xref: /third_party/glfw/docs/quick.md

# Getting started {#quick_guide}

[TOC]

This guide takes you through writing a small application using GLFW 3.  The
application will create a window and OpenGL context, render a rotating triangle
and exit when the user closes the window or presses _Escape_.  This guide will
introduce a few of the most commonly used functions, but there are many more.

This guide assumes no experience with earlier versions of GLFW.  If you
have used GLFW 2 in the past, read @ref moving_guide, as some functions
behave differently in GLFW 3.


## Step by step {#quick_steps}

### Including the GLFW header {#quick_include}

In the source files of your application where you use GLFW, you need to include
its header file.

```c
#include <GLFW/glfw3.h>
```

This header provides all the constants, types and function prototypes of the
GLFW API.

By default it also includes the OpenGL header from your development environment.
On some platforms this header only supports older versions of OpenGL.  The most
extreme case is Windows, where it typically only supports OpenGL 1.2.

Most programs will instead use an
[extension loader library](@ref context_glext_auto) and include its header.
This example uses files generated by [glad](https://gen.glad.sh/).  The GLFW
header can detect most such headers if they are included first and will then not
include the one from your development environment.

```c
#include <glad/gl.h>
#include <GLFW/glfw3.h>
```

To make sure there will be no header conflicts, you can define @ref
GLFW_INCLUDE_NONE before the GLFW header to explicitly disable inclusion of the
development environment header.  This also allows the two headers to be included
in any order.

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


### Initializing and terminating GLFW {#quick_init_term}

Before you can use most GLFW functions, the library must be initialized.  On
successful initialization, `GLFW_TRUE` is returned.  If an error occurred,
`GLFW_FALSE` is returned.

```c
if (!glfwInit())
{
    // Initialization failed
}
```

Note that `GLFW_TRUE` and `GLFW_FALSE` are and will always be one and zero.

When you are done using GLFW, typically just before the application exits, you
need to terminate GLFW.

```c
glfwTerminate();
```

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


### Setting an error callback {#quick_capture_error}

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

In case a GLFW function fails, an error is reported to the GLFW error callback.
You can receive these reports with an error callback.  This function must have
the signature below but may do anything permitted in other callbacks.

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

Callback functions must be set, so GLFW knows to call them.  The function to set
the error callback is one of the few GLFW functions that may be called before
initialization, which lets you be notified of errors both during and after
initialization.

```c
glfwSetErrorCallback(error_callback);
```


### Creating a window and context {#quick_create_window}

The window and its OpenGL context are created with a single call to @ref
glfwCreateWindow, which returns a handle to the created combined window and
context object

```c
GLFWwindow* window = glfwCreateWindow(640, 480, "My Title", NULL, NULL);
if (!window)
{
    // Window or OpenGL context creation failed
}
```

This creates a 640 by 480 windowed mode window with an OpenGL context.  If
window or OpenGL context creation fails, `NULL` will be returned.  You should
always check the return value.  While window creation rarely fails, context
creation depends on properly installed drivers and may fail even on machines
with the necessary hardware.

By default, the OpenGL context GLFW creates may have any version.  You can
require a minimum OpenGL version by setting the `GLFW_CONTEXT_VERSION_MAJOR` and
`GLFW_CONTEXT_VERSION_MINOR` hints _before_ creation.  If the required minimum
version is not supported on the machine, context (and window) creation fails.

You can select the OpenGL profile by setting the `GLFW_OPENGL_PROFILE` hint.
This program uses the core profile as that is the only profile macOS supports
for OpenGL 3.x and 4.x.

```c
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
GLFWwindow* window = glfwCreateWindow(640, 480, "My Title", NULL, NULL);
if (!window)
{
    // Window or context creation failed
}
```

When a window and context is no longer needed, destroy it.

```c
glfwDestroyWindow(window);
```

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


### Making the OpenGL context current {#quick_context_current}

Before you can use the OpenGL API, you must have a current OpenGL context.

```c
glfwMakeContextCurrent(window);
```

The context will remain current until you make another context current or until
the window owning the current context is destroyed.

If you are using an [extension loader library](@ref context_glext_auto) to
access modern OpenGL then this is when to initialize it, as the loader needs
a current context to load from.  This example uses
[glad](https://github.com/Dav1dde/glad), but the same rule applies to all such
libraries.

```c
gladLoadGL(glfwGetProcAddress);
```


### Checking the window close flag {#quick_window_close}

Each window has a flag indicating whether the window should be closed.

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.

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

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

You can also set it yourself with @ref 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 {#quick_key_input}

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, create a key
callback function.

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

The key callback, like other window related callbacks, are set per-window.

```c
glfwSetKeyCallback(window, key_callback);
```

In order for event callbacks to be called when events occur, you need to process
events as described below.


### Rendering with OpenGL {#quick_render}

Once you have a current OpenGL context, you can use OpenGL normally.  In this
tutorial, a multicolored rotating triangle will be rendered.  The framebuffer
size needs to be retrieved for `glViewport`.

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

You can also set a framebuffer size callback using @ref
glfwSetFramebufferSizeCallback and be notified when the size changes.

The details of how to render with OpenGL is outside the scope of this tutorial,
but there are many excellent resources for learning modern OpenGL.  Here are
a few of them:

 - [Anton's OpenGL 4 Tutorials](https://antongerdelan.net/opengl/)
 - [Learn OpenGL](https://learnopengl.com/)
 - [Open.GL](https://open.gl/)

These all happen to use GLFW, but OpenGL itself works the same whatever API you
use to create the window and context.


### Reading the timer {#quick_timer}

To create smooth animation, a time source is needed.  GLFW provides a timer that
returns the number of seconds since initialization.  The time source used is the
most accurate on each platform and generally has micro- or nanosecond
resolution.

```c
double time = glfwGetTime();
```


### Swapping buffers {#quick_swap_buffers}

GLFW windows by default use double buffering.  That means that each window has
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, the buffers need to be swapped with one
another, so the back buffer becomes the front buffer and vice versa.

```c
glfwSwapBuffers(window);
```

The swap interval indicates how many frames to wait until swapping the buffers,
commonly known as _vsync_.  By default, the swap interval is zero, meaning
buffer swapping will occur immediately.  On fast machines, many of those frames
will never be seen, as the screen is still only updated typically 60-75 times
per second, so this wastes a lot of CPU and GPU cycles.

Also, because the buffers will be swapped in the middle the screen update,
leading to [screen tearing](https://en.wikipedia.org/wiki/Screen_tearing).

For these reasons, applications will typically want to set the swap interval to
one.  It can be set to higher values, but this is usually not recommended,
because of the input latency it leads to.

```c
glfwSwapInterval(1);
```

This function acts on the current context and will fail unless a context is
current.


### Processing events {#quick_process_events}

GLFW needs to communicate regularly with the window system both in order to
receive events and to show that the application hasn't locked up.  Event
processing must be done regularly while you have visible windows and is normally
done each frame after buffer swapping.

There are two methods for processing pending events; polling and waiting.  This
example will use event polling, which processes only those events that have
already been received and then returns immediately.

```c
glfwPollEvents();
```

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,
@ref 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.  This saves a great deal of CPU cycles and is useful for,
for example, many kinds of editing tools.


## Putting it together {#quick_example}

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

This program creates a 640 by 480 windowed mode window and starts a loop that
clears the screen, renders a triangle and processes events until the user either
presses _Escape_ or closes the window.

@snippet triangle-opengl.c code

The program above can be found in the [source package][download] as
`examples/triangle-opengl.c` and is compiled along with all other examples when
you build GLFW.  If you built GLFW from the source package then you already have
this as `triangle-opengl.exe` on Windows, `triangle-opengl` on Linux or
`triangle-opengl.app` on macOS.

[download]: https://www.glfw.org/download.html

This tutorial used only a few of the many functions GLFW provides.  There are
guides for each of the areas covered by GLFW.  Each guide will introduce all the
functions for that category.

 - @ref intro_guide
 - @ref window_guide
 - @ref context_guide
 - @ref monitor_guide
 - @ref input_guide

You can access reference documentation for any GLFW function by clicking it and
the reference for each function links to related functions and guide sections.

The tutorial ends here.  Once you have written a program that uses GLFW, you
will need to compile and link it.  How to do that depends on the development
environment you are using and is best explained by the documentation for that
environment.  To learn about the details that are specific to GLFW, see
@ref build_guide.