed, so you are expected to monitor this flag and either destroy the window or give some kind of feedback to the user.

while (!glfwWindowShouldClose(window))
{
// Keep running
}

You can be notified when user is attempting to close the window by setting a close callback with glfwSetWindowCloseCallback. The callback will be called immediately after the close flag has been set.

You can also set it yourself with glfwSetWindowShouldClose. This can be useful if you want to interpret other kinds of input as closing the window, like for example pressing the escape key.

Receiving input events

Each window has a large number of callbacks that can be set to receive all the various kinds of events. To receive key press and release events, a key callback is set using glfwSetKeyCallback.

static void key_callback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
glfwSetWindowShouldClose(window, GL_TRUE);
}

For event callbacks to actually be called when an event occurs, you need to process events as described below.

Rendering with OpenGL

Once you have a current OpenGL context, you can use OpenGL normally. In this tutorial, a multi-colored rotating triangle will be rendered. The framebuffer size, needed by this example for glViewport and glOrtho, is retrieved with glfwGetFramebufferSize.

int width, height;
glfwGetFramebufferSize(window, &width, &height);
glViewport(0, 0, width, height);

However, you can also set a framebuffer size callback using glfwSetFramebufferSizeCallback and call glViewport from there.

void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
glViewport(0, 0, width, height);
}

Reading the timer

For the triangle to rotate properly, a time source is needed. GLFW provides glfwGetTime, which returns the number of seconds since glfwInit as a double. The time source used is the most accurate on each platform and generally has micro- or nanosecond resolution.

double time = glfwGetTime();

Swapping buffers

GLFW windows always use double-buffering. That means that you have two rendering buffers; a front buffer and a back buffer. The front buffer is the one being displayed and the back buffer the one you render to.

When the entire frame has been rendered, it is time to swap the back and the front buffers in order to display the rendered frame, and begin rendering a new frame. This is done with glfwSwapBuffers.

Processing events

GLFW needs to communicate regularly with the window system both in order to receive events and to show that it hasn't locked up. Event processing must be done regularly and is normally done each frame before rendering but after buffer swap.

There are two ways to process pending events. glfwPollEvents processes only those events that have already been received and then returns immediately. This is the best choice when rendering continually, like most games do.

If instead you only need to update your rendering once you have received new input, glfwWaitEvents is a better choice. It waits until at least one event has been received, putting the thread to sleep in the meantime, and then processes all received events just like glfwPollEvents does. This saves a great deal of CPU cycles and is useful for, for example, many kinds of editing tools.

Putting it together: A small GLFW application

Now that you know how to initialize GLFW, create a window and poll for keyboard input, it's possible to create a simple program.

#include <GLFW/glfw3.h>
#include <stdlib.h>
#include <stdio.h>
static void error_callback(int error, const char* description)
{
fputs(description, stderr);
}
static void key_callback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
glfwSetWindowShouldClose(window, GL_TRUE);
}
int main(void)
{
GLFWwindow* window;
glfwSetErrorCallback(error_callback);
if (!glfwInit())
exit(EXIT_FAILURE);
window = glfwCreateWindow(640, 480, "Simple example", NULL, NULL);
if (!window)
{
exit(EXIT_FAILURE);
}
glfwSetKeyCallback(window, key_callback);
while (!glfwWindowShouldClose(window))
{
float ratio;
int width, height;
glfwGetFramebufferSize(window, &width, &height);
ratio = width / (float) height;
glViewport(0, 0, width, height);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-ratio, ratio, -1.f, 1.f, 1.f, -1.f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glRotatef((float) glfwGetTime() * 50.f, 0.f, 0.f, 1.f);
glBegin(GL_TRIANGLES);
glColor3f(1.f, 0.f, 0.f);
glVertex3f(-0.6f, -0.4f, 0.f);
glColor3f(0.f, 1.f, 0.f);
glVertex3f(0.6f, -0.4f, 0.f);
glColor3f(0.f, 0.f, 1.f);
glVertex3f(0.f, 0.6f, 0.f);
glEnd();
glfwSwapBuffers(window);
}
exit(EXIT_SUCCESS);
}

This program creates a 640 by 480 windowed mode window and runs a loop clearing the screen, rendering a triangle and processing events until the user closes the window. It can be found in the source distribution as examples/simple.c, and is by default compiled along with all other examples when you build GLFW.

To learn more about how to compile and link programs that use GLFW, see Building programs that use GLFW.